Figure 1. Head rot symptoms start as yellow spots and then turn brown and black (all photos courtesy Y. Wang.)
Broccoli head rot, also known as pin rot, continues to increase in the Salinas Valley, especially in fall broccoli production. Two types of head rot, bacterial head rot and Alternaria head rot, are affecting broccoli (Koike 2010). Here we focus on Alternaria head rot caused by the fungi Alternaria spp.
Head Rot Symptoms
All aboveground parts of broccoli are subject to infection including heads and leaves. Head rot symptoms start as yellow spots and then turn brown and black (Figure 1).
Figure 1. Head rot symptoms start as yellow spots and then turn brown and black (all photos courtesy Y. Wang.)
The infection can spread from buds to stems (Figure 2).
Figure 2. The infection can spread from buds to stems.
With secondary bacteria or fungi infection, further decay occurs. The initial yellow spots resemble brown bead (Figure 3), a broccoli disorder that can potentially be caused by excessive temperature, poor growth or nutrient and water deficiency. However, the brown bead doesn’t rot the stem, and no sign of fungi is presented on the buds. For uncertain cases, scraping the buds to see if the stem rot or fungi are presented is a useful technique. Leaf spot symptoms start as small yellow spots on the old leaves and then form dark, concentrical rings like a target (Figure 4). The old spots may become brittle and split open or fall out as shot holes. The high number of leaf spots per plant indicates a higher disease pressure and could be a signal for fungicide application.
Figure 3. Brown bead, a broccoli disorder.
Management Options
The disease is favored by prolonged wetness from rain, dew and fog. The wetness, a thin layer of water, is required for fungal spore germination. In addition, fungal spores are spread by winds and splashing water. Cultural practices to promote leaf drying or prevent leaf wetness may reduce disease severity. Some growers have seen the benefits of using drip irrigation instead of overhead irrigation to avoid wetting the foliage. An early harvest before rainfall could also reduce disease risk. Variety effects on disease tolerance play a role. Lumpy broccoli heads tend to accumulate water which may further weaken the plant tissues and become a suitable target for the pathogens. Finally, there are a number of fungicides that have activity against the disease. Preventative fungicide applications should be considered for wet weather that is favored by the disease.
Figure 4. Leaf spot symptoms start as small yellow spots on the old leaves and then form dark, concentrical rings like a target.
Research Update: Fungicide Evaluation
This study was conducted to evaluate some new and common fungicides in fall broccoli to support the growers and ag industry.
One fungicide trial was conducted in a commercial broccoli field to test the efficacy of select fungicides for controlling broccoli head rot in fall 2023. Broccoli ‘Centennial’ was direct-seeded on July 27, 2023. Seven fungicide treatments and a nontreated control were arranged in a randomized complete block design with four replications. Each plot consisted of two seedlines of broccoli 30-ft long on 40-inch-wide beds. On each side of the plot was a nontreated guard bed. Treatments were applied with a CO2-pressurized backpack sprayer calibrated to deliver 35 gpa at 30 psi using double TeeJet 8004E flat fan nozzles. Fungicide applications were made on October 4 and October 16. All treatments were applied with non-ionic surfactant Dyne-Amic 0.08% v/v. Alternaria head rot incidence was evaluated at harvest on October 23. Disease incidence was expressed as the percentage of the number of plants with Alternaria head rot in the total number of plants within the middle 15 ft of the plot. Data were analyzed using analysis of variance (ANOVA) and the Tukey test to separate means at P<0.05. The total rainfall received one month before harvest was 0.57 inches. The average, minimum and maximum temperatures were 62 degrees F, 53 degrees F and 75 degrees F, respectively.
Table 1
The disease pressure in this trial area was low with nontreated control having 14.0% head rot (Table 1). However, significant differences occurred among treatments for the % Alternaria head rot. All treatments reduced % Alternaria head rot numerically, while Pydiflumetofen+Fludioxonil, Azoxystrobin, Fluxapyroxad+Pyraclostrobin, Fluopyram+Trifloxystrobin and Pyraclostrobin had significantly lower % Alternaria head rot than nontreated control. And they had statistically similar % Alternaria head rot. These results also showed single FRAC 11, premixes with FRAC 7 and 11, and premixes with FRAC 7 and 12 provided good control of Alternaria head rot. Single FRAC 7 provided fair control of Alternaria head rot.
Thanks to the cooperating growers and PCAs for assisting the trial. Thanks to agrochemical companies for funding and the technical assistance from Carlos Rodriguez.
References
Koike, S. T. 2010. Looking ahead: head rot can be issue for winter and early spring broccoli. Salinas Valley Agriculture blog. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=3861
Olives produce numerous small flowers (a) on panicles (b). Each panicle contains 12 to 20 flowers (b). Perfect flowers contain two anthers and a pistil in the center (c) (photos by E. Andrews, illustration by S. Hishinuma.)
Olive orchards entering an ‘off’ year in 2024 may benefit from pre-bloom foliar boron applications to support reproduction and yield. Because the 2023 California olive crop varied widely both within and between olive-growing regions, the value of B applications should be considered at the individual orchard level. For example, in the southern San Joaquin Valley, the 2023 ‘Manzanillo’ table olive crop was off due to the high temperatures at bloom whereas many oil cultivars in the region were unaffected by the heat and had heavy production. Those orchards that had a heavy ‘on’ crop in 2023 may benefit from pre-bloom B application in the 2024 season.
B is an essential micronutrient for plant growth and reproduction. B deficiency affects plant reproduction by reducing pollen viability and germination and limiting pollen tube growth. Deficiency also limits the proportion of flowers that set fruit and reduces the retention of developing fruit. The influence of B deficiency on multiple stages of reproduction may negatively impact yield. B also plays a role in vegetative growth and metabolism, ensuring cell wall and membrane integrity and facilitating sugar transport and cell division. Because it plays a crucial role in reproduction, B is translocated from vegetative tissues to reproductive tissues, resulting in higher concentrations of the nutrient in reproductive organs than leaves. Due to this high demand, reproductive B deficiency can occur even when vegetative B and available soil B are sufficient.
Figure 1. Olives produce numerous small flowers (a) on panicles (b). Each panicle contains 12 to 20 flowers (b). Perfect flowers contain two anthers and a pistil in the center (c) (photos by E. Andrews, illustration by S. Hishinuma.)
Benefits of Boron
Studies conducted across numerous global olive-growing regions demonstrate the beneficial effects of foliar B application on yield, particularly in advance of an off crop. The influence of B application on productivity in olive orchards may relate to increases in photosynthesis, an increase in the number of perfect flowers (those with both male and female reproductive parts) (Figure 1) and an increase in pollen viability, or pollen tube growth. Olives are considered andromonoecious, a reproductive strategy in which plants bear both hermaphroditic (perfect) flowers and male flowers. Stress prior to bloom may cause pistil abscission in a fraction of buds resulting in a higher percentage of male flowers. Several research studies have demonstrated pre-bloom foliar B application can increase the percent of perfect flowers on trees, thus increasing the number of flowers capable of producing fruit. In olive, B is readily mobilized from both young and old vegetative growth to support flower and fruit production; therefore, a portion of B applied throughout the year may be utilized to support reproductive processes. During the pre-bloom season, however, cool temperatures and the corresponding reduced physiological activity may limit the uptake and translocation of B in olive. Additionally, flowers are not as strong a B sink as fruit; therefore, the pre-bloom foliar application may render the micronutrient available at a short-lived yet critical time in crop development.
Both oil olives and ‘Manzanillo’ table olives have been shown to benefit from foliar B applications. For example, ‘Arbequina’ receiving pre-bloom foliar application of B exhibited increased bloom and a 27% increase in yield in an off year. In the ‘Arbequina’ study, no value of B was observed in an on year, and B was found to have no effect on vegetative growth. In another study, B applications to ‘Frantoio’ resulted in increased concentration of chlorophyll and soluble sugars as well as changes in the profile of endogenous plant growth regulators within the leaves. In California, pre-bloom B applications on ‘Manzanillo’ resulted in increased percentage of perfect flowers and improved fruit set and yield, particularly during an off year.
Recommended Application
The recommended foliar B concentration for olives ranges from 19 to 150 ppm. Values below 14 ppm B may result in B deficiency, whereas values above 185 ppm may result in B toxicity. A foliar nutrient analysis only provides a snapshot of the status of the plant at the time of leaf collection; however, low B status of leaves has been found to correlate well with symptoms of deficiency. Symptoms of B deficiency in olive include dead leaf tips with a characteristic yellow band and green leaf base as well as twig and limb dieback (Figure 2). B deficiency may first become apparent in the meristems, the growing tips of shoots. B deficiency may also result in misshapen and defective fruit (Figure 2), low fruit set and premature fruit drop. The value of B application for improved fruit set is not limited to orchards with visual symptoms of B deficiency or foliar B levels below the recommended range. In fact, the numerous research studies that demonstrate the value of pre-bloom foliar B applications for enhanced fruit set and yield were conducted in orchards with no B deficiency. Based on these findings, foliar analysis alone may not be a useful predictor of benefits from pre-bloom foliar B application.
Figure 2. Symptoms of boron deficiency in olive include dead leaf tips with a characteristic yellow band and a green leaf base (a) and misshapen fruit (b) (photos courtesy J. Connell.)
