Figure 1. Magnesium deficiency in Scarlet Royal table grapes. Photo courtesy of Matthew Fidelibus.
When most California grape growers think of macronutrients, nitrogen (N) and potassium (K) are rightfully at the top of list, as these two mineral nutrients are needed in relatively high amounts and are commonly supplemented with fertilizer applications. However, magnesium (Mg) is also considered a macronutrient, though it is needed in much lower amounts than N or K. Even so, it is not uncommon to observe Mg deficiency symptoms, especially in certain grape varieties which appear to be particularly prone to Mg deficiency, including Barbera, Grenache, Redglobe, Thompson Seedless, and Zinfandel. Recently, I have heard from several growers that some of the newer table and raisin grape varieties also appear to be prone to Mg deficiency. Rootstocks also differ in their ability to take up Mg. For example, 1103P is considered to be good at amassing Mg, whereas Riparia Gloire (Vitis riparia), and some V. riparia hybrid stocks are less effective at amassing Mg.
Magnesium
Magnesium is a central component of chlorophyll, and by mid to late summer, the leaves of Mg-deficient vines typically develop a distinctive creamy-white chlorosis along the margin of basal leaves. The primary and secondary veins of the leaves retain a dark green color, resulting in a Christmas-tree pattern on the leaf (Figure 1). In red varieties, the leaf margins may develop red color (Figure 1), and in severe deficiencies, the margin may become necrotic, brown colored, and dry. Analysis of petiole samples can be useful in verifying Mg deficiency. Petioles collected at bloom should contain >0.3 percent Mg.
Magnesium plays a critical role in enzymatic reactions, including the activation of adenosine triphosphate (ATP). Magnesium deficiency impairs the loading of sucrose into phloem in leaves, thereby causing carbohydrates to accumulate in leaves, while reducing the supply of carbohydrates to other organs that need them. Thus, Mg deficiency could theoretically limit the vine’s ability to produce and distribute carbohydrates. Australian research has linked low Mg levels in rachises with bunch stem necrosis (BSN), and in such cases, application of Mg reduced BSN. Vines with Mg-associated BSN sometimes, but not always, had leaves with Mg deficiency symptoms. However, studies in California have not verified a link to Mg and BSN.
Magnesium is Moderately Leachable in Soil
Magnesium is moderately leachable in soil and tends to be most abundant in subsoil and least abundant in the surface layers, especially on weathered soils. Young vines are more susceptible to Mg deficiency than older vines, probably because the roots of young vines have likely not explored as much of the subsoil as the roots of older vines. Thus, vine age, particularly the age of the root system (if on topworked vines), should be considered when assessing the relatively susceptibility of new varieties to Mg deficiency. Removal with the harvested crop can further reduce Mg in a vineyard, though previous studies suggest that grape berries only amass about 0.2 lbs Mg/ton of fruit. The Mg concentration in soils can be easily measured, but critical soil values have not been established, and it would be very difficult to account for the possible supply of Mg in the subsoil that may be available to the vine.
As noted above, Mg deficiency can occur due to insufficient Mg in the root zone, a limited root system, or both. However, Mg deficiency can also be induced by soil acidification (pH < 5.5), which can occur after years of irrigation and fertilization. High levels of other cations, especially K and calcium (Ca) compete with Mg for uptake by roots. Thus, an imbalance in K or Ca may induce Mg deficiency. Peacock and Christensen (1996) suggests Mg deficiency is most likely when the Mg saturation of cation exchange capacity of the soil is <5 percent, or when total exchangeable Mg concentration drops below 25 mg/kg. In addition, Peacock and Christensen (1996) notes that exchangeable Mg should be two to three times as high as exchangeable K.
Mild Mg Deficiencies
Mild Mg deficiencies, defined as the appearance of symptoms on a few basal leaves in localized vineyard areas, do not contribute to economic loss, and do not require correction. More serious deficiencies should be corrected. To correct Mg deficiencies, growers should consider the various factors, outlined above, which can contribute to Mg deficiency. For example, if soil acidification is found to be a contributing factor, then incorporating lime into the soil can help address Mg deficiency. Dolomitic limestone can increase pH and add Mg. Fertigation and foliar application of Mg fertilizers are effective and may be needed in cases where the deficiency is due to insufficient Mg in soil. Magnesium sulfate may be used for either fertilization method, though other Mg fertilizers are also available. Christensen and Peacock recommended ½ to 2 lbs of MgSO4/vine for fertigation, and 4 lbs MGSO4/100 gallons for foliar application.
Further Reading:
Christensen, L.P. and W.L. Peacock. 2000. Mineral nutrition and fertilization. In Raisin Production Manual. L.P. Christensen (Ed.), pp. 102-114. University of California Agriculture and Natural Resources, Oakland.
Peacock, B. and P. Christensen. 1996. Magnesium deficiency becoming more common. UCCE Pub. NG5-96
Figure 1 – Neofabraea leaf lesions on Arbosana olive.
Neofabraea leaf and twig lesions were first detected in California super-high-density oil olive orchards in 2016. Since then the disease was found in Glenn, San Joaquin, and Stanislaus Counties. Causal agents of this new disease of olive were identified as Neofabraea kienholzii and Phlyctema vagabunda (syn: Neofabraea vagabunda). Phlyctema vagabunda is known in Spain as the causal agent responsible for the olive leprosy or lepra fruit rot, causing fruit malformation as well as leaf lesion and twig canker. This disease is of increasing concern in Spain, Portugal and Italy. Dr. Trouillas at UC Davis has outlined the disease epidemiology, disease cycle, and determined best spray timings and materials that will help to control this disease.
Disease Symptoms
Neofabraea leaf and twig lesions are primarily associated with wounds, such as those sustained during mechanical harvest. Leaf lesions are circular to elongate, necrotic, approximately 0.5 to 1cm in diameter and normally do not number more than one lesion per leaf (Figure 1). Twig lesions are reddish-brown in color mainly affecting the bark tissues (Figure 2). The disease may occasionally cause fruit rot near the time of harvest. In severely infected orchards, defoliation and fruit loss may occurr.
Disease Biology
Two fungal pathogens have been identified using morphological and molecular techniques: Neofabraea kienholzii and Phlyctema vagabunda (syn: Neofabraea vagabunda). These pathogens have been associated with bull’s eye rot and canker of apples and pears in the Pacific Northwest.
In olive, the disease has been detected primarily from super-high-density oil olive orchards in Glenn, San Joaquin and Stanislaus counties. The cultivar ‘Arbosana’ is the most susceptible but the disease has also been isolated on occasion from ‘Arbequina’ olives in the Central Valley. It was not found in the Koroneiki cultivar. Previous reports of the disease in California olive have included fruit spots in ‘Cortina’, ‘Picholine’ and ‘Frantoio’ varieties in Sonoma county. To date, table olive varieties (Manzanillo and Sevillano) in the Central Valley have not tested positive for Neofabraea leaf and twig lesions.
Infection occurs at the site of plant injuries. In super-high-density oil olives, these wounds are typically associated with damage caused by mechanical harvesters but may also include abrasion sites where leaves or twigs rub against each other. Following mechanical harvest, rain events allow for fungal inoculum to be released in the air, leading to infection of the fresh wound sites. Leaf spot symptoms are most visible in March, with defoliation occurring in April and May. Infected leaves and fruits act as inoculum sources for infection the following year.
Disease Management
Field trials have been conducted for three consecutive years in the highly susceptible Arbosana cultivar to determine fungicide efficacy. Results showed that several products were effective in reducing infection by the pathogens and limiting disease incidence. Overall, best disease control was achieved by Topsin M, Vanguard, Inspire Super, Bravo and Ziram fungicides, which provided up to 75 percent reduction in disease incidence. Copper fungicides did not control the disease. Comparison of different fungicide application regimes showed that one to two applications after harvest significantly reduce disease incidence. Two independent wound susceptibility trials were conducted also to determine the duration (0, 1, 2, 3, 4 or 5 weeks) when wounds on leaves remain susceptible to infection, and thus determine the number and timing of fungicide applications required to control Neofabraea and Phlyctema diseases. Results showed that leaves inoculated immediately after wounding (harvest) and those inoculated one week after wounding were the most susceptible to infection. Overall, leaf wound susceptibility to infection declined substantially after four weeks following wounding. This suggests that wounds had healed after four weeks following a wounding event at harvest and that one fungicide application after harvest followed by a second application two to three weeks later should suffice to protect olive trees from infection.
Figure 2 – Neofabraea twig lesions on Arbosana olive.
Next Steps
Two fungicides were nominated to the IR-4 program in 2018: Ziram (Ziram 76WDG) and difenoconazole/cyprodinil (Inspire Super). These fungicides were approved for residue trials at the National Food Use Workshop in September for registration on olives. Strong support was provided based on the after-harvest and winter season usage with expected zero to limit-of-detection residues on the crop in the following harvest season. Ziram is a FRAC Code M3 whereas Inspire Super is a FRAC Code 3/9. Thus, integration of multi-site modes of action for both products was also established as an effective anti-resistance strategy. Ziram and Inspire Super were also submitted for section 18 emergency exemptions, which are expected to come into effect during the course of 2020. The availability of these two fungicides in olive will improve control of Neofabraea and Phlyctema leaf and shoot lesions and will allow for management of fungicide resistance by rotating modes of action.
Acknowledgements
We are thankful to the Olive Oil Commission of California for funding this research.