B is typically introduced to orchards either as a solid mineral broadcast on the soil surface, or in solution as a foliar spray. The pre-bloom foliar application is designed to specifically enhance fruit set and yield and should be applied three weeks prior to bloom. B is generally sold as borax, sodium borate, sodium tetraborate, boric acid, or Solubor® (Table 1). The B content varies between formulations; therefore, all calculations should be based on the equivalents of active ingredient (i.e., pounds of B). For example, for soil-applied B in olive, 5 to 10 lbs/acre B is broadcast, which equates to approximately 45 to 49 lbs/acre borax (11% B) or 24 to 48 lbs/acre Solubor® (20.5% B). In California, foliar application of B three weeks prior to ‘Manzanillo’ bloom, particularly in off years, at rates of 1 or 2 lb./acre Solubor® in a 100 gallon/acre (246 or 491 mg/L B at 935 L/hectare) was demonstrated to improve yield by approximately 30%. The baseline B level in this California study site was 16 ppm B, a level just below the established critical level, but high enough to avoid deficiency symptoms.
The value of B applications on orchard health and economic return varies based on the status of the alternate bearing cycle in the year of application, the baseline B status of the tree and soil, and other climate factors that may influence yield. Plants have a narrow range between B deficiency and toxicity. Be sure to read the product label carefully to avoid over-application and conduct annual leaf tissue analyses to gather baseline information on the B status of orchards. More information on fertilizer rates for olives and other California crops may be found on the CDFA FREP California Crop Fertilization Guidelines website at cdfa.ca.gov/is/ffldrs/frep/FertilizationGuidelines/.
Because of their comprehensive knowledge in soil, water and nutrient management, crop consultants have been asked to play a role in helping California growers meet requirements of the Irrigated Lands Regulatory Program (photo by Vicky Boyd.)
As water quality regulations intensify for California growers, the industry has drawn attention to crop consultants as a source of trusted knowledge and advice. Rigorous certifications, such as the Western Region Certified Crop Adviser (WRCCA), ensure the most qualified agricultural professionals are available to support growers in improving their farming operations through efficient practices and resource use. Because of their comprehensive knowledge in soil, water and nutrient management, crop consultants have been asked to play a role in helping California growers meet requirements of the Irrigated Lands Regulatory Program (ILRP).
Precedential Water Quality Requirements for Growers
ILRP regulates discharges such as nitrate from irrigated agriculture to protect surface and groundwater quality. ILRP covers over 6 million acres of irrigated lands in growing regions across California. Due to the diversity in production systems and water quality impairments across the State, regulations are generally handled on a regional basis. However, some reporting requirements set by the State Water Resources Control Board (SWRCB) serve as precedential requirements across all regions.
In 2018, SWRCB amended the waste discharge requirements for the East San Joaquin River Watershed. This ruling is referred to as the Eastern San Joaquin (ESJ) Order. SWRCB designated portions of the ESJ Order as precedential and directed the Regional Water Boards to revise their programs to be consistent with the precedential requirements. So far, almost all Regional Water Boards have updated their regulatory language to reflect the precedential requirements.
The precedential regulations include the requirement for all growers to complete Irrigation and Nitrogen Management Plans (INMP) and submit summary data to a third-party coalition or the Regional Water Board.
INMP reporting requirements serve two main purposes: The first is to help growers project the total amount of N a given crop will require over the season. Such planning can increase application efficiency and reduce the loss of N to surface and groundwater. Second, the data made available through the summary reports enable third-party coalitions to analyze the range of N application rates for each crop in the region. This allows the coalitions to identify any parcels that may be outliers and implement follow-up action to help reduce overapplication.
There are several trainings and resources available to crop consultants that cover the information needed to prepare a detailed and accurate INMP including the University of California’s Nitrogen Management Training for Crop Consultants.
Role of Crop Consultants
The precedential requirements have increased the need for agricultural professionals trained in irrigation and nutrient management practices and technologies. CCAs have been identified to fill this need along with certified professional soil scientists, agronomists and agriculture irrigation specialists. Being familiar with the precedential requirements and completing INMP worksheets and summary reports adds value to your professional toolkit.
Additionally, growers in your region may be required to have INMPs certified by a specialist in irrigation and N management. The Regional Water Boards have discretion on requiring if all growers’ INMPs must be certified, or just a subset of growers, based on a risk categorization such as the low/high vulnerability area distinction. Currently, the Central Valley and Los Angeles Regional Boards require certification of plans for parcels located in high vulnerability areas.
To certify INMPs for growers in these regions, you must hold one of the following certifications:
By signing off on a grower’s INMP, you are certifying the INMP was prepared under your direction and supervision and that the data reported is, to the best of your knowledge, accurate and complete. Additionally, you are certifying that you used sound irrigation and N management planning practices to develop your recommendations and that the recommendations are informed by applicable agronomic training.
As a certifier, you are not responsible for any damages, loss or liability arising from subsequent implementation of the INMP by the grower in a manner inconsistent with INMP’s recommendations. The certification does not create any liability or claims for environmental violations.
Nitrogen Management Course and Specialty Certification for Crop Consultants
There are several trainings and resources available to crop consultants that cover the information needed to prepare a detailed and accurate INMP including the University of California’s Nitrogen Management Training for Crop Consultants.
The self-paced course covers N and irrigation management practices that reduce environmental impacts while maintaining crop productivity. Participants receive access to online videos covering a variety of topics, including N cycling, sources, budgeting, dynamics in California cropping systems, the environmental impacts of N loss, irrigation management and barriers to adoption of environmental practices.
The course offers 10 hours of CCA continuing education credits. While the course is designed for CCAs, anyone interested in learning more about N and irrigation management in California agriculture is welcome to participate. The course is open for enrollment through May 31, 2024 at https://ucanr.edu/sites/nitrogencourse/.
The course was created to provide CCAs with the specialized training and education necessary to obtain the California Nitrogen Management Specialty Certification (CA-NSp). The CA-NSp is for California-based CCAs who provide N management planning services to their clients and are interested in certifying INMPs. CCAs must pass the exam to obtain the specialty certification. After passing the exam, specialists must maintain their certification by meeting the continuing education requirements set by the CCA Program.
Individuals that participated in CDFA’s Nitrogen Management Training for CCAs prior to Oct. 1, 2020 were automatically awarded the specialty certification.
CDFA Irrigation and Nitrogen Management Training Program
CDFA’s FREP also offers a training that covers the basics of irrigation and N management and provides step-by-step instructions in completing INMP worksheets and summary reports.
The self-paced course and accompanying workbook were designed for growers required to complete INMPs and interested in self-certification. However, the training workbook is also available to crop consultants developing and certifying plans for growers. The digital workbook covers all the information provided in the training and includes exercises and additional resources (https://www.cdfa.ca.gov/is/ffldrs/frep/training.html).
Supporting Growers into the Future
As regulatory pressure increases, the demand for trained professionals will continue to rise. Taking part in a certification program like WRCCA helps demonstrate to growers that you have the knowledge and experience to provide sound advice in irrigation and nutrient management practices that will help them meet regulatory requirements. As crop consultants, it is important to stay up to date on the latest research and technology so we can support growers as they face the challenges of balancing profitability with environmental stewardship.
CDFA’s FREP provides funding for research, education and outreach projects focused on advancing agricultural nutrient and irrigation management practices. To receive updates on current and completed projects, FREP’s grant cycle and relevant irrigation and nutrient management events, subscribe to the FREP newsletter at www.cdfa.ca.gov/subscriptions/.
Nitrogen Use Efficiency (NUE), or the amount of N supplied versus the amount removed in the crop, is a key metric of crop nutrition management efficiency and improves the closer applications are matched to demand and the 4Rs of Nutrient Stewardship for delivery are leveraged (photo by Cecilia Parsons.)
I know what you’re thinking; another green-washed lecture from someone who has no understanding of farming practices and wants to tell you how to run your operation. I assure you this is not that type of op-ed.
The topic of this discussion is understanding the practical relevance of improving your almond production carbon footprint (“CFP”). We begin by defining CFP as a measure of CO2-equivalent pounds per unit of food produced, or pounds of almonds in this case. There are two critical components of that statement. The first is “CO2-equivalent pounds.” This metric is used to harmonize units across industries that emit various types of greenhouse gas emissions. In almond production, as with other crop production systems, the primary greenhouse gas concern is N2O typically lost via crop nutrition management practices. The reason N2O is a primary concern is because each pound of N2O gas is equivalent to almost 300 pounds of CO2. This equivalency is also why crop nutrition management (primarily nitrogen management) is typically the single largest contributing factor to total production emissions. However, it’s also the lowest-hanging fruit for CFP improvements. The second critical element is these values are expressed on a per-unit-of-food-produced basis. This allows for efficiency gains in our management strategies to be accounted for more appropriately.
California Senate Bills 253 and 261 were signed into law this past October. These bills define a requirement for entities conducting business in California that meet specific revenue thresholds to report on their greenhouse gas emissions beginning in 2026. Entities within thresholds will have to look upstream and downstream in their supply chains to accurately quantify their emissions, which means they’ll likely need some verification of production CFPs from farms that are supplying ingredients.
What Can Be Done Right Now Nitrogen management
With an expectation of new levels of scrutiny in the future, it’s important to focus on the practical relevance of these emissions. In many ways, emissions can be related to the overall efficiency of farming practices to produce food as previously noted. For example, if you were able to produce 2500 kernel lbs/ac using 200 lbs of N fertilizer instead of 250 lbs, you will have reduced your CFP of production. A second example might be adding a berm blower to your spray rig so that you could make half as many passes through the orchard for preemergent sprays. These improvements enhance our overall efficiency and lower emissions. Ultimately, N and irrigation management are by far the most significant contributors and offer the greatest opportunities for CFP reductions by efficiency improvements.
N management and irrigation are intimately tied together as N is taken up by the crop via mass flow with water. This characteristic is helpful in that its movement in the soil profile is predictable. It can, however, be detrimental when we mismanage our irrigation applications. Nitrogen Use Efficiency (NUE), or the amount of N supplied versus the amount removed in the crop, is a key metric of crop nutrition management efficiency. NUE improves the closer we get to matching our applications to demand and leveraging the 4Rs of Nutrient Stewardship for delivery. Traditional practices of slugging on 200+ pounds N in four to five calendar-based applications typically reduce our NUE to around 60% to 70%. The environmental fate of 30% to 40% N applied in this manner is aboveground losses due to volatilization and below-rootzone losses from leaching. These losses are counted in a production carbon footprint. Even in a scenario where greenhouse gasses like N2O are not emitted but the product is leached below the rootzone, that “wasted” input carries a CFP value with it that has not contributed to yield.
Many growers have switched to applying smaller doses more often, but they’re still adhering to the historic practice of concluding their N applications by early June. This is an improvement in that they are getting closer to matching demand at each application though they are still operating by an arbitrary calendar and not applying to true demand. The ultimate management strategy for NUE optimization is matching demand with every irrigation by adopting season-long continuous fertigation.
Tools like CoolFarm Tool and Comet Planner con help growers take inventory of emissions related to various farm management activities.
Continuous fertigation has been well documented to achieve 90% or better NUE with proper execution. This is achieved by minimizing environmental losses. Consider the following example to demonstrate the practical benefits. You’ve been traditionally applying 242 lbs/ac N to meet demand for a 2500-lb crop at 70% NUE. You move to a continuous fertigation strategy and from 70% to 90% efficiency. Those 242 N units can now support a 3200-lb crop. In this example, you’ve purchased and applied the same amount of N for a 2500-lb crop at 70% efficiency, but the management improvements support increased production by 700 lbs/ac, reduced CFP per unit of food produced (due to more crop with same N rate) and increased gross profit by over $1000/ac (using $1.5/lb crop price). The opportunity costs for NUE improvement are promising, even in a depressed market.
Crop nutrition management plays a critical role in improving NUE and thus reducing CFP, but input selection is equally important. There are three common forms of N fertilizer, each with their own strengths and weaknesses. Urea contains CO2, which can gas off immediately after application, ammonium can volatilize, and nitrates can denitrify under specific conditions. Selecting the right input source for the right conditions supports NUE improvements. There are also inherent manufacturing processes that influence the product CFP at the farm gate. Speak with a trusted CCA to learn more about these product-specific attributes.
Irrigation management
In a close second to nitrogen management, irrigation management accounts for about 1/3 of the total production CFP and can be maximized by irrigating in smaller doses more frequently. Water management improvements can, in many cases, lead to less total water applied per pound of kernel weight and less relative pumping time, which reduces your CFP due to fewer engine-related emissions. Optimizing irrigation efficiency and crop nutrition delivery also relies on distribution uniformity (DU). System tests should be conducted periodically because poor DU will result in overapplication of water and nutrients in some areas of the field while other areas suffer due to inadequate resources. While this all sounds good on paper, there is one of fundamental hurdle for adoption of continuous fertigation and irrigations strategies, which is the availability of water. Those that have productive wells with good-quality water are best positioned to achieve these goals as they can apply water as needed to support their efficiency goals. Those that are beholden to district water delivery schedules don’t have quite the same opportunity to feed water and nutrients in this fashion.
Irrigation management accounts for about one-third of the total almond production carbon footprint and can be maximized by irrigating in smaller doses more frequently (photo by Curt Pierce, UCCE.)
Remember: It is when we look at CFP as energy and resources used that did not support yield improvements that we are able to connect it back to production efficiency.
To begin assessing your operations, there are tools like CoolFarm Tool and Comet Planner to help you take inventory of your emissions related to various farm management activities. There is, however, no one-size-fits-all methodology to make improvements; strategies should be rooted in sound agronomy based on site-specific conditions to achieve the maximum benefit. Ultimately, don’t be afraid to ask questions or start taking inventory of your emissions as it will likely provide insights to your most costly farming activities in terms of energy and resources and guide you toward a more profitable future.
Bacterial canker symptoms appear early in the growing season (all photos by F. Niederholzer.)
In the Sacramento Valley orchard business, the grass is blue and the sky is green. The primary nut crops, almonds and walnuts, are struggling with poor grower returns while cling peaches and prunes are making (some) money. That’s a general reversal of fortunes for the stone fruit crops from the last 15 to 20 years. How long this success will continue is far from certain as both peach and prunes require high management focus and production costs and both face significant international competition. Still, the stone fruit crops in the region are, at least for now, a lifeline for those growers able to put in the management effort to grow a large, quality crop while controlling costs and so help support their entire farming business.
Careful pest management is a key part of delivering a quality crop. Partnering with a knowledgeable PCA/CCA is valuable for all growers, but particularly so for prune growers. Prunes generally have fewer and more manageable pests compared to other crops in the region but have major issues in slow-acting, chronic diseases.
The focuses of this article are the relatively unique pests of prune compared to other common fruit and tree nut crops in the northern growing regions of the state. Many prune pests are familiar to PCAs who grow/work in almonds and/or peaches. These include blossom brown rot, leaf rust, fruit brown rot (monilinia hull rot in almonds), scales (especially San Jose scale), peach twig borer (PTB) and webspinning mites. The common pests of almonds and stone fruit, which should be familiar to PCAs, will be covered briefly.
It is worth noting that, more than any crop grown in the north state, prunes can suffer weather-related crop failures. Since 2004, there have been six years with regional or statewide prune crop failures related to sudden hot or extended cool weather at bloom. In those years, alert PCAs are often the key to careful use of a limited budget to maintain orchard health and prepare for next year.
Insect and Mite Pests
The key insect and mite pests of prunes are aphids, peach twig borer (PTB), webspinning mites, and scale. Two of the four general pest groups (webspinning mites and scale) are often controlled by natural predators as long as they are preserved by careful, selective pesticide use to control the remaining pests of note (aphids and PTB).
Aphids
Two aphids, mealy plum aphid (MPA) and leaf curl plum aphid (LCPA), are the key pests of prune. High populations of plum aphid feeding in prune trees in the spring and into the summer can reduce tree size, fruit sugar and return bloom.
These aphids have similar life cycles and so are managed similarly. Both species move into prune orchards in the fall, mate and lay eggs in the late fall. Eggs hatch during prune bloom. Aphid populations can increase rapidly in warm springs, ‘outrunning’ biological control from many natural enemies including ladybugs, lacewings, etc. Plum aphids move out of prune orchards in the late spring (LCPA) or summer (MPA) as shoot growth slows, although they can remain into the summer in young, vigorous orchards with active shoot growth. MPA spend their summers in an alternative host such as reed grass or cattails. LCPA spend the summer on plants of the Asteraceae (daisy) plant family (daisies, sunflowers, thistles, etc.)
No need to turn over leaves once the colonies grow. Honeydew will show you where the aphids are in the trees.
There is no economic threshold for MPA when monitoring before bloom. Finding a single aphid egg in an orchard means that orchard should be sprayed based on UC IPM Guidelines. Finding no eggs in the dormant season doesn’t mean they aren’t present, so many growers and PCA manage aphid using orchard history. Where there is a history of aphid feeding in the orchard, many PCAs treat preventatively with a research-proven low rate of pyrethroid in the late fall or winter (the bottom of the labeled rate for Asana or Warrior were very effective in controlling both species in many UC field studies in fall or dormant timings the last 20 years.)
Because pyrethroids that drift or runoff to creeks and ditches can harm aquatic life (see article at the end of the column), effective but not excessive use rates are an easy first step to reducing environmental risk with these materials. Solid but not excessive pyrethroid rates (for example, 6 to 8 oz/acre Asana) delivered excellent peach twig borer control in multiple UC trials over many years. These rates also deliver excellent aphid control. What about pesticide resistance? Alternating peach twig borer control chemistries (e.g., dormant spray one year, B.t. at bloom the next year) is an effective way to manage pesticide resistance without the environmental risk of high rates of pyrethroid use.
Two applications of 4% oil during bloom is also effective in smothering aphid and does not harm the prune flowers or fruit set. If no pre-bloom or bloom sprays are applied, growers and/or PCAs should monitor orchards after bloom for signs of aphid colonies. See post-bloom monitoring practices in the references at the end of this column. Even if aphid spray(s) were applied, it’s a good idea to spot-check areas of the orchard where spray coverage might have been sketchy (e.g., row ends near a road, structures, etc.) Systemic or translaminar materials such as neonicotinoids (Assail, Actara, imidacloprid), BeLeaf or Movento are effective on post-bloom colonies as the curled leaves can shelter the pests from direct contact with pesticides. Use of neonics and/or pyrethroids after bloom can harm beneficial insects that control spider mites or scale. Post-bloom, oil does give good control where it reaches aphids (won’t get into curled leaves). However, using oil after bloom risks smothering parasitoid larvae along with aphid.