Experimental tank wagon for band application of liquid digestate in a walnut orchard. The digestate is pumped via a custom nozzle underneath the tree row. Food hygiene guidelines need to be observed.
Large amounts of organic wastes of food or animal origin accrue in cropping systems and in the food industry. Traditionally, many of these byproducts could remain in the agricultural production chain. For example, almond hulls may be used as dairy feed. Others ended up in landfills. With the continually increasing amounts, and for other market changes, alternative uses are urgently needed. When converting these energy-rich materials to biogas, organic matter from food waste or animal manure are processed through anaerobic digestion by microorganisms in specialized biodigesters. The resulting biogas is then used as fuel for electricity and heat generation or put into cars and other vehicles as transportation fuel. The anaerobic digestion process has been favored to reduce the emissions of methane and other gases from organic waste materials during natural decomposition. Although animal manure is probably the most widely used substrate for anaerobic digestion worldwide, food waste is another organic substrate due to its high methane production potential. Besides biogas, a liquid effluent called anaerobic digestate is also produced from digestion processes. The disposal of such residues represents an environmental and economic challenge. A meaningful use of this material would favorably impact environmental stewardship by reducing waste disposal issues, and could benefit agriculture by recycling the nutrients in the digestate for plant growth benefits.
Plant-parasitic Nematodes
Plant-parasitic nematodes are a constraint in crop production, especially in perennial crops in California. Long cropping cycles, soils that favor high nematode densities, and favorable climate conditions, increase nematode reproduction. In the past, nematode-infested fields have been effectively treated with soil fumigants before planting or with various post-plant nematicides. The use of fumigants and non-fumigant nematicides is challenged by human and environmental health concerns. For example, regulation limits the use of 1,3-dichloropropene materials under a so-called township cap—so quantity restriction based on the entire amount used in an area. Clearly, more environmentally friendly alternatives to the use of these chemicals are urgently needed.
Environmentally Friendly Alternatives
A number of studies have investigated the potential of these digestates as bio-fertilizers. Because these wastes originate from plant material they are nutrient rich and their use fits into a cyclic production of returning byproducts to the primary field production. Such cycling has positive environmental effects. In some studies, the potential of these digestate for managing pests and diseases in different crops were explored. In a study in Germany, anaerobically digested maize silage suppressed the sugar beet cyst nematode, a major pest of sugar beet production in Central Europe. Using organic materials as nematode management tool is challenging because such materials can vary greatly in their physico-chemical composition. This composition likely will impact the nematode-suppressive potential of digestates. It probably depends not only on the substrate but also on the conditions during anaerobic digestion.
Experiment with pepper in microplots. Microplots are contained areas of two foot diameter and five feet long culvers perpendicularly inserted in the ground, and filled with test soils. Each of these plots allow for precise application amounts of digestates or other treatments.
In a project supported by the Department of Pesticide Regulation (DPR), digestates from different sources of different processing conditions and substrate base as well as varying chemical constitution showed differences in nematode suppressive potential. This illustrated the challenge of working with organic materials, and the need to quickly and easily characterize the nematode suppressive potential of digestate. For this purpose, a robust fast turn-around bioassay was tested in three different incubation environments, two different growing containers, and with two different nematode life stages as inoculum. In this test, a single radish seed is planted into nematode-infested soil in small containers after a small amount of digestate has been added. After four to five days, a staining procedure is used to visualize the nematode that have penetrated the young radish roots. Low numbers compared to roots that did not receive the digestate suggest some suppression of nematode infection. In this project, results were similar in the different contexts, and the digestate tested was able to suppress nematodes in all contexts. Based on these results, this bioassay may be useful as a quality control tool for measuring nematode suppressivenesss of organic liquids that could possibly be implemented by commercial laboratories.
Watermelon experiment for testing for efficacy of digestates to suppress nematode population densities. Watermelon seeds are grown in root-knot nematode-infested soil after at-plant application of digestates for one month. Then roots are harvested and examined for nematode-induced galling.
Temperature
Temperature is one of the most significant parameters influencing anaerobic digestion. Biogas generation through the anaerobic digestion process can take place over a wide range of temperatures, from as low as 50 F (10 °C) to 135 F (55 °C), corresponding to psychrophilic <68 F (< 20°C ), mesophilic 68 to 104 F (20-40°C ), and thermophilic >104 F (>40°C ) conditions. Because of an increased biogas yield, in most cases, digesters are operated under mesophilic or thermophilic conditions. Temperature does influence the activity and composition of microorganism groups. This influences the methane yield and likely the constitution of the resulting digestate possibly influencing the nematode suppressive potential. Of course the substrate, which can vary between different organic wastes will impact this constitution as well. The substrate and the process may therefore impact what secondary metabolites are produced during digestion, and thus nematode suppressive potential. Therefore, liquid manure and food waste both processed either mesophilically or thermophilically were used in a number of experiments to study the influence of these two factors.
Radish seedling four days after seeding into nematode-infested soil and digestate amendment. This seedling has sufficient roots to allow for examination of nematode infection.
Food Waste Versus Manure
In the radish bioassay with the sugar beet cyst nematode, no difference in root penetration was found between the two substrates (food waste vs manure) but a significant difference was found between the two processes. The thermophilic digestates were able to reduce nematode root penetration by more than 50 percent compared to the mesophilic digestates. In greenhouse experiments, the digestates of different substrates and processes were used to treat watermelon in soil infested with Meloidogyne incognita (root-knot nematode, RKN) to test the versatility of nematode suppression. After five-weeks incubation, plants were harvested and roots evaluated for nematode damage (root galling, and number of egg masses). Nematode-induced galling was similar or higher in plants from the digestate treatments than for plants from the control. A numerically small but significant reduction in root galling was found in food waste compared to manure. None of the digestates resulted in better plant growth when compared to the control.
Small Field Experiments
Microplot and small field experiments were conducted to implement the findings of controlled conditions into practical field contexts. Application strategies included drench application of the digestates as pre- or post-planting treatments. In a bell pepper microplot trial in RKN-infested soil, five different digestates were applied at planting. Three months later, plants were harvested and roots assessed for nematode suppression. The digestates did not result in improved plant growth compared to the control treatments. Nematode damage in roots was not reduced after treatment with digestates. Although, populations for RKN after harvest, were lower in plots treated with mesophilic manure and similar to the nematicide control. Similar studies were conducted with almond and walnut and ring nematode, root-knot and root lesion nematodes but results were somewhat variable indicating the need for improved application strategies.
In summary, some beneficial effects of thermophilic digestates were observed on plant growth and nematode suppression compared to mesophilic digestates under controlled conditions. In preliminary tests in the greenhouse, nematode suppression was observed but under field conditions with different nematode pests of different crops, inconsistent results were obtained. Further experimentation is needed to elucidate the chemical nature of compounds conferring nematode suppression, and how to make use of this beneficial capacity of the waste product digestate. The environmental and economic benefits of cycling plant nutrients and concomitantly suppressing soil pests make this a valuable endeavor.
Root-knot nematodes are known for their root changing effects. Galls or the name-giving knots are visible on young seedlings, and older plants. Water and nutrient uptake are impeded by such unusual roots.
Weeds are problematic in crops, primarily because they compete with commodities for water, light, and nutrients, which can result in yield loss. Weeds can also impact crops, indirectly, by serving as alternate hosts for insects and pathogens (Del Pozo-Valdivia 2019; Petit et al. 2011), providing habitat for vertebrate pests (White et al. 1998), or by impeding harvest operations (Morgan et al. 2001; Smith et al. 2000), among many other effects.
Resistant Weeds
Consequently, growers employ a variety of control strategies, including the application of herbicides, to manage unwanted vegetation in their production systems. Although herbicides can be extremely effective, weeds may escape chemical control for a variety of reasons, including the evolution of herbicide resistance. Currently, there are 500 confirmed cases (species x site of action) of herbicide resistance, worldwide (Heap 2019). With respect to the United States, 164 unique instances of resistance have been documented. Most resistances (52 cases) are to the acetolactate synthase (ALS) inhibitors followed by the photosystem II (PS II) inhibitors (26 cases), 5-enol-pyruvyl-shikimate-3-phosphate synthase (EPSPS) inhibitors (17 cases), and the acetyl-CoA carboxylase (ACCase) inhibitors (15 cases) (Heap 2019). Examples of active ingredients for these sites of action would be rimsulfuron (ALS-inhibitor), atrazine (PS II-inhibitor, glyphosate (EPSPS-inhibitor, and sethoxydim (ACCase-inhibitor), respectively.
Herbicide Resistance
Herbicide resistance is an evolutionary process. Herbicides do not directly cause the mutations that lead to herbicide resistance, rather their repeated use over space and time ‘selects’ for the genetic mutations that result in reduced herbicide efficacy. In short, the genetic mutations that confer herbicide resistance are already present before the herbicide is applied. The herbicide treatment eliminates all the weeds that do not contain the mutated gene (i.e. the susceptible plants); if no further intervention is undertaken, the resistant survivors will continue to grow, flower, and set seed, which will be added to the soil seedbank. Over time, the resistant trait becomes dominant in the population as susceptible individuals die out without successfully reproducing (Figure 1) (Hanson et al. 2013).