Mealy plum aphids prefer to feed on the backs of leaves. Turn leaves over when scouting for early signs of infection.
Peach Twig Borer
Peach twig borer (PTB) is a regular pest of prune and could be considered a key pest, and is one that often results in unacceptable damage and so frequently must be controlled. Fruit skin damaged by PTB feeding is an entry point for brown rot fungi. Growers report a good PTB management program helps control fruit brown rot, especially in years and/or locations where orchard humidity and infection risk is increased as fruit mature (late July/early August). As mentioned above, dormant pyrethroid sprays deliver effective PTB control. Another option are bloom sprays containing B.t. (DiPel, Javelin, etc.). This is the only recommended bloom insecticide; others harm bees.
Miss dormant or bloom spray for PTB? Post-bloom applications of Intrepid or Altacor can control PTB too without flaring mites when orchard monitoring shows a need and used to time sprays. Monitor PTB after bloom using pheromone traps and degree day calculations using the program described in the Prune Pest Management Guidelines (ipm.ucanr.edu/agriculture/prune). A treatment threshold is included in the guidelines.
Mites and Scale
Webspinning (spider) mites and scale can be controlled by natural predators in many orchards by avoiding in-season use of broad-spectrum pesticides such as pyrethroids or neonics. See monitoring details for spider mites in the references including treatment thresholds. Scale, especially San Jose scale, should be monitored during the dormant period and treated if populations exceed the threshold described.
Diseases of Fruit, Flowers and Foliage
Three major fungal diseases of fruit, flowers and foliage can impact prunes: blossom brown rot, fruit brown rot and leaf rust. All are best controlled by fungicide(s) applied before rainfall and the disease infections promoted by the wetness.
Blossom brown rot is controlled by one or two fungicide sprays during bloom. In a dry year, one spray around 50% bloom is needed to control infections promoted by dew. See the UC IPM Fungicide Efficacy and Timing publication (ipm.ucanr.edu) for information on fungicide selection.
Wet weather at or soon after full bloom can contribute to lacey scab. This is not a disease, but a scar-tissue that forms on the skin of rapidly growing fruit soon after bloom. Captan or Bravo at full bloom reduces this cosmetic damage that if extreme can reduce fruit grade (strange that a fungicide would reduce damage not from a fungus, but that’s the research results.)
Leaf rust is controlled by sulfur or synthetic fungicides (FRAC 3 or 11) applied ahead of rain in the late spring or summer.
Wet or humid conditions promote fruit brown rot infections as the fruit “sugars up” just ahead of harvest. Fungicides, especially those with FRAC 3 materials (Tilt, Tebuzol, Quash, etc.) give the best control possible when sprayed ahead of infection. Good spray coverage is needed to protect fruit from infection, so higher spray volumes (e.g., 140 to 150 gallons/acre vs 70 to 100 gallons/acre) deliver the best control in UC trials. Adding 415 spray oil (1 to 2 gallons of oil/100 gallons water) helps cut the waxy coating on the fruit and allows better coverage. Every other row spraying doesn’t control disease on the far side of the tree from the sprayer.
Bark cankers and wood rot diseases
The 900-pound gorilla in prune pest management is bark or wood disease(s). Prune trees can be killed outright by these diseases, suddenly or slowly depending on the pathogen. Many of these diseases do the most damage to stressed trees.
Wood rot weakens trees, and crop weight can break trees as a result.
Bacterial canker (bac canker) is just that, a bacterial infection of tree bark which enters trees stressed by ring nematodes (usually), poor nitrogen nutrition and/or other stressors like soil hardpan. This disease is active in wet, cold weather. Activity dies down as temperatures warm in the spring. There is no effective control measure when the stressors listed are present. Bac canker often hits young trees two to five years old. Where ring nematode is found in ahead of planting, preplant fumigation is recommended but may not completely control the nematodes. The best approach for bac canker control is to use rootstocks more tolerant of ring nematode than the Mariannas (M2624, M29C, Myro root, etc.) What are those? In recent prune rootstock trials, ‘Improved French’ trees on Lovell or Viking rootstock had a higher survival percentage than the current industry standard rootstocks M2624 and M29C. In the same study, trees on Krymsk 86 rootstock also survived better than those on M2624 and M29C.
Fungal cankers are another major killer of prune trees. Cytospora (and other pathogen species such as Botrysphaeria and Phomopsis) enter prune trees though damaged bark such as sunburn, oil burn and/or pruning cuts. Practices to protect prune trees from infection include protecting trunk bark with white paint, leaving the interior of the tree “shaggy” to shade scaffold wood and/or spraying thiophanate-methyl (Topsin-M, etc.) fungicide on pruning cuts immediately after pruning. While other fungicides may provide some protection of pruning wounds from canker pathogens, thiophanate-methyl most consistently controlled infections in UC trials.
Finally, wood rot fungi (Phellinus pomaceus, previously known as Phellinus tuberculosus) causes rapid and early decline of prune orchards. This slow-growing fungus is believed to enter trees through pruning wounds. Broken limbs and scaffolds generally begin to appear after 10th leaf, but infections are believed to occur years earlier. To maintain a productive orchard, pruning practices early in the life of the orchard should include thiophanate-methyl during dry weather to avoid infection. This might seem crazy, but not as much (or as painful) as a perfectly good prune orchard falling apart after year 10.
Other pests that damage prune orchards are not unique to prunes such as gophers and weeds. Information on the control of those can be found in the usual spots including ipm.ucanr.edu.
The major, unique pests of prunes are aphids and canker diseases as reviewed above. The UC IPM Pest Management Guidelines for prunes and plums are being revised this spring, so more information and resources (images, effective pesticides) will soon be available.
In the meantime, here’s to hoping the prune crop will set well this spring and orchardists in the Sacramento Valley will have decent crops across all acres.
References
Monitoring aphids
ipm.ucanr.edu/agriculture/prune/springsummer-monitoring-for-aphids/
Pyrethroids in surface water
sacvalleyorchards.com/blog/common-concerns/protecting-sacramento-valley-waterways-from-pyrethroid/
Dormant season scale monitoring
ipm.ucanr.edu/agriculture/prune/dormant-spur-sample/
Prune disease references
ipm.ucanr.edu/agriculture/prune/brown-rot-blossom-and-twig-blight/
ipm.ucanr.edu/agriculture/prune/russet-scab/
ipm.ucanr.edu/agriculture/prune/brown-rot-on-fruit/
sacvalleyorchards.com/prunes/diseases-prunes/cytospora-signs-management/
sacvalleyorchards.com/prunes/diseases-prunes/update-on-heart-rot-in-prunes/
https://www.growingthevalleypodcast.com/podcastfeed/phellinus
Multiple industry groups and federal agencies came together to recognize the CCA Conservationist of the Year Award.
In early December 2023, Russell Taylor received the 2023 Certified Crop Adviser Conservationist of the Year Award in a ceremony in Washington, D.C., hosted by USDA. The CCA Conservationist of the Year Award recognizes a crop advisor who has demonstrated leadership in conservation within the agriculture industry.
Taylor is a CCA and vice president of Live Earth Products, Inc., which mines and manufactures humic and fulvic acid-based products. To him, conservation in ag means doing more with less.
2023 CCA Conservationist of the Year Award recipient Russell Taylor speaking at the award ceremony in Washington D.C. hosted by USDA (all photos courtesy R. Taylor.)
“Agriculture is going to deal with the human population doubling by 2050,” Taylor said. “And how will we do that with the same amount of resources? That’s going to be using the same amount of water and the same amount of land, or even less land due to loss to development, to feed that growing population. So as somebody who’s looking at conservation, we need to maximize our output and make better use of our inputs.
“Conservation is also reshaping the rules that prevent farmers from accessing products and information that aid in conservation,” he said. “So, getting CCAs’ and researchers’ expanded knowledge is essential to accomplish the goal of helping [growers].”
Taylor has contributed to the conservation movement for years through various educational avenues. He advocates for using humates to reduce fertilizer loss and add organic matter to the soil to improve conservation. He’s also worked with states and the federal government to change rules restricting label claims and products growers can use.
“Some of the rules we’re working under are 1950s laws,” Taylor said. “And through agricultural advancement, we have new technologies that these old laws don’t cover. So, the laws were written for DDT and pesticides, and that was it. No other beneficial crop inputs like plant biostimulants were included.
“The efforts that both me and many individuals within the regenerative agriculture and the biostimulant communities have worked hard toward is just changing these rules.”
While receiving the award, Taylor got the chance to speak with multiple leaders in the ag industry about conservation, including USDA NRCS’ national nutrient management specialist and national agronomist as well as members of the Senate and House Ag committees. Reflecting on these conversations, Taylor agreed that in terms of conservation, the ag industry is headed in the right direction.
“You see the grind on the news about Congress and how things aren’t getting done,” Taylor said. “Then you go meet with those in the Ag committee, and it’s a lot of positivity because we’re making significant changes.
“There’s a lot of positivity going on… we’re doing great things.”