Resistance in Other Weed Management
Herbicides are not, however, the only selective forces that can alter the structure of weed populations and communities. Any weed management or crop production practice can select for weed species that are adapted to the resulting environment. For example, repeated and consistent mowing (Pirchio et al. 2018) can favor the development of species that are naturally prostrate or spreading in habit, like clovers (Trifolium spp.) (Figure 2). The use of drip-irrigation in processing tomatoes can lower the numbers of annual weeds that emerge and compete with the crop (likely due to reduced surface wetting that stimulates germination) while facilitating the establishment of field bindweed (Convolvulus arvensis), a deep-rooted and drought-tolerant perennial weed (Shrestha et al. 2007; Sosnoskie and Hanson 2015; Sutton et al. 2006). The adoption of reduced tillage in processing tomatoes favors the spread of field bindweed which can be suppressed by frequent soil disturbance. Even hand-weeding can serve as a selective pressure; Echinochloa crus-galli subsp. Oryzicola, a form of barnyardgrass that mimics cultivated rice both in physical form and phenology, is difficult to visually identify and may escape removal in labor-intensive production systems (McElroy 2014).
Figure 2: Herbicide resistance is an evolutionary process. Herbicides do not actively mutate the target weeds, rather, the repeated use of an active ingredient over space and time eliminates susceptible individuals (plain green patches) from a population leaving only the resistant plants (orange patches with the “R”) to reproduce and set seed. Over time, the resistant trait becomes dominant in the population as susceptible individuals die out without successfully reproducing.
Managing Herbicide Resistance
When it comes to managing herbicide resistance, the Weed Science Society of America (WSSA) has a list of strategies to employ in order to increase the diversity of tools in a production system. However, these tools have value beyond the prevention and mitigation of resistance; varying the types and timing of disturbances should help to combat difficult to control species that arose in response to the repeated use of a weed control strategy. Some of the best management practices endorsed by the WSSA include:
• Using multiple herbicide modes of action and applying herbicides at the proper rates and times
• Adopting mechanical weed control when appropriate
• Rotating crops to diversify the type and timing of weed control and production practices
• Emphasizing cultural practices that are suppressive to weeds
• Preventing the movement of weeds within and between systems
• Reducing weed seed production and seed return to the soil seedbank
• Understanding the biology and ecology of troublesome species and identifying the forces that could allow them to become dominant in a given production environment
Source
Del Pozo-Valdivia (2019) Weeds serving as alternative hosts for diamondback moth. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=29228. Last accessed on May 14, 2019.
Hanson et al. (2013) Selection pressure, shifting populations, and herbicide resistance and tolerance. ANR Publication 8493. https://anrcatalog.ucanr.edu/pdf/8493.pdf. Last accessed on May 16, 2019.
Heap (2019) The International Survey of Herbicide Resistant Weeds. http://weedscience.org/ Last accessed on May 14, 2019.
McElroy (2014) Vavilovian mimicry: Nikolai Vavilov and his little-known impact on weed science. Weed Sci. 62:207-216.
Morgan et al. (2001) Competitive impact of Palmer amaranth (Amaranthus palmeri) in cotton (Gossypium hirsutum). Weed Tech 15:408-412.
Petit et al. (2011) Weeds in agricultural landscapes. A review. Agron. Sustain. Develop. 31:309-317.
Pirchio et al. (2018). Autonomous mower vs. rotary mower: effects on turf quality and weed control in tall fescue lawn. Agronomy 8:15.
Shrestha et al. (2007) Sub-surface drip irrigation as a weed management tool for conventional and conservation tillage tomato (Lycopersicum esculentum Mill.) production in semi-arid agroecosystems. J. Sustain. Agric. 31:91–112.
Smith et al. (2000) Palmer amaranth (Amaranthus palmeri) impacts on yield, harvesting, and ginning in dryland cotton (Gossypium hirsutum). Weed Technol. 14:122-126.
Over the past several years, few irrigated regions of the country have escaped the impacts of drought and in many places this has translated to increased costs and reduced availability of irrigation water. California growers in particular have experienced dramatic reductions in surface water deliveries and have increasingly turned to using groundwater to make up the surface water shortfall. Consequently growers have experienced unprecedented water table level reductions that have increased the price of pumping and maintenance on their water wells. Growers have also responded to the drought by increasing personal and personnel resources dedicated to water management and have increased their efforts to better understand the complexities of irrigation water management. Efforts to improve on-farm water management practices must include these key elements:
Elevating irrigation system design and management expectations.
Better exploit our understanding of crop development and physiology including crop sensitivity to water stress.
Increasing our capacities to measure and manage soil water storage.
Miro-sprinkler irrigation system in a young almond orchard.
Proper evaluation and integration of these key water management elements can be complex but can result in operating whole farm and field irrigation systems at peak efficiency over time. For example, it may not help us to increase the monitoring and measurement soil moisture if we do not have a more complete understanding of how specific changes in soil moisture content influence crop water stress and crop performance. Similarly it would not be difficult to misinterpret plant water stress or soil moisture information in a field where water is not applied in a uniform manner. And how might field indicators of soil water availability be used in different field management settings? Interpreting soil moisture readings in a drip irrigated field that applies water multiple times per week will differ from that of a furrow or flood irrigated field that is irrigated using two to three week intervals. These complexities help to point out that each field is unique from a water management standpoint and that irrigation decision makers should not rely too heavily on any one piece of information to guide their water management program without also considering broader systems information.
Post-filtration pressure monitoring in drip irrigation.
Irrigation system design and management
Improperly designed irrigation systems are incapable of achieving high performance levels making efficient water management an impossibility regardless of how well water might be scheduled. Application efficiency is a fundamental measure of irrigation system performance defined as the amount of beneficially applied water in relation to the amount of total irrigation water applied to the field. One of the biggest obstacles to developing field systems with high application efficiency comes from the fact that many irrigation systems do not apply water uniformly. Water applied in a non-uniform manner which commonly leads to larger soil water deficits on low water application areas and over-irrigating areas of the field that have higher than average application rates. To maintain preferred soil moisture levels in all areas of the field, water managers typically compensate by over-applying water on portions of the field which causes losses to leaching or run-off both considered to be non-beneficial uses of water. Properly designed irrigation systems work to achieve a high degree of uniformity and deliver near equal amounts of water throughout the field.
One of the more challenging design issues in drip irrigation systems is achieving near uniform pressures throughout the field. Even when fitted with pressure compensating emitters, fields that have large differences in line pressure are susceptible to significant differences in emitter output causing non-uniform applications. This problem is more commonly associated with long field lengths which stretch the design capacities of our current drip irrigation products. And while many drip irrigation fields have relatively high distribution uniformity, maintenance and management issues continue to leave many fields with less than optimal efficiencies. Field lengths of less than 800 feet are less prone to this concern while it is a much more common issue in field lengths exceeding 1100 feet. This emphasizes the need to carefully consider the products being purchased and the pressure requirements of that product. Uniformity issues can also be exagerated as the system ages or in systems that are not properly filtered and maintained to avoid biological and mineral contamination.
Occasionally simple modifications in surface irrigation systems can also result in significant improvements in distribution uniformity and application efficiency. Many flood and furrow irrigated systems experience slow water advance times down the furrow or irrigated strip before the irrigation is completed resulting in long infiltration periods at the head end of the field relative to the infiltration period at the lower end of the field resulting in higher amounts of applied water at the head end of the field. Solutions to this problem have been achieved by increasing the on-flow rate of irrigation water, reducing the size of the irrigation check and by modifying the surface soil roughness to allow water to more freely move to the bottom of the field. These relatively modest system design changes can have a significant impact on delivering water with greater uniformity and efficiency.
Water meters are an essential system evaluation tool.
Crop development, physiology and water stress management
Crop sensitivities to water stress are not constant throughout the growing season depending on crop type, growth stage and atmospheric conditions. Most field and row crops are highly susceptible to the impacts of limited water availability during the germination and seedling development stages and require high soil moisture availability in the surface soil during this period. However, as the early vegetative growth period is initiated crops like cotton and small grains can go many weeks before the first seasonal irrigation water is required. This is partially due to the rapid root growth that occurs in these plants and soil water is more easily extracted during these low evapotranspiration days. Alternatively, many vegetable crops including tomatoes, carrots and the brassicas require more frequent irrigation events early season to relieve mild water stress during this period needed to support a rapidly expanding plant canopy. Later in the season when roots are well established and the lower portions of the soil profile are exploited, many deep-rooted crops are less sensitive to water stress and water management strategies can be incorporated that take advantage of deep soil water reserves resulting in delayed irrigation scheduling.
Similar issues can be observed with the permanent crops as early season root flushes occur at different intervals allowing soil moisture to be exploited at varied depths depending on crop type. Generally, crop water stress sensitivities in permanent crops are often more acute during early leaf out periods if winter rains have not fully charged the soil profile and again during the rapid fruit growth periods. Monitoring the developmental stage of the crop often provides insights to the physiological periods of the crop that are more sensitive to water deficits and points to periods when water deficiencies are more likely to result in impacts to yield and quality. Understanding these more sensitive water stress periods also allows us to better plan on the time of year to increase crop water stress monitoring by using tools such as the pressure chamber or canopy temperature tools that are used to evaluate the relative degree of crop stress. And while water stress limits may differ during the growing season, using established water stress guidelines when available, can provide sound guidance on irrigation scheduling decisions.
Groundwater well development and re-development has become more common as groundwater levels decline and surface water availability is limited.