USDA-Natural Resources Conservation Service, Agricultural Retailers Association, American Society of Agronomy, CropLife America, Crop Science Society of America, National Association of Conservation Districts, National Association of State Departments of Agriculture, Soil Science Society of America and The Fertilizer Institute all came together to recognize the CCA Conservationist of the Year award.
“You’ve got multiple industry groups and federal agencies working together to recognize this award, so it is a big deal,” Taylor said. “The Conservationists Award is a collaboration of the leading conservation groups in agriculture.”
Taylor encouraged others within the ag industry to support the conservation movement where possible, highlighting the Plant Biostimulant Act as an important measure. The Plant Biostimulant Act, authored by U.S. Representative Jimmy Panetta (D-CA-20) and introduced with Rep. Jim Baird (R-IN-4), “would create a uniform process for approving commercial plant biostimulant use and require more federal research on the technology’s benefits for soil health,” according to a press release on Congressman Panetta’s website.
“The No. 1 thing a grower could do is contact a congressman and say, ‘I support this. Please get it in the farm bill,’” Taylor said.
Trials suggest best cover crop benefits for almonds in the Central Valley come from late termination (photo by J. Kratt.)
According to the Sustainable Agriculture Research and Education (SARE), “a cover crop is a plant that is used primarily to slow erosion, improve soil health, enhance water availability, smother weeds, help control pests and diseases, increase biodiversity and bring a host of other benefits to your farm.”
Why Use Cover Crops?
The first answer I give: Nature fights against bare ground. Andre Lu wrote, “Bare ground is the best way to encourage weeds as most weeds are pioneer species. They rapidly germinate to cover disturbed and bare ground. Nature always regenerates disturbed soil by rapidly covering it with plants. Weeds are nature’s way of healing disturbed soil. Living plants feed the soil microbiome with the molecules of life so they can regenerate healthy soil.”
The way nature regenerates bare ground is through a process called plant succession, which is the change in the species structure of an ecological community over time. A great example of this is the area around Mt. Saint Helens the past 43 years since its eruption. A plant community gradually or rapidly replacing another can result from developmental changes in the ecosystem itself or from disturbances such as wind, fire, volcanic activity, insects and disease, or harvest. Different plants require different ratios of fungi and bacteria based on their successional growth traits. Cover crops are a way to dramatically speed up this process on the farm.
Different plants require different ratios of fungi and bacteria based on their successional growth traits (photo courtesy Earthfort, LLC.)
In my opinion, the more species you have in your cover crop from different plant families, the better it performs. The regenerative agriculture department at CSU Chico wrote: “Dr. Christine Jones lays out a strong case for the importance of nurturing the biodiversity in the soil using multispecies cover crops. The most diverse mix produces the best results both for the soil microbiome and for the productivity and resiliency of crops grown together in that soil.”
Side-by-side blocks on the same farm, same day, 24 hours after a heavy spring rain showing cover crops’ ability to improve water infiltration (photos by J. Kratt.)
I’ve had the benefit of meeting Dr. Jones twice in person and sat through her lectures on how cover crops can signal something called “Quorum Sensing,” which is when a very diverse mix of plant species enable the soil microbiome to become more active, which then exponentially speeds up soil health processes. According to Dr. Jones, multispecies mixes that induce Quorum Sensing need to include species from different functional plant groups (grasses, brassicas, legumes and broadleaves).
Measurable Outcomes
The first thing we saw was dramatically larger and more diverse microbial communities using a variety of soil biology tests, including Direct Microscopy, MicrobiometerTM, and BiomeMakers DNA testing. We also saw physically improved soil structure and water infiltration rather quickly. Often, our growers report seeing more earthworms and a diversity of arthropods in the soil.
After initial stunted growth at planting and through spring, whole orchard recycling cover-cropped almond trees (top) eventually outperform older conventional plantings (bottom) (photos by J. Kratt.)
In a Wasco, Calif. almond trial, samples collected June 2023 showed an orchard farmed regeneratively the past three years had 5.5x more fungi, 1.7x more bacteria, a 3.3x higher fungi-to-bacteria ratio, 3x more biologically active carbon, 2x more biologically active nitrogen, and a 1.5x higher carbon-to-nitrogen ratio than the conventional almond orchard. In addition, the regenerative orchard used to be full of several diseases, is over 22 years old, yielded 1800 to 2200 lbs. per acre the previous five years and was ready for the bulldozer, but last year and this year yielded 3600 and 3000 lbs. per acre, respectively, vs the conventional orchard which averaged 2700 and 2300 lbs. per acre. Both orchards were in the same soil series, and both were Nonpareil/Monterey on Hansen hybrid rootstock, and the conventional orchard is nine years old. The regenerative orchard used cover crops, biologicals made by Earthfort and 60% less conventional N compared to the conventional orchard which had no cover crop, no biologicals, a full NPK program per UC Davis guidelines and bare orchard floors using herbicides.
In 2021, Fenster et al. published a study titled, “Regenerative Almond Production Systems Improve Soil Health, Biodiversity and Profit” in Frontiers in Sustainable Food Systems. The study included another orchard we work with in the Wasco area. One of the more interesting findings was the water infiltration rate of regenerative orchards in California versus conventional orchards (each conventional orchard was adjacent to each regenerative orchard to control for soil type variances.) The conventional orchards averaged 0.04 ml water infiltration per second, whereas the regenerative orchards averaged 0.8 ml/s, and the orchard in Wasco we work with averaged 1 ml/s. We believe the combination of having a 16-species diverse cover crop mix, keeping it alive until June and using fungal-dominant biologicals were the reasons why this orchard (which used to have sealed ground prior to becoming regenerative) had superior water infiltration.
As mentioned before, we see improved disease suppression when using cover crops, primarily due to improved water infiltration, but we also see a dramatic increase in microbial diversity including microbes that are predatory against bacterial and fungal soil pathogens.
After mowing in late spring or early summer, cover crop residue acts as an armor to protect against water evaporation and can keep soil microbes alive during hot summer months (photo by J. Kratt.)
Plant-parasitic nematode suppression is one of the more impressive benefits we have seen. In past research, wild mustard reduced a wide range of parasitic nematodes, velvetbean reduced root-knot nematodes, and hairy indigo and joint vetch with mulched cowpea maintained low populations of B. longicaudatus and M. incognita nematodes. In a grower trial we did in Paso Robles, Calif. in wine grapes, a combination of cover crops and Earthfort biologicals were used to try and improve vineyard health and productivity. In 2021, there were 2.8 times more plant parasitic nematodes than beneficial nematodes. After three years on our regenerative program, there were 85.5x more beneficial nematodes than plant-parasitic nematodes. This was a 196% increase in beneficial nematode species and a 98.7% decrease in parasitic species.
One of the most popular reasons almond growers in California have been using cover crops is to have a food source for bees. This is important, but typically in the southern end of the Central Valley there aren’t many (or any) flowers open at the time of almond bloom due to the lateness of the planting after orchard operations or the lateness of winter rains. Then, many growers mow it all down in March either from fear of frost damage or fear that if they let the cover grow too large it will interfere with harvest operations. In contrast, we have found our best benefits come from late termination as this allows for more beneficial insects to assist with mite control, and it also helps with improved water infiltration because the roots were allowed to go much deeper.
This brings us to the most frequently stated benefit we hear: improved water infiltration. This past spring when much of the Central Valley was flooded, orchards with cover crops were able to resume orchard activities sooner than those without. Cover crops have been widely shown to reduce erosion and runoff, and in many cases we find water penetration is improved so much that gypsum is no longer needed.
After mowing in late spring or early summer, cover crop residue acts as an armor to protect against water evaporation and can keep soil microbes alive during hot summer months. Some growers and irrigation GSAs are fearful that cover crops increase water use, but recent studies by UC Davis point to no net increase of water inputs and in some cases measurable decreases in water use.
Cover crops might assist in the breakdown of wood chips in whole orchard recycling (WOR). A Kern County almond grower who did WOR in November 2022 and planted trees in January 2023 seeded a cover crop immediately after. Initially, the trees were significantly stunted from a lack of N tied up by the excess C (wood). The cover crop grew incredibly well due to the heavy spring rains. The trees were fed monthly with amino N and a microbial package from Earthfort, and the cover crop was disc-incorporated in July 2023. By the end of September, the trees outgrew the neighbor’s trees that were from the same nursery and rootstock and planted two months earlier in fumigated ground, kept bare with herbicides and fertilized with UN32. Our trees didn’t have long whippy growth either; they had thicker caliper trunks and more lateral branches than the neighbor’s. Further work needs to be done in this area to determine how cover crops and biologicals can best be used in WOR plantings.
In the final analysis, with water and weather issues continuing to be problematic, cover cropping in California is proving to be a practice that growers can’t afford not to do.
References
Lu, Andre; Growing Life: Regenerating Farming and Ranching. ACRES USA, 2021.
Frontiers in Sustainable Food Systems, Volume 5, Article 664359, August 2021.
Bending and Lincoln 1999
Rodriguez-Kabana et al. 1992
Rhoades and Forbes 1986
Resources
https://www.csuchico.edu/regenerativeagriculture/ra101-section/cover-crop-biomass.shtml
https://www.plantsciences.ucdavis.edu/news/Mitchell-Gaudin-cover-crops-video
https://www.csuchico.edu/regenerativeagriculture/blog/water-use-and-cover-crops.shtml
A side cut of a leaf. Sap analysis analyzes available nutrients from the xylem and phloem. Tissue testing analyzes available and unavailable (total) nutrients from the entire leaf (photo by Jose Aburto, NEWAGE Labs.)