Managing and measuring soil water storage
The measurement of soil water can appear to be an uncomplicated process that involves placing soil moisture sensors in the soil and reading the values to provide an irrigation management decision. And while this may be satisfactory for relatively simple, uniform and well understood field systems, there is much that should be considered in developing an approach to soil moisture monitoring if the goal is to maximize the beneficial uses of applied irrigation water. Successful growers and consultants that regularly monitor soil water and use the information as an irrigation scheduling tool agree that there are several important questions to consider before purchasing, installing and using the sensors to make informed irrigation decisions. Before investing the time and resources in soil moisture monitoring, establish basic achievable goals with the understanding that the higher the expectation and more detailed the goal, greater effort and resources will be required.
As goals are established for field monitoring, consideration of the type of sensor to be used is a good place to start as well as the price of those sensors and systems. Field soil moisture systems can start with an investment of a few hundred dollars or less and can run into several thousand dollars for a single monitoring site. Soil water sensors can monitor soil water content more directly or they can provide direct or indirect measures of soil water potential. Each sensor type can be useful depending on the goals and information needs established. Identifying a field location or locations and placing the sensors at appropriate depths is also important and care should be taken to select locations that are representative of soil water conditions in the crop root zone. Monitoring multiple soil depths can also provide information on soil water storage and percent soil profile water depletion level. Depending on the time of year, many growers have turned to using different soil depths as triggers to initiate irrigation events using sensors placed at more shallow depths to schedule early season irrigation events.
Evaluating the readings from soil moisture devices also requires some experience and understanding of soil water retention characteristics of the field being monitored. Although estimated values of field capacity, permanent wilting point and plant available water can be referenced for various soil textural classes, these values are very generic and can misrepresent actual site values that can be used for irrigation scheduling purposes making the information less reliable. Developing individual field or soil type data for your fields often aids in providing more specific and repeatable information that can improve irrigation decision making.
Subsurface drip irrigation line configuration for vegetable crops including onions and garlic.
Integrating the information
Recognizing the need to integrate key water system information into a coherent field and farm water management plan system requires work and experience in evaluating multiple system elements and will assist in avoiding the tendency to place the focus and reliance on singular management indicators. Integration of these primary water management elements will have impacts on the time and amount of field applied water and can have significant impacts on farm water use by limiting the application of water that does not directly benefit the crop. Recognizing the importance of integrating appropriate information from multiple sources to manage farm water requires that we begin using the tools available to us to document and interpret a wide variety of information of our irrigation systems, soil systems and our individual cropping systems.
Independent evaluations of an irrigation system performance can provide feedback on the current issues of concern for individual systems and can provide an assessment of whether system maintenance is badly needed or if help determine if the system requires improved pressure regulation, higher flow rates or other design modifications to operate more optimally. Water applied uniformly and with high efficiency limits water and nutrient losses to the groundwater and results in more uniform crop yield and quality. But information related to application efficiency or distribution uniformity can also aid in targeting locations for soil moisture monitoring sites and locating sites to monitor plant stress and crop growth.
Using available knowledge of plant development and physiology can aid in identifying periods when the crop is particularly sensitive to water stress events and aid in irrigation scheduling decisions by either limiting the drawdown of soil moisture reserves during those periods or by extending the irrigation cycle. In a similar manner, the use of plant water stress indices can be used to hasten or delay irrigation events and help establish the intensity and duration of water stress events. Numerous university publications are available that provide sound information on the development and physiology of specific crops. This information often includes water management studies that can provide tools to identify crop developmental stage as well as information on water stress sensitivity and periods of relative tolerance to water stress.
Water flow monitoring is an essential system evaluation component.
Establishing reasonable goals and expectations for monitoring soil water status and using soil sensor information for irrigation decision making purposes can assist in tightening irrigation schedules thereby reducing the likelihood of excessive losses while also reducing the risk of crop losses that result from elevated crop water stress levels. This information is particularly useful when combined with other irrigation scheduling tools such as crop evapotranspiration estimates and crop water stress indicators.
Tensiometer installation in a citrus orchard. Tensiometers provide a direct measure of soil water matric potential.
The inaugural Crop Consultant Conference will be a gathering place for all who are dedicated to caring for California specialty crops.
Pest control advisors (PCA), certified crop advisors (CCA), applicators and agriculture retailers are all invited to participate in this two-day event, September 26th – 27th in Visalia.
September 26th – 27th in Visalia
This event at the Visalia Convention Center packs a full menu of educational workshops and seminars, professional networking opportunities plus multiple hours of PCA and CCA credits into 24 hours. The program begins at 1 p.m. on Thursday and concludes after lunch and a final speaker, at 1 p.m. on Friday.
The workshop and seminar topics at the CCC have been chosen to help all crop advisors keep informed about new regulations, pest and disease control and management updates, label information and new technologies. In addition to the educational component, this conference will feature an early evening mixer and networking opportunities to be followed by a full gala dinner and entertainment.
Why Attend?
“Where else can a PCA or CCA get that many hours of credit, receive useful information plus meals and entertainment and not have to drive long distances?” says Jason Scott, publisher of West Coast Nut, Progressive Crop Consultant and Organic Farmer magazines and host of this event.
“This event is right in their back yard, where specialty crops addressed in this conference are grown. It is designed to present the ‘big picture’ of specialty crop production, innovative technology, regulations and challenges here in California,” Scott added.
Citrus
Greg Douhan University of California Cooperative Extension (UCCE) area citrus advisor for Tulare, Fresno and Madera counties, said the conference will be a valuable forum to communicate important research and information regarding many aspects of various crops grown in California. Agriculture industry personnel, PCAs, CCAs, and so on and so forth, benefit from these meetings tremendously to keep abreast of the latest challenges that face California Agricultural producers.
Douhan, whose territory includes a major portion of California’s citrus belt, will be one of the featured conference speakers and will present current information on HLB and Asian Citrus Psyllid management.
Aerial Drone Technology
A presentation on aerial drone technology is also expected to drive attendance.
Chris Lawson, Business Development Manager for Aerobotics, will speak on optimizing integrated pest management (IPM) and nutrient management using drones.
Agronomist Nick Canata with Ingleby USA/Eriksson LLC of Visalia reports that the CCC agenda looks interesting, especially the drone technology presentation. His company, he added, is presently using aerial flyovers to obtain irrigation information.
Mating Disruption
Crop advisors who are evaluating their mating disruption choices will hear a panel of experts that includes United States Department of Agriculture (USDA) researcher Chuck Burks, Dani Casado chemical ecologist with Suterra and Peter McGhee, research entomologist with Pacific BioControl Corp. This panel will evaluate mating disruption as part of an IPM program.
Soils
Thursday’s program starts on the ground with sustainability specialist Richard Kreps who will explain how to get the most out or your soils.
Kreps, with Ultagro said making soils work at an optimal level requires a quite a bit of dedication. Attacking it from all sides: amending, nutrition applications, increasing organic matter, biology and proper irrigation require a lot of coordination. The upside is orchard longevity, higher returns with less disease and pest pressure.
Paraquat Guidelines
Thursday’s education agenda ends with new EPA guidelines for 2020 for Paraquat closed transfer system. Speaker will be Charlene Bedal, West Coast regional manager with Helm AGRO US.
Trade Show and Mixer
The conference mixer and trade show begin at 5 p.m. Thursday, and dinner will be served at 6 p.m. The keynote speech will be Trécé on the Future of Navel Orangeworm Management and Solutions. At 7 p.m., Las Vegas entertainer and illusionist Jason Bird will perform. One of the most innovative and prolific minds in the magic industry, Bird continuously advances the boundaries of his craft while making connections with his audiences. Bird will also perform small group illusions during the trade show/mixer.
Friday
Friday morning’s agenda kicks off at 7 a.m. with breakfast and a presentation by Patty Cardoso of Gar Tootelian on keeping growers compliant with local and state regulations. The trade show opens at 7:30.
Friday’s topics include A New Approach to IPM by Surendra Dara, UCCE entomologist; a panel discussion major crop pests affecting specialty crops; and an update on labels.
To register for this event and see a complete agenda, Click Here.
Figure 1: Farmers and consultants examining crop residue and root growth and development in strip till corn in Chowchilla, California. Photo courtesy of Jeff Mitchell.
Soils are essential for life on earth. In addition to the fundamental role of soil in agriculture, soils support building and recreation, filter and store water, recycle nutrients, protect our communities from flooding, sequester carbon, and due to the wide microbial diversity of soils, have even been a source of antibiotic and prescription drug discovery. Soils are alive! In fact, up to one billion bacterial and several yards of fungal hyphae can live in a single gram of soil. These microbes, invisible the naked eye, are at the core of many of our soil building management practices.
Despite being one of our most important natural resources, we may not often think about soil as something that needs to be built or protected. Unfortunately, soils globally and in the United States are being destroyed at a rapid rate. Soil is a non-renewable resource and once a soil has been degraded to the point where it cannot be used to produce crops, it is very challenging, if not impossible, to restore. As President Franklin Delano Roosevelt once said, “the nation that destroys its soil, destroys itself.” The United States Department of Agriculture (USDA)/National Resources Conservation Service (NRCS) estimates that the annual cost of soil erosion in the United States alone is $44 billion. While we cannot re-create soil once it is destroyed, we can employ on farm management practices to reduce the risk of soil destruction and to increase soil function.
The main principles of soil health are to maintain soil cover, minimize soil disturbance, keep a living root in the soil, and incorporate plant diversity. These principles are intended to keep soils alive by encouraging flourishing soil microbial communities and physically protecting soils from either loss or structural damage. Soil microbes are a critical part of soil health because of the role they play in nutrient cycling and building soil aggregates. Soil aggregates are clumps of soil particles that are bound together, leaving more available space for air and water. Aggregates are held together by organic matter (like roots), organic compounds (produced by soil microbes), and fungal hyphae. Microbes get nutrients and energy from the carbon found in soil organic matter. This is the reason that many soil health practices involve increasing soil organic matter—it provides food for soil microbes which increases their activity and population. Consider this: soil microbes are necessary for the conversion of nitrogen from one form to another (like ammonium to nitrate) and they need carbon to thrive.
Maintaining Soil Coverage
Bare soil is more susceptible to wind and water erosion, as well as to surface compaction. Our top soil is the most nutrient-rich part of the profile so when we lose soil to erosion we are losing our most valuable soil to the environment. Although loss of soil to erosion may seem minor from year to year, when we consider that it takes 500 years to form an inch of topsoil, if we are losing just 1/100 of an inch of topsoil to erosion a year, we are still losing soil five times faster than it is being formed. Surface compaction develops when rain and irrigation water hits tilled soil, which forms a soil crust. Maintaining soil coverage throughout the year physically protects soil and provides a range of other benefits like reducing soil evaporation rates, moderating soil temperature, and suppressing weed growth. In annual systems we can keep our soil covered by planting a cover crop during our fallow season. Keeping crop residue on the field is also effective (Figure 1).
Minimize Soil Disturbance
As described above, soil microbial activity is critical for soil aggregate formation and stability. Tillage practices disrupt this activity and break the fungal hyphae and roots that are holding aggregates together. Although it may seem that tillage increases soil pore space, this benefit is short lived. This is because individual particles break off aggregates in recently tilled soils and can fill in pore spaces. A healthy soil has about 50 percent open pore space (half filled with air and half filled with water). Soil pores are where roots grow and microbes thrive. There are other management practices to minimize soil disturbance. These include not working or driving over soil when it is wet, distributing tractor weight over a larger surface area to reduce pressure on specific points in the field, and reducing the number of trips over a field when possible. Even if converting to no-till isn’t realistic for your farm, reducing the number of passes with the disc over a field and using vertical instead of horizontal tillage are ways to minimize soil disturbance. California farmers in a number of regions are now experimenting and sharing their experiences with reduced disturbance production systems like direct seeding into crop residue from the previous crop with little soil disturbance (Figure 2). Ongoing summaries of this work may be found at http://casi.ucanr.edu/
Figure 2: Direct seeding corn into wheat stubble in Five Points, California. Photo courtesy of Jeff Mitchell.
Keep a Living Root in the Soil
Plant roots release small carbon-based compounds called root exudates, which are a mix of sugars, amino acids, enzymes, organic acids and other compounds. These exudates can help breakdown mineral nutrients, leading to increases soil fertility. They also serve as a source of food for soil microbes. Maintaining a living root in the soil helps keep our soil alive throughout the year. Many beneficial microbes cannot survive in a low-carbon environment and keeping a living root is yet another tool for maintaining consistent soil carbon levels. In general, keeping our soils alive throughout the year will increase soil function. Cover crops that are inserted into rotations as possible are a means for achieving this soil health principle. As part of a California Department of Food and Agriculture Healthy Soils Program grant, my colleagues and I are experimenting with planting cover crops during the fallow season. In San Joaquin County, UC Cooperative Extension farm advisors Michelle Leinfelder-Miles and Brenna Aegerter, are researching the effects of a warm-season legume cover crop between winter small grains rotations. In Sutter County, Amber Vinchesi-Vahl and I are researching the effects of a winter cover crop, at different seeding rates, between summer cash crops (Figure 3). Our projects are entering the second of a third-year project, and we look forward to reporting the results on soil health and crop yields in the future.
Incorporate Plant Diversity
In annual systems this is called crop rotation. Crop rotation is beneficial for many reasons. It breaks disease cycles by starving out pathogens that can only thrive on specific plants (or plants in a specific family). In addition, crops with different rooting depths will mine nutrients, release carbon compounds, and improve soil structure at different depths of the soil profile. Finally, plants form symbiotic relationships with various microbes, but these relationships are often species specific. When we incorporate plant diversity into our systems we also build up the diversity in our soil microbial populations. In perennial systems, plant diversity can be achieved by planting a cover crop in between crop rows.
Although soil biology is an important component of soil health, it is not the only consideration. It’s also important to remember the rules of soil fertility including the 4Rs and the Law of the Minimum. As a reminder, the 4Rs refer to the Right Rate, the Right Source, the Right Timing, and the Right Mode of Application. These principles allow us to optimize our fertilizer application by ensuring the greatest nutrient use efficiency. This reduces the risk of loss to the environment and can increase the bottom line. Liebig’s Law of the Minimum refers to the idea that the limiting factor has to be addressed first. In other words, the soil issue (pH, nutrient status) that is most restricting yield is the one that has the greatest potential to improve yield. If the pH is yield limiting, no amount of fertilizer application will fix this problem.
Maintaining soil health is the long game and changes may not be apparent for several years. However, the more the management practices outlined above are incorporated into our farming systems, the greater the likelihood of long-term viability and protection of arable land. In addition to the benefits already discussed, soil water dynamics can be improved by increased water infiltration and water storage. Every farming system is unique and some of the practices may be cost-prohibitive or not viable for some other reason. The goal should be to incorporate as many of the practices that will work in our farm systems as often as we can. Every opportunity to build and protect our soil will ensure the long-term economic viability of our farms, as well as food security for our growing global population.
Sprinkler and flood irrigation often generates runoff that transports sediment from agricultural fields to downstream rivers, lakes, and estuaries. Additionally, some classes of pesticides, such as pyrethroids, bind to the suspended sediments in tail water which can cause toxicity to aquatic organisms in these receiving waters. Currently numerous rivers and creeks in California are considered impaired by pesticides and sediment transported with drainage from agricultural land. As water quality regulations become stricter, growers will need to implement practices on their farms that treat potential pollutants in runoff.
Tail water can be a particularly challenging water quality problem on the central coast of California where overhead sprinklers are widely used for vegetable production. Sprinklers can contribute to high concentrations of suspended sediment in tail water because the force of the falling water droplets erode soil aggregates and allow fine particles to be carried with runoff water. Research that we have conducted on the central coast has shown that adding a low concentration of polyacrylamide to irrigation water can dramatically reduce sediment loads and sediment-bound pesticides in agricultural tail water (Fig. 1). Across a range of soil types, polyacrylamide treatment reduced sediment concentration in runoff by more than 90 percent on average. On some highly erosive soils polyacrylamide reduced sediment concentration in sprinkler induced runoff by more than 98 percent. Total phosphorus and nitrogen concentrations in sprinkler runoff were also reduced by 40 percent to 70 percent using polyacrylamide.
Despite dramatic improvements in water quality, polyacrylamide, also called PAM, has been slow to be adopted as a management practice on the central coast and in much of California. One reason may be because of misunderstandings about how to most effectively use PAM for treating runoff, especially for sprinkler irrigation. Another reason is that several of the physical properties of polyacrylamide make it challenging for handling and applying to fields.
Figure 1. Runoff from overhead sprinkler water untreated (left) and treated with 5 ppm PAM (right).
Brief background on PAM
Polyacrylamide (PAM) is a simple polymer molecule made of carbon, nitrogen, and oxygen. Various forms of PAM exist, but the type used to stabilize soil and prevent erosion is a very large, mostly negatively charged molecule (12-15 megagrams per mole). Agricultural PAM is commercially available in dry powder (granular), emulsified liquid, and dry tablet forms, and costs as little as $4 to $6 per pound of active ingredient depending on the formulation, supplier, and cost of the raw materials used for manufacturing PAM (i.e. natural gas). PAM is used for many nonagricultural purposes such as a flocculant for waste and potable water treatment, processing and washing of fruits and vegetables, clarification of juices, and paper production. It is also a component of makeup.
Use of PAM for Irrigation and Erosion Control
Because PAM is a very long, linear molecule it easily binds to soil aggregates, thereby preventing soil erosion during irrigation events. Beginning in the early 1990’s numerous studies demonstrated that low application rates of PAM (1 to 2 lb/acre) reduced runoff and improved water quality in furrow systems by stabilizing the aggregate structure of soil, improving infiltration, and flocculating out suspended sediments from irrigation tail water. Most of the research and demonstrations of PAM were conducted in furrow systems in Idaho and Washington states where soils are very erodible. By 1999, almost 1 million acres of land were annually treated with PAM in the northwest of the United States. Additionally, growers in the San Joaquin Valley and the Bakersfield areas of California used PAM to reduce soil erosion under furrow and flood irrigation.
Working with PAM
PAM can be very difficult to use if it is not handled correctly. Wet PAM is very slippery, and because it solubilizes slowly in water, PAM spills should be cleaned up with a dry absorbent rather than washing it with water. Although it is not toxic to humans, some precautions should be taken when handling PAM: Use gloves to avoid irritation to skin. Goggles will prevent eye exposure. Also, a dust mask is recommended when pouring or handling dry granular or powder forms of PAM to avoid inhalation.