Utilizing lab data successfully comes down to knowing what is being measured and why. Take for example soil testing; there is a broad range of analyses. A Total Nutrient Digest uses strong acids to provide a combined (available and unavailable) total nutrient assay, whereas a Saturated Paste test utilizes no acid and provides insight on nutrients that are water soluble in solution. Both are soil tests but provide drastically different data based on the type of lab analyses performed. The relationship between the two types of soil tests is analogous to the differences between leaf tissue and sap analysis for plant nutrition assessment.
Sap extractions ready to be analyzed. Increased understanding of nutrient mobility from sap analyses can often identify nutrient deficiency, excess or toxicity long before any symptoms become visible (photo by Sierra Wall, NEWAGE Labs.)
Leaf tissue testing (whole leaf or petiole) quantifies the total accumulated nutrients that are both available and unavailable inside the plant. Unavailable nutrients have been taken up and used to build the leaf and its cellular structures. Because this test is measuring the nutrient content of the entire leaf, the results are mostly illustrating what the nutrient status was during the growth and development of those tissues. Sap analysis is measuring the liquids actively flowing in the vascular tissues (xylem and phloem) and provides a near real-time assessment of the nutrients currently available in the plant.
Leaf tissue analysis is carried out by drying, heating and pulverizing tissue for consistency in size and then employing ashing and strong acids to create a total nutrient assay solution. Sap analysis only employes linear pressure to extract the sap but keep the integrity of the leaf with no leaf mastication, for a liquid sap extraction, no heat, acids or solvents are used in preparation of sap for analyses. Leaf sap analysis identifies mostly available nutrients located inside the xylem and phloem. Nutrients within the vascular bundle are not yet incorporated into the leaf structure. The proprietary sap extraction process used at NEWAGE Laboratories in South Haven, Mich. uses differing pressures for different crop types so as not to violate the integrity of the leaves and cellular structures.
Table 1. Nitrogen parameters from a sap report showing three nitrogen parameters and nitrogen conversion efficiency comparison.
With these two different types of leaf tests also comes differing sampling protocols. Leaf tissue samples are collected from a singular age of leaf and placed in a paper bag to facilitate dehydration. Sample dehydrating is a key component for tissue analysis but in so doing one loses the ability to assess nitrogen species such as nitrate or ammonium, as well as sugars, brix, pH and electrical conductivity. With sap analysis, it is critical to maintain the samples’ integrity from the moment leaves are removed from the plant. Protocols for sampling and handling are available to help ensure leaves maintain a condition of stasis (the least amount of moisture lost and low respiration until received by the lab). To alleviate this issue, NEWAGE provides an overnight shipping program and protocol to keep samples fresh all the way to the lab.
Sap samples are collected from both new and old age leaf sources and placed in separate plastic zip style locking bags in a cooler. Collecting a new yet fully formed leaf plus petiole and an old yet still viable leaf plus petiole sample set allows for plant nutrient uptake and mobility to be observed. Nutrient mobility assessment is something Leaf Tissue testing cannot offer though an important feature of sap analysis.
How to Analyze and Interpret Nutrient Mobility from Sap
Increased understanding of nutrient mobility from sap analysis can often identify nutrient deficiency, excess or toxicity long before any symptoms become visible. As an example, N, being highly phloem-mobile, will produce a deficiency symptom initially in the old leaves if the plant has been nitrogen deficient for a prolonged period. Old leaves start to turn yellow due to lack of chlorophyll synthesis. On a sap report, N deficiency is identified, many times long before visual symptoms occur, when higher amounts of N are measured in the new leaf than the old leaf, meaning the plant is remobilizing N from old leaves to meet the high demand in the new leaves when not enough N uptake is available. An excess of N is commonly identified in sap results as well. In an excess N condition, the sap report shows more N in the old leaves than in the new. When an excess of N is persistent in sap, this points to an area where fertilizer rates can be potentially decreased and production problems with pests and diseases associated with excess N can be avoided. This comparison of new vs old leaves is applicable to identify deficiencies and excesses of all phloem-mobile nutrients which include phosphorus, potassium, magnesium, chloride, sodium, molybdenum, nickel and, of course, N. The unique N information available from sap analysis also includes N conversion efficiency analysis, something unique to NEWAGE Labs’ sap analysis and again something not available from leaf tissue analysis.
A new and old leaf sap set. Sap sampling is normally taken in pairs (photo by Finn Telles, Penny Newman.)
Nitrogen Conversion
Consideration with N fertilization is the type of N being applied and how to measure it. Sap reports from NEWAGE have four N parameters to provide an enhanced picture of N uptake and its conversion, ultimately to more complex forms like amino acids and proteins. NEWAGE measures Total Nitrogen (all species of N in the plant), N in nitrate and N in ammonium and compares all three. To compare Total Nitrogen to the N in nitrate and ammonium, the oxygen in NO3 and hydrogen in NH4 need to be removed from the equation so they are directly comparable in terms of N concentration.
Now that the different forms of N have been normalized for the analysis, this apples-to-apples N comparison can be translated into an algebraic equation to solve for ‘x’. Total Nitrogen is the product, N in NH4 and N in NO3 are known values and x is the amino acids and proteins portion of this Nitrogen Conversion equation. NEWAGE Labs has labeled this as Nitrogen Conversion Efficiency % (NCE%). The NCE% provides insight on how N is converted inside the plant. If the NCE% is at or above 90%, it means 90% of total N being taken up by the plant is being converted to amino acids and/or proteins and 10% is staying in NO3 and/or NH4 forms. A 90% or higher conversation rate is the target. As seen in Table 1, see page 30 NCE% in new and old leaves are less than 80%, and there is more Total Nitrogen in the old leaves by >80%; therefore, the color code in the report is blue to indicate an excess condition. The plants from this example are potentially being overfertilized, and the nitrogen isn’t transforming well inside the plant, which can ultimately lead to increased diseases and pests, reduced grain or fruit quality (and fruit shelf life) and an undue burden on your fertility budget.
Quality of Data
When deciding which sap lab to work with, here are some laboratory attributes to evaluate that will have a direct bearing on the quality of the reports. These laboratory attributes to evaluate include:
Report turnaround time
Quality training
Support of sample collection staff to ensure quality samples are collected, handled and shipped properly
Sample transit time to the lab
Report interpretation support and training
Easy-to-fill-out sample documentation is also important as a practical nutrient management tool. NEWAGE’s sample collection sheet is simple, capturing the essential information of collection date and time, field name, and who the sample was collected by. All this information is then transferred onto a NEWAGE report. With sap sampling it’s imperative the samples are kept cool to prevent N volatilization and degradation of the sample. If samples get too warm, usually by poor shipping, results of the 25 nutrient parameters NEWAGE analyses can be adversely affected. NEWAGE Labs’ overnight shipping decreases turnaround time to approximately 48 hours. Same-day sampling and shipping is recommended. If samples are held over to the next day for shipping, keep samples in refrigerator or cooler and let the air out of the plastic bags. Make sure samples do not freeze. When cellular structures burst, sap analysis is no longer a viable testing method. Leaves freeze mainly when samples are stored right up against a frozen gel back without a barrier between them like a paper towel or thin bubble wrap. Request a crop-specific sampling guide as how and when to sample are of great importance and impact data quality.
Leaf sap analysis is a tool to help growers make informed in season decisions by looking at nutrient uptake in greater detail using nutrient mobility and N conversion efficiency. Go to www.newagelaboratories.com or contact Info@newagelaboratories.com for more information.
Figure 2. Adult Carpophilus truncatus (blue circles) inside of a hull-split almond (photo by J. Rijal.)
Growers and PCAs should be on the lookout for a new pest called carpophilus beetle (Nitidulidae: Carpophilus truncatus) (Figures 1 and 2).
Figure 1. Adult Carpophilus truncatus as seen from the (a) dorsal, (b) ventral, (c) left lateral and (d) anterior end (photos by Sarah Meierotto, UC Riverside.)
Damage occurs when adults and larvae of this pest feed directly on the developing kernel, causing reductions in both crop yield and quality. This pest was initially found infesting almonds and pistachios in the northern and central part of the San Joaquin Valley, and a broader survey is now underway to verify the extent of its spread in California.
Figure 2. Adult Carpophilus truncatus (blue circles) inside of a hull-split almond (photo by J. Rijal.)
The carpophilus beetle is recognized as one of the top two pests of almond production in Australia, where growers typically experience 2% to 5% infestation, but it can be closer to 30% to 40% in more extreme cases (Madge 2022). In addition to feeding on new crop nuts, the carpophilus beetle can also likely facilitate the spread of Aspergillus fungi that can lead to the production of aflatoxins, which are known human carcinogens that are heavily regulated in key markets.
Biological and chemical control options are very limited or unknown. As such, the key to managing carpophilus beetle is crop sanitation since this pest overwinters on remnant mummy nuts in the orchard like navel orangeworm (Pyralidae: Amyelois transitella) (NOW). Growers and PCAs can monitor carpophilus beetle by directly inspecting mummy or new crop nuts, although there are no known economic thresholds for this pest.