One rule of thumb to keep in mind is that it is much easier to add water to PAM than to add PAM to water. Dry PAM rapidly absorbs water, increasing its original volume many times to become a slimy, gooey substance. Dissolving dry PAM in water can be challenging. Because PAM is a very large molecule it does not dissolve readily into water and requires many hours of agitation to dissolve. It will often stick to the side of a tank when being mixed. Also, mixing up concentrations greater than 0.15 percent in water is nearly impossible because the solution becomes very viscous and difficult to agitate. Some manufacturers sell effervescent PAM tablets which aids dissolution in water, but still only relatively dilute solutions can be mixed up.
For these reasons it easiest to use liquid PAM products that have been emulsified with a carrier such as mineral oil or humectants, or work with dry PAM products, such as granular PAM or PAM in a tablet form. The emulsified liquid products generally have active ingredient concentrations ranging from 25 to 50 percent.
Application Methods
For applications in furrow systems dry or liquid product can be added to water flowing in a head ditch or main line (if gated pipe is used) at a rate to achieve a 2.5 to 10 ppm (parts per million) concentration. Automated equipment can be used to spoon feed granular PAM into flowing canal water. The application can be made continuously during the irrigation or until the water advances almost to the end of the furrows. An alternate application method, called the “patch method” involves spreading granular PAM to the first 3 to 5 feet of each furrow. The granular PAM slowly dissolves as water flows down the furrows. A similar strategy is to add a PAM tablet at the beginning of each furrow. Applications into sprinkler systems require specialized equipment for injecting either liquid PAM or dry PAM into pressurized pipe which will be discussed in more detail later.
Environmental and Food Safety
Only PAM products labeled for application to food crops should be used in agriculture. Also, the buyer/processor/shipper of the produce should be informed that PAM is being applied to the crop, especially if the application is made near harvest.
Agricultural PAM used for soil erosion is not toxic to mammals. Environmental studies of anionic (negatively charged) PAM have not shown any detriment to fish, algae, and aquatic invertebrates such as Ceriodaphnia dubia, and Hyalella azteca. Polyacrylamide is sometimes confused with acrylamide monomer, a precursor in the manufacturing of PAM. Acrylamide monomer, a potent neurotoxin, has a high, acute toxicity in mammals. The Federal Environmental Protection Agency (EPA) requires that PAM sold for agricultural uses contain less than 0.05 percent acrylamide monomer. In soil, PAM degrades rapidly by physical, chemical, biological, and photochemical processes, but it does not decompose into the acrylamide monomer. A previous study of the movement of PAM from agricultural fields showed that less than three percent of the applied product remained in the runoff leaving the field. The remaining PAM in the tail water was almost completely removed through adsorption to suspended sediments as the water flowed a distance of 300 to 1000 ft in the tail water ditch.
One concern with using emulsified liquid PAM is that the mineral oil carrier can have toxicity to downstream aquatic invertebrates. However, toxicity from the mineral oil can be avoided by using PAM formulations with either high concentrations of PAM (>50 percent active ingredient) or with non-oil carriers such as humectants.
Optimizing PAM for Sprinklers
Although many research studies have evaluated the efficacy of PAM in furrow systems, fewer studies have evaluated the use of PAM with sprinklers. Applications of PAM made before irrigating with sprinklers, such as by spraying PAM solution or broadcasting dry product on the surface of the soil were far less effective than continuously injecting PAM at a low rate into the irrigation water. Injecting PAM only at the beginning of an irrigation was also less effective in controlling sediment in runoff than a continuous application at a low concentration during the entire irrigation. Our studies on the central coast show that injecting PAM to achieve a 5 ppm concentration in the irrigation water provided the highest reduction in sediment, nutrients, and pesticides in the tail water using the least amount of product. In some fields 2.5 ppm PAM provided equal efficacy for control of suspended sediments as 5 ppm PAM. Treatment with PAM should be started with the first irrigation after planting and continue during the following two to three irrigations. PAM should be reapplied when sprinkler irrigating after the field has been cultivated. Product can be saved in fields where very little runoff occurs during the first few hours of an irrigation by making an initial application for the first half hour and then applying product again when runoff becomes significant.
Injecting PAM into Pressurized Irrigation Systems
The chemical characteristics of PAM that make it so effective as a flocculant, also make it difficult to inject into pressurized irrigation systems. Liquid PAM solutions are viscous and will clog chemigation equipment with valves such as diaphragm and piston pumps, as well as venturi injectors. Also, because the desired PAM concentration in the irrigation water is low, injection rates as low as one to three ounces per minute are needed to treat 10 to 15-acre fields. Most small, gas-powered centrifugal pumps usually cannot be easily calibrated to inject at these low rates. Peristaltic pumps can be adjusted to inject slowly but often are not designed to operate under high pressures that are common in sprinkler main lines. The best type of pump that we have identified for injecting liquid PAM is an auger pump (Fig. 2.) which has no valves and can inject viscous solutions at very low rates. These pumps also maintain a consistent injection rate at pressures as high as 100 psi (pounds per square inch).
Figure 2. Trailer outfitted for injecting liquid PAM into the main line of an irrigation system using an auger metering pump.
Although liquid formulations of PAM are the best option for pressurized irrigation systems, we are currently exploring methods of injecting dry PAM into pressurized sprinkler systems. The advantage of dry PAM is that it is generally cheaper than liquid products and the possibility of introducing toxicity from the inactive emulsifying ingredients in the liquid products is eliminated. Another advantage of this approach is that it may require less labor since no pump must be calibrated and managed during an irrigation. The dry PAM applicator is loaded with cartridges containing either granular or tablet forms of PAM (Fig 3.). A portion of the water from the main line is diverted through the applicator chambers and then added back to the main water stream. Although the PAM concentration is lower than can be achieved with liquid PAM, preliminary tests have shown that as much as 90 percent of the suspended sediments can be eliminated in the runoff (Fig 4). Further studies during the upcoming season will evaluate the practicality of use this applicator in commercial fields.
Figure 3. Prototype dry PAM applicator for pressurized irrigation systems (left) and PAM cartridge that inserts into the applicator (right).
Other Potential Benefits of PAM
In addition to water quality benefits, we have observed or measured agronomic benefits from the use of PAM. Because PAM stabilizes soil aggregates, soil is less likely to crust under the impact of sprinkler droplets, which improves infiltration and decreases the volume of runoff. In one field trial, PAM reduced runoff from four successive sprinkler irrigations from 4000 gallons per acre to less than 1500 gallons per acre. Less crusting of the soil may also improve germination of small seeded vegetables such as lettuce. In one of four replicated field trials conducted in commercial lettuce fields, we were able to measure an increase in yield and plant weight with the use of PAM. This yield increase may be a result of better infiltration of moisture or because the seed emerged earlier than in the non-treated plots. Although we have not conducted long-term studies, anecdotal reports from growers who applied PAM to their fields over successive years were that soil structure was improved by keeping the fine particles in place.
Figure 4. Samples of field runoff from irrigation water treated using the dry PAM applicator (left) and untreated (right).
An additional benefit of PAM is saving costs associated with cleaning out ditches and retention ponds that become clogged with sediment during the irrigation season. Often once or twice per year growers on the east side of the Salinas valley who farm on soils prone to crusting and runoff must schedule a backhoe and crew to remove sediment from ditches and ponds and redistribute the material in their fields. To reduce costs with using PAM, growers also can receive cost-share payments under the United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) Environmental Quality Incentive Program) (EQIP).
In summary here are a few key concepts on using PAM for controlling sediment in runoff:
Polyacrylamide is a long linear molecule that binds to soil and can flocculate suspended sediments in water.
PAM does not readily solubilize in water and increases the viscosity of water (thickens).
Concentrations of 2.5 to 5 ppm PAM in irrigation water are ideal for optimizing erosion control benefits under sprinkler and furrow irrigation.
For agricultural purposes only use anionic PAM products for erosion control and labeled for use on food crops.
Small amounts of granular PAM can be applied to the beginning of furrows before flood irrigating (1 to 3 lbs/acre).
For sprinklers PAM needs to be injected into the irrigation water during the entire irrigation event.
Applying PAM to the soil before sprinkle irrigating or only at the beginning of an irrigation will not maximize control of sediment in runoff.
PAM should be applied during the first three to four irrigations after planting or transplanting and when irrigating after soil cultivation.
Auger pumps are ideal for metering liquid PAM products into pressurized sprinkler water.
PAM itself is not toxic, but the mineral oil in some liquid PAM products can be toxic to aquatic organisms.
A prototype applicator is being developed to inject dry PAM into pressurized sprinkler systems, although it is not yet commercially available.
Further information on using polyacrylamide is available on the UC Cooperative Extension Website for Monterey County (http://cemonterey.ucanr.edu/Custom_Program567/Polyacrylamide_PAM/)
Figure 1. Nitrogen stabilizers applied at sidedress fertilizer application. Photo courtesy of Michelle Leinfelder-Miles.
Introduction
Nitrogen (N) is part of a balanced, natural cycle in the environment among the atmosphere, soil, plants, animals, and water. Nitrogen is the most important element needed by crops, and we often add nitrogen fertilizer to optimize crop productivity. Nitrogen use in agricultural systems must be reported for regulatory compliance under the Irrigated Lands Regulatory Program and the Dairy Order to help ensure that a greater fraction of the applied N is recovered in the harvested crop and not lost to the environment. Nitrogen management gives consideration to the four R’s:
Right source: selecting a fertilizer source that matches with crop need and minimizes losses.
Right rate: applying the right amount based on crop need and nutrient availability through other sources.
Right time: applying the nutrient when the crop can use it.
Right place: fertilizer placement that optimizes the crop’s ability to use it.
The four R’s address management considerations (e.g. fertilizer program, irrigation), but site characteristics (e.g. soil, cropping system, weather conditions) also influence N recovery in the crop. Also important to improving crop N recovery is understanding barriers to adopting best management practices, such as costs or risks to crop quality or yield.
While the four R’s articulate four principles for nitrogen management, the N cycle in cropping systems is complicated. Nitrogen can be introduced and lost by various paths. We generally add N with fertilizer or organic matter amendments—such as crop residues, compost, or manure. Fertilizers provide N in plant-available forms—ammonium (NH4+) and nitrate (NO3–). Organic matter amendments must be mineralized before the N is available for plant uptake. Mineralization is a process that involves soil biology converting organic N to NH4+. The timeline of this conversion will depend on the properties of the amendment, environmental conditions—such as soil temperature and moisture, and the activity and abundance of soil microbes.
In the soil, NH4+ has different fates. It can be immobilized by microorganisms, taken up by plants, fixed to soil particles due to its positive charge, volatilized to ammonia gas (i.e. lost from the system), or converted to NO3–—a process known as nitrification. Nitrification is a two-step process. The first step is the conversion of NH4+ to nitrite (NO2–) by Nitrosomonas bacteria. The second step is the conversion of NO2– to nitrate (NO3–) by Nitrobacter bacteria. These two steps generally occur in close succession to prevent the accumulation of NO2– in the soil. Conditions that affect nitrification include soil aeration, moisture, temperature, pH, clay and cation content, NH4+ concentration, among others. Just as NH4+ has different fates in the soil, so too does NO3–. Plants preferentially take up NO3–, but if NO3– is present when plants are not in need of it, then NO3– may be immobilized by microorganisms, volatilized to nitrogen gas (i.e. lost from the system), or leached out of the root zone (i.e. lost from the system).
Technologies have been developed to mitigate N losses from the system. These technologies are collectively known as enhanced efficiency fertilizers (EFF) and include additives, physical barriers, and chemical formulations that stop, slow down, or decrease fertilizer losses. Nitrogen stabilizers, slow-release fertilizers, and polymerized fertilizers are examples of EEF. Nitrogen stabilizers are fertilizer additives intended to improve crop N use efficiency and reduce N losses to the environment by interrupting the microbial processes that change N to its plant-available forms. We developed a trial to evaluate two N stabilizer products with the objective of determining whether the treatments improved corn silage yield or plant N status compared to fertilizer alone. We did not attempt to measure N losses from the system (e.g. leaching, denitrification), as these are very challenging to quantify.
The products in our trial were Vindicate (Corteva Agriscience) and Agrotain Plus (Koch Agronomic Services). Vindicate delays the nitrification process by inhibiting the Nitrosomonas bacteria that converts NH4+ to NO2–. Vindicate has bactericidal activity, and the active ingredient is nitrapyrin. Agrotain Plus has two modes of action—reducing ammonia volatilization and delaying nitrification. Ammonia volatilization is the conversion of NH4+ in the soil to ammonia gas (NH3) in the atmosphere, and it is reduced by inhibiting the urease enzyme. Ammonia volatilization is most problematic when the N source is urea-based and not incorporated or watered into the soil. The active ingredients of Agrotain Plus are Dicyandiamide (DCD), which delays nitrification, and N-(n-butyl)-thiophosphoric triamide (NBPT), which reduces volatilization. DCD has bacteriostatic activity, which means it slows the metabolism of Nitrosomonas. We hypothesized that N stabilizers would improve yield and N uptake over the fertilizer-only treatment, providing growers with a tool for nutrient stewardship.
Methods
The trial took place in San Joaquin County on a DeVries sandy loam soil. The field had a winter wheat crop that was cut for forage in the late spring. Dry manure was applied to the field between wheat harvest and corn planting, which occurred on May 24, 2018. The variety was Golden Acres 7718. At-planting fertilizer provided approximately 12 lb N per acre (4-10-10). Sidedress fertilizer application occurred on June 21st and provided approximately 105 lbs N per acre (UAN 32). Four treatments were applied at sidedress, when plants were at V3-4 stage of development (Fig. 1). The N stabilizers were applied at the label rates, and the treatments were: 1) Vindicate at 35 fluid ounces per acre, 2) Agrotain Plus at 3 pounds per acre, 3) combination of Vindicate and Agrotain Plus at aforementioned rates, and 4) fertilizer-only, no stabilizer product (“untreated”). Plots were 35 feet across (i.e. fourteen 30-inch rows), in order to adapt to equipment of different widths, by 900 feet long. Treatments were randomly applied in three replicate blocks. Aside from the treatments, the trial was managed by the grower in the same manner as the field.
We evaluated soil N status, plant N status, and silage yield. Prior to planting, 20 soil cores were randomly collected from across the trial and aggregated by foot-increments, down to two feet. Mid-season leaf and soil samples were collected when the corn was in the R1 stage (i.e. silking). Soil was collected from 10 in-row locations in each treatment, and aggregated by foot-increments, down to two feet. Leaves were sampled from ten plants in each treatment, sampling the leaf one-below and opposite the earleaf. Harvest occurred on September 20th. All fourteen rows were harvested for weight, and samples were collected at the silage pit for aboveground biomass N analysis. The samples were dried at 60⁰C for 48 hours for calculating dry matter (DM). Post-harvest, 10 in-row soil cores were collected to one-foot depth and aggregated for each treatment. Laboratory analyses were conducted by Ward Laboratories (Kearney, NE; https://www.wardlab.com/). We used Analysis of Variance to detect differences in treatments and Tukey’s range test for means separation (JMP statistical software). Treatments were considered statistically different if the P value was less than 0.05.
Results and Discussion
There were no statistically significant differences among treatments for plant tissue N, yield, dry matter, or total N removed at harvest (Table 1). Mid-season leaf N averaged 2.88 percent across treatments, and aboveground biomass N at harvest averaged 1.12 percent. At mid-season, leaf N from 2.7 to 3.5 percent indicates that the plants had sufficient N to carry the crop to harvest, and at harvest, whole plant N from 1.0 to 1.2 percent indicates that the N fertilization program was adequate for maximizing yield [1]. Calculated to 30 percent dry matter, average yield across treatments was 38.8 tons/acre, and dry matter was 35 percent. There was a trend for the two treatments with Vindicate to have a higher N removed than the two treatments without it, but the difference was not statistically significant. The low coefficient of variation (CV), which is a measure of variability in relation to the mean, indicates low variability among replicates for all of these parameters.
Treatment
Midseason (R1) Leaf Total N
(%)
Aboveground
Biomass Total N
(%)
Yield at
30% DM (tons/acre)
DM
(%)
Total N Removed at Harvest
(lbs N/acre)
Vindicate
2.97
1.12
40.4
0.37
272
Agrotain Plus
2.97
1.11
37.7
0.34
250
Vindicate and Agrotain Plus
2.71
1.16
38.7
0.34
269
Untreated
2.87
1.09
38.3
0.35
251
Average
2.88
1.12
38.8
0.35
261
CV (%)
4
2
3
3
5
P value
0.32
0.18
0.48
0.20
0.39
Table 1. Plant N, yield, dry matter (DM), and N removed results for the 2018 N stabilizer efficacy trial. There were no significant differences among treatments.
The pre-plant (post-dry manure application) soil nitrogen status was 17 parts per million (ppm) NO3-N and 4 ppm NH4-N for the 0-12 inch depth, and 7 ppm NO3-N and 2 ppm NH4-N for the 12-24 inch depth. When soil NO3-N is below 25 ppm in the top foot of soil, it is recommended to apply N fertilizer in order to prevent yield reductions [2]. There were no differences among treatments in soil N status at the mid-season sampling, but there were differences at the post-harvest sampling (Table 2). Mid-season soil NO3-N averaged 32 ppm across treatments in the top foot of soil, and 10 ppm in the second foot of soil, which is an adequate concentration to carry the crop through to harvest. Soil NH4-N averaged 4 ppm and 2 ppm across treatments for the top foot and second foot, respectively. The CV was high for mid-season soil data, which indicates high variability among replicates. Post-harvest soil NH4-N averaged 2 ppm across treatments in the top foot of soil, but soil NO3-N was higher than at any other time during the season, averaging 46 ppm across treatments. These results may indicate that the dry manure mineralized later in the season, after the peak demand of the corn. Post-harvest soil NO3-N above 20 ppm is considered high and indicates that this crop was not deficient in N [2]. The low CV for NO3-N indicates low variability among replicates. The significant differences among treatments are not well-understood, particularly as the control (fertilizer-only) treatment had soil NO3-N that was not different from any of the treatments. Interestingly, Vindicate had the lowest post-harvest soil NO3-N and a trend toward higher N removed (though not statistically higher), which may indicate that product use made N available at a time that optimized N uptake.
Treatment
Mid-season NO3-N (ppm) 0-12 inches
Mid-season NO3-N (ppm) 12-24 inches
Mid-season NH4-N (ppm) 0-12 inches
Mid-season NH4-N (ppm) 12-24 inches
Post-harvest NO3-N (ppm) 0-12 inches
Post-harvest NH4-N (ppm) 0-12 inches
Vindicate
33
10
4
2
38 b
2
Agrotain Plus
32
10
4
2
44 ab
2
Vindicate and Agrotain Plus
32
9
3
2
57 a
2
Untreated
31
12
4
2
43 ab
2
Average
32
10
4
2
46
2
CV (%)
25
31
20
36
7
19
P value
0.99
0.75
0.58
0.97
0.04
0.89
Table 2. Soil N status (as NO3-N and NH4-N) at mid-season and post-harvest samplings for the 2018 N stabilizer efficacy trial.