Global Distribution and Initial Detections in California
Carpophilus beetle has been well-established in Australia for over 10 years, where it is considered a key pest of almonds. More recently, the beetle was reported from walnuts in Argentina and Italy. Carpophilus truncatus is a close relative to other beetles in the genus Carpophilus, such as the driedfruit beetle (C. hemipterus) that is known primarily as a postharvest pest of figs and raisins in California.
In California, populations of carpophilus beetle were first detected in September 2023 in almond and pistachio orchards in Madera and Kings counties, respectively. Pest identification was subsequently confirmed by CDFA. A broader survey of orchards throughout the San Joaquin Valley is now underway to determine the extent of the outbreak as well as confirm additional hosts such as walnuts and pecans. So far, almond or pistachio orchards infested by carpophilus beetle have been confirmed in Stanislaus, Merced, Madera and Kings counties, suggesting the establishment of this new pest is already widespread. In fact, some specimens from Merced County were from collections that were made in 2022, suggesting the pest has been present in the San Joaquin Valley for at least a year already.
Host Plants
While in Australia the carpophilus beetle is primarily found in and around almond orchards, alternative host plants have been identified that the beetle can use for feeding and reproduction. These include Brazil nuts (Lecythidaceae: Bertholletia excelsa), candlenuts (Euphorbiaceae: Aleurites moluccanus), cashews (Anacardiaceae: Anacardium occidentale), pistachios (Anacardiaceae: Pistacia vera), quandong seeds (Santalaceae: Santalum acuminatum), walnuts (Juglandaceae: Juglans regia), acacia seeds (Fabaceae: Acadia spp.), pine nuts (Pinaceae : Pinus spp.), desiccated coconut flesh (Arecaceae: Cocos nucifera), sunflower seeds (Asteraceae: Helianthus annuus) and granulated or powdered pollen (Madge 2022). While adults have also been shown to survive on dried apricots (Rosaceae: Prunus armeniaca), dates (Arecaceae: Phoenix dactylifera) and cup mushrooms (Pezizaceae: Peziza spp.), the larvae are unable to complete development on these hosts (Madge 2022). In California, this pest has so far been confirmed feeding on almonds, pistachios and walnuts.
Seasonal Ecology
Carpophilus beetles overwinter in remnant nuts (i.e., mummy nuts) that are left in the tree or on the ground following the previous year’s harvest. In contrast to NOW, carpophilus beetles tend to prefer remnant nuts on the ground, likely due to increased moisture, and they generally tend to forage closer to ground level (Madge 2022). That said, carpophilus beetles can also be found in tree mummies as well.
The beetles become active when temperatures and photoperiod increase in the spring. They can have multiple generations per year, although there is no information available on the upper and lower temperature thresholds for this pest, much less degree-day requirements. While more research is still needed to better characterize beetle activity in the spring prior to hull split, they likely make use of mummy nuts as a reproductive host during that early part of the season, similar to NOW.
At hull split, carpophilus beetles will move onto new crop nuts, although some fraction of the population does remain on mummies all year. Prior to egg deposition on new crop nuts, the adult beetles will chew through the shell (Figure 3) and/or feed on the kernel, which is thought to both facilitate larvae access to the developing kernel, as well as inoculate the nut with a symbiotic gut-associated yeast that likely help the larvae feed (Madge 2022). Recent studies in Australia have demonstrated adults prefer to oviposit onto nuts previously fed upon by other carpophilus beetles and/or inoculated with yeasts (Madge 2022). While carpophilus beetles have been found infesting nuts throughout the tree, they tend to concentrate in the lower canopy (Madge 2022). This contrasts with NOW, which tend to initially infest nuts higher in the tree canopy.
Figure 3. Prior to laying eggs, adult carpophilus beetle can chew a hole through the shell (yellow circle) (photo by J. Rijal.)
Following egg deposition, the larvae that emerge feed on the developing kernels, leaving the almond kernel packed with a fine powdery mix of nutmeat and frass that is sometimes accompanied by an oval-shaped tunnel (Figures 4 and 5).
Figure 4. Almond with fine powdery frass due to infestation by Carpophilus truncatus (photo by H. Wilson.)
Figure 5. Carpophilus truncatus feed directly on the nut kernel, which can sometimes result in a distinct oval-shaped tunnel (photo by H. Wilson.)
Damage from carpophilus beetle may be confused with NOW and/or ants, but these can be differentiated. For example, NOW tends to feed all over the kernel (rather than tunneling) and produces a darker and larger type of frass (i.e., excrement) along with webbing (Figure 6).
Figure 6. NOW damage to almonds results in larger, darker frass as well as webbing (photo by Jack Kelly Clark, courtesy UC Statewide IPM Program.)
In contrast, ants tend to chew through the skin of the kernel and feed primarily on the white nutmeat, leaving the papery skin behind (Figure 7).
Figure 7. While ant damage to almonds can also produce a fine white powder, damage to the nut tends to be broad and superficial in contrast to carpophilus beetles which tunnel into the nut (photo by Jack Kelly Clark, courtesy UC Statewide IPM Program.)
Ant feeding is also associated with the presence of a fine white powder (similar to carpophilus beetle damage) that can be seen while sampling in the field but disappears in the hulling/shelling process prior to the inspection of processed kernels (Figure 8).
Figure 8. Damage from ants can result in a fine white powder like what is produced by carpophilus beetles (photo by D. Haviland.)
Monitoring
Monitoring for carpophilus beetle is currently limited to direct inspection of hull-split nuts for the presence of feeding holes and/or larvae or adult beetles. Given their tendency to infest nuts lower in the canopy, this is a good area to focus on for initial inspection of new crop nuts. That said, to get an idea of average overall infestation levels, it is best to take a sample from freshly shaken new crop nuts on the ground at harvest, since this is more representative of the entire tree canopy.
Over the winter, mummy nuts can also be inspected for the presence of carpophilus beetles. Initial studies from Australia suggest that monitoring points should be at least 200 yards apart (Madge 2022). While no specific economic thresholds have been developed, summer infestations on new crop nuts tend to reflect the distribution of infested mummy nuts during the winter.
A lure based on male-produced aggregation pheromone from three related species of Carpophilus spp. was previously developed in Australia and used for monitoring as well as an attract-and-kill strategy in stone fruit orchards (Hossain 2018). With the arrival of C. truncatus in Australia, this monitoring and management strategy was tested in almond orchards. Unfortunately, in its current form, this trap and lure system is not very attractive to C. truncatus, and so it has no utility for either monitoring or as a control strategy (Hossain 2018).
As such, Australian researchers are now working to develop a pheromone lure that is specific to C. truncatus as well as one or more co-attractants based on host plant volatiles and/or gut-associated yeasts. Preliminary studies have shown a lot of promise, and this new lure may soon provide a better monitoring tool for growers, PCAs and researchers, but it is not yet commercially available (Madge 2022). Initial studies with the new species-specific pheromone lure have demonstrated that carpophilus beetles appear to forage mostly at ground level, and beetle catch with the candidate pheromone was improved when traps were moved from the tree canopy (i.e., hung at about 5 ft height) to ground level (Madge 2022).
Biological Control
While there are certainly parasitoids and predators that attack Nitidulids and species in the genus Carpophilus in particular, very little is known about the specific natural enemies of C. truncatus, much less their efficacy in an orchard setting. Much more information exists on the parasitoids of C. hemipterus, the driedfruit beetle, a key pest of figs and stone fruit in California. Key parasitoids of C. hemipterus include the Encyrtids Zeteticontus spp. and Cerchysiella spp. as well as a Proctotrupid Brachyserphus abruptus, all of which attack the larvae. Additional parasitoids include the Braconid Microctonus nitidulidis and the Bethylid Pseudisobrachium flavinervis, which attack the adult and pupal life stages, respectively. These parasitoids have a wide host range and may attack C. truncatus, but more research will be necessary to confirm this as well as their efficacy for population control.
Documented predators of Carpophilus spp. include the Staphylinid Atheta coriaria and the Reduviid Peregrinator biannulipes. Other generalist predators commonly found in tree nut orchards (e.g., spiders, lacewings) may also contribute to biological control of C. truncatus, but again more research is needed to further characterize this, much less determine their impacts on C. truncatus specifically.
In the absence of specialist parasitoids or predators, researchers in Australia have focused on the use of entomopathogenic fungi (EPF) for carpophilus beetle, particularly the EPF Beauvaria bassiana. While preliminary studies found that in some cases B. bassiana could cause up to 70% mortality of C. truncatus larvae (and to a lesser extent adults), the use of this EPF under field conditions is still being developed (Madge 2022).
Finally, vertebrates like birds and rodents may provide some degree of control by consuming or damaging remnant mummy nuts infested by carpophilus beetle, but the impacts of this remain unclear in California orchards.
Chemical Control
The ability to control carpophilus beetle with insecticides is limited, primarily due to challenges with spray coverage. Most of the beetle’s life cycle is spent protected inside the nut, with relatively short windows of opportunity available to spray the adults while they are exposed. In Australia, the use of bifenthrin for control of carpophilus beetle has produced inconsistent results, and experiments to improve coverage with various adjuvants did not lead to improved control (Madge 2022). Furthermore, the continued presence of carpophilus beetles on remnant mummy nuts throughout the season presents an additional challenge to control with insecticides.