Summary
N is part of a balanced, natural cycle in the environment and is the most important nutrient in cropping systems. Giving consideration to N management will help ensure that a greater fraction of the applied N is recovered in the harvested crop and not lost to the environment, and keeps growers in regulatory compliance. Enhanced Efficiency Fertilizers, such as N stabilizers, have been shown to improve crop yield in regions like the Midwest and the Northeast, and may help to mitigate N losses from the environment. In our trial, we evaluated the efficacy of N stabilizer products for improvements in corn silage yield or plant N status compared to fertilizer alone. Under the management and environmental conditions of this trial, we found no differences in yield or plant N status; however, plant and soil tests indicated that N was never limiting in the trial. If N was lost from the system, the loss was not large enough to result in N limitation in the control. Future study should test these products using different N sources and N rates (e.g. grower rate and grower rate minus 50 lbs N/acre). It may be possible to reduce the fertilizer N rate without sacrificing yield.
First known as Anaheim grapevine disease or California vine disease, Pierce’s disease (PD) has impacted California’s grape production since the late 1880’s. Grape growers had been losing vineyards to an unidentifiable disease, which prompted the US Government to hire Newton B. Pierce, the first United States Department of Agriculture (USDA) plant pathologist and namesake of the disease. Pierce had spent considerable time walking vineyards in Los Angeles, San Bernardino and Orange Counties where Muscat of Alexandria used for raisins and Mission and other wine grape varieties were dying at alarming rates (2). At the time, approximately 25,000 acres had been infected and or lost to an unknown “malady”. Pierce also spent time traveling to France, Italy and other Mediterranean grape growing regions studying plant disease symptom expression and declining grapevines to compare with those found in California. After researching all aspects of California viticulture production, including pests, diseases and associated symptoms, Pierce could never correctly identify the cause of California vine disease. For many years, a plant virus was thought to be the culprit of PD. It wasn’t until the mid-1970’s when University of California (UC) researchers isolated a bacterium from diseased vines. Once isolated, the bacteria were reintroduced to healthy grapevine plants that developed Pierce’s disease symptoms within 2-4 months (1). The bacterium is known to move from vine to vine with the help of insect vectors representing the sharpshooter (Cicadellidae) and spittlebug (Cercopidae) families. Infected insects ease of movement into a vineyard can be devastating in a few seasons when a PD susceptible grape variety is planted.
Although Pierce’s disease outbreaks occurred in California vineyards from time to time since being ID’d it was not a primary issue for the grape industry. Major raisin, table and wine grape growing regions had moved north into the San Joaquin and Sacramento Valleys and the central and north coast. The mild, southern California weather was the perfect environment for the pathogen, vectors and disease development. In contrast, the seasonality of the interior valleys and coastal grape growing regions seemed to result in a lower PD incidence year to year. However, there were PD “hot spots” located near riparian areas (i.e. Napa and Kings Rivers) or alfalfa planting that experienced significant vine deaths. Those hot spots were costly to individual farming operations but was not a concern for the industry. That changed when the disease/vector dynamics shifted.
In 1999, the nonnative PD vector, glassy-winged sharpshooter (GWSS), arrived in Temecula Valley. At that time a once thriving southern California wine industry was experiencing rapid vine death. GWSS turned out to be an effective vector and superior flyer when compared to native sharpshooters. Additionally, vineyards planted next to citrus proved to be a deadly combination. GWSS used citrus groves to feed, breed and for protection from potential predators from late fall to early spring. California grape growers were concerned about their future as they watched Temecula Valley vineyards die. As GWSS spread to other parts of California, PD became a much greater concern and problem. According to K.P. Tumber et. al (3) California growers are paying $56+ million in lost production and vine replacement annually.
Photo courtesy of University of California
Pierce’s Disease: Cause, Symptoms and Management
Cause of Pierce’s Disease
A gram-negative bacterium was found to be the causal organism of Pierce’s Disease in 1975 (1). Prior to Xylella fastidiosa being identified, it was thought that a virus was responsible for the demise of southern California vineyards. Newton B. Pierce, the diseases namesake, began researching the cause in the late 1880’s but never properly identified it as a bacterium.
Living in the xylem of grapevines, X. fastidiosa blocks the movement of water and nutrients throughout the plant. Once infected, the bacterium moves systemically from the point of infection to other parts of the plant. Early season symptoms can be confused with nutrient deficiencies (i.e. Zinc), displaying interveinal leaf chlorosis and stunted growth. Late season symptoms have a scorched foliage appearance resulting from the plants inability to move water through the xylem vessels. Young vines are more susceptible than older vines to infection and may die by the end of the season, while older plants may display symptoms over several seasons. However, when bacteria populations increase to a level that restricts significant sap movement, foliage and fruit will dehydrate and die. Geographical location, time of year and variety (Table 1) will determine how severe the symptoms become. As temperatures increase, fruit will shrivel, and green shoots mature poorly and never cure prior to winter. At this point, financial losses are expected to impact vineyard viability.
Pierce’s Disease Symptoms
Springtime symptoms consist of grapevine leaves displaying interveinal chlorosis
Late-summer or fall symptoms consist of grapevine leaves displaying concentric rings of drying from the leaf margin towards the center. Leaf margins of red or black grape varieties turn red and then brown
Leaves that have turned brown will detach, leaving only the petioles attached to canes
A unique disease symptom is the irregular, patchy bark maturity, leaving half the shoot brown and half green, displayed as islands of green and mature brown coloration
Berries on clusters will shrivel and/or raisin
Pierce’s disease symptoms can often be confused with nutritional deficiencies, water related issues or other diseases. Multiple tissue samples should be shared with your pest control advisor (PCA), certified crop advisor (CCA), local farm advisor or university plant pathologist to correctly ID the symptoms. Once properly identified, a treatment plan can be devised to improve the vineyard’s health.
Management
Management strategies will depend on several factors. Insect vector, grape variety and location will have a significant impact on the success of managing PD. The four main insects that transmit PD are the blue-green sharpshooter (Graphocephala atropunctata), native to coastal regions near riparian areas; the green sharpshooter (Draeculacephala minerva) and red-headed sharpshooter (Xyphon fulgida), native to interior valleys; and the glassy-winged sharpshooter (Homalodisca vitripennis), a non-native species to California and the most dominate vector of X. fastidiosa. It is important to monitor for sharpshooter insects if PD is to be managed. Sticky cards, sweep nets and visual observations of the vineyard and nearby properties will help in determining the population size and what control measures will be needed. Once identified, properly timed insecticide applications will help reduce the population. Vineyards located in areas where the PD bacterium is common, and temperatures are mild will be a challenge at keeping PD under control. In this case, identifying and managing the insect vector will be most important. If GWSS is the primary vector, insecticides will need to be timely to keep insects from moving into the vineyard. Citrus planted next to a vineyard will have to be sprayed as well to keep populations in check. Citrus should be visually checked for adults, nymphs and eggs. Visit the UC Pest Management Guidelines online for the most current insecticide management strategies (4,5).
Locations that have a history of PD should not be planted to highly susceptible varieties like Chardonnay if possible. Finding a more tolerant PD variety will improve the health of the vineyard. Newly developed PD resistant varieties have been released from UC Davis. These numbered wine grape selections (Table 1.) can be planted in areas with a high incidence of PD and used for blending with traditional varieties. Unfortunately, there are not any PD resistant varieties for raisin or table grape production, but research is ongoing.
Table 1. Variety Susceptibility to X. fastidosa
Highly susceptible
Moderately susceptible
PD resistant*
· Chardonnay
· Redglobe
· Fiesta
· Riesling
· Chenin blanc
· Cabernet Sauvignon
· Ruby Cabernet
· Muscat of Alexandria
· Thompson Seedless
· 07355-075 (50% Petite Sirah, 25 % Cabernet Sauvignon)
*UC Davis PD resistant wine grape varieties released in 2017 from Dr. A. Walker
Grapevines showing unusual foliar symptoms should be taken to your local UC Cooperative Extension office for identification. Plant tissue suspected of having Pierce’s disease can be sent to a diagnostics lab for positive identification using molecular tools. Leaf blades and petioles sampled from green portions of the cane in the late summer to early fall will give the best results. Vineyard insects should be caught and identified, too. A sweep net or sticky cards strategically placed in the vineyard can be used to survey insect populations in areas displaying foliar symptoms. Unique insects can be taken to your local Agriculture Commissioners office or the California Department of Food and Agriculture—Plant Health and Pest Prevention Services Division for identification. These first steps are paramount for developing a management plan.
References
Pierce’s Disease
Davis, MJ, Purcell, AH, Thomson, SH, 1977. Pierce’s Disease of Grapevines: Isolation of the Causal Bacterium.
Pierce, NB. 1892. The California vine disease: a preliminary report of investigations. U.S Dep. Agric. Div. Veg. Pathol. Bull. 2, 222.
Tumber K, Alston J, Fuller K. 2014. Pierce’s disease costs California $104 million per year. Calif Agr 68(1):20-29. https://doi.org/10.3733/ca.v068n01p20