Cultural Control
In the absence of clear chemical or biological control strategies, the most important tool for managing carpophilus beetle is crop sanitation. In Australia, this is currently the primary method for managing this pest. While carpophilus beetle can be found overwintering on remnant mummy nuts both in the tree canopy and on the ground, they tend to prefer ground mummies, likely due to elevated moisture conditions. After removing remnant mummy nuts from trees, it is critical all ground mummies be thoroughly broken apart and destroyed. Simply disking mummies under the soil will not be effective, since research in Australia demonstrated even when mummies are buried as deep as 3 ft down, adult carpophilus beetle can still survive and crawl up to the soil surface (Madge 2022). As such, make sure to use a flail mower to thoroughly destroy mummy nuts. It might need a double pass to ensure all nuts are shredded.
Carpophilus beetle is a new pest in California that will need to be addressed by both researchers and growers alike. Within the research community, new research and extension activities are being developed by UCCE personnel in collaboration with their counterparts in Australia. Until more is known about this pest in California, growers are advised to monitor for its presence and follow the Australian model of focusing on winter sanitation as the primary means for its control.
If you suspect that you have this beetle in your orchard, please contact your local UCCE Farm Advisor (https://ucanr.edu/About/Locations/), County Agricultural Commissioner (https://cacasa.org/county/) and/or the CDFA Pest Hotline (https://www.cdfa.ca.gov/plant/reportapest/) at 1-800-491-1899.
Selected References
Hossain, M. 2018 “Final Report – Management of Carpohilus Beetle in Almond” Project Code AL15004, Horticulture Innovation Australia, North Sydney, Australia.
https://www.horticulture.com.au/growers/help-your-business-grow/research-reports-publications-fact-sheets-and-more/al15004/
Madge, D. 2022 “Final Report – An Integrated Pest Management program for the Australian almond industry.” Project Code AL16009, Horticulture Innovation Australia, North Sydney, Australia.
https://www.horticulture.com.au/growers/help-your-business-grow/research-reports-publications-fact-sheets-and-more/al16009/
Soil pH affects the availability of many nutrients, but the optimum pH for plant growth depends on which nutrient is the most limiting (photo courtesy Danny Klittich, Mission Produce, Inc.)
Soil acidity and soil alkalinity in relation to plant growth has been well-studied. Soil pH is often used as an indicator of the chemical fertility of the soil, and it is believed that most major and minor plant nutrients are best available around a slightly acid pH. This concept of soil pH-nutrient availability, the Achilles heel of soil fertility studies, was first developed in the 1930s and 1940s based on field trials, observation and various assumptions.
Early Conceptions
In 1936, a bulletin entitled, “A useful chart for teaching the relation of soil reaction to the availability of plant nutrients to crops” was published (Pettinger 1936) which stated, “…the effect of the degree of acidity or alkalinity on the availability of plant foods, or the relation between lime and fertilizers is one of the most widely discussed subjects in agriculture.” Soil reaction was perceived to be “…one of the pulses which indicates the state of health of the soil.” In the bulletin and diagram that came with it, Pettinger discussed the range of soil pH in relation to the availability of potassium, nitrates, magnesium, calcium, phosphates, iron, aluminium and manganese. A color diagram was presented that composed a series of bands representing the availability of plant nutrients in relation to a pH range of 4 to 10. The changes in width of the bands represent changes in the availability of the nutrient. It was stated that the diagram was designed to illustrate basic principles in the availability of nutrients in relation to soil reaction and did not “…portray the situation in a quantitative or absolute manner for any particular soil.” The diagram was considered only valid for well-drained soils of humid regions and not for alkali soils of arid regions or poorly drained or organic soils. The availability of some nutrients was directly affected by soil reaction whereas for other nutrients the availability was controlled by processes not related to the soil reaction. The bulletin noted, “…when the discovery of new evidence makes it necessary to discard present beliefs either wholly or in part, or when better methods of representing the facts are developed, the diagram will be revised and re-issued in improved form.”
The bulletin was not widely distributed, and it was received by Emil Truog at the University of Wisconsin-Madison who, by the 1930s, was a national leader in soil fertility and plant nutrition.
Relationship between soil pH and nutrient availability from Emil Truog’s 1946 paper in the Soil Science Society of America Proceedings (top), and a modern depiction of the relationship (bottom).
His work on the availability of plant nutrients emphasized the availability of plant nutrients was a relative matter and ‘available’ should be replaced by ‘readily available’, and ‘unavailable’ by ‘difficulty or slowly available’ (Truog 1937a), and that different cropping systems and crops have different levels of nutrient requirements and sufficiency levels.
Truog liked the soil pH-nutrient availability diagram, and considered it very useful and “…the subject of tremendous importance in connection with liming, fertilizing and soil management” (Truog 1946). He expanded the diagram to 11 nutrients and made it “…more simple in form but more complete in several aspects” (Truog 1946). The diagram illustrated the relation of the soil pH to plant nutrients in which the width of the band at any pH value indicates the relative availability of the nutrient. The band did not present the actual amount as that was affected by other factors such as the type of crop, soil and fertilization. For the 11 nutrients on the diagram, a pH of around 6.5 was most favorable but did not mean a satisfactory supply; it indicated as far as the soil reaction was concerned, the conditions were favorable. Likewise, it did not mean outside the favorable range that a deficiency would prevail. Nutrients outside the optimal range could be adequately supplied as other factors than the soil pH affected plant growth or as some plants had low requirements for a particular nutrient at a high or low pH (Truog 1946).
Previous research shows a direct effect of acidity on plant roots and on soil microorganisms, and pH at the root surface may differ from that of the bulk soil (photo courtesy Danny Klittich, Mission Produce, Inc.)
Limitations
The soil pH-nutrient diagram was presented as conceptual in 1937 and 1946 and contained several assumptions. It assumed the availability of nutrients was the same to all plants in all soils and it was best to have the soil around pH 6.5. However, many acid soils are highly productive as are some soils that have an alkaline pH. The diagram suggested deficiencies of micronutrients did not occur at low pH and there were no problems with the availability of potassium or sulfur at high pH (Blamey 2005). There are plants that require a high soil acidity such as tea, pineapple, blueberry and cranberry, and others that require a high soil pH (Hartemink and Barrow 2023).
There are numerous cases in the availability of plant nutrients that do not match the diagram, and some of them were already highlighted by Truog (e.g., the toxicity of copper and zinc in acid soils, and the fact that calcium may not be a limiting factor in acid soils, which is not uncommon). It was often found that despite the low availability of calcium at low pH, liming had limited effect as calcium was taken up from the subsoil, other nutrients were limiting (in particular phosphorus), or soil drainage was the problem (Truog 1937b). Improved crop performance with liming is often from the reduction in aluminum toxicity, and calcium deficiency is not always the major cause of poor growth (Blamey and Chapman 1982). Other exceptions to the diagram include manganese toxicity at low soil pH, iron toxicity on acid soils, boron deficiency in alkaline soils and sulfur deficiency on alkaline soils (Hartemink and Barrow 2023). Some of these exceptions to the pH-nutrient availability concept have been explained as “…simply due to methodology” (Penn and Camberato 2019).
Sources of soil acidity include urea- and ammonium-containing nitrogen fertilizers, sulfur soil amendments and biological soil processes (photo courtesy Danny Klittich, Mission Produce, Inc.)
The availability of phosphorus is often assumed to be problematic in low-pH soils where it is said to be fixed by iron and aluminium, or in soils with a high pH when phosphorus is precipitated by calcium. Of all the plant nutrients, this is probably the most widely accepted pH-availability relationship, and in a recent review it has been termed the “…the classic understanding of the effect of pH on P uptake from soils” (Penn and Camberato 2019). Barrow recently summed up the problems with this model: It makes wrong predictions, there is very little evidence for the existence of the separate postulated sinks for phosphate and it has no facility for explaining other aspects of the behavior of phosphates (Barrow 2017). There are different effects of pH on the P availability. When the pH is decreased from 6 to 4, the rate of uptake of phosphate by roots increases, the amount desorbed from soil increases and the amount sorbed by soil often also increases. The first two increase the P availability while the third effect decreases it. The pH-phosphorus availability diagram fails the most fundamental test of science and is difficult to understand why it persists (Barrow 2017).
Soil pH is a useful indicator of the soil condition, and it affects numerous soil chemical reactions and processes. But it cannot be used to predict or estimate plant nutrient availability, and different plants respond differently as nutrients interact which can be synergistic as well as antagonistic (Barrow and Hartemink 2023). Soil pH influences solubility, concentration in soil solution, ionic form, and mobility of most plant nutrients. Soil pH affects the availability of many nutrients, but the optimum pH for plant growth depends on which nutrient is the most limiting (Barrow 2017). Furthermore, the activity of microbial communities and a range of chemical reactions in soil are affected by fluctuating pH. The bulk pH of the soil (commonly measured in a soil-water ratio) may not reflect the pH in the rhizosphere where nutrients are taken up by the plant. The soil solution pH is relevant for soil and plant biogeochemical processes, and better a predictor of crop yields than the soil pH measured in a soil-water mixture. Too seldom have theories been tested by actually measuring the effects of pH on uptake of nutrients by plants growing in soil (Barrow and Hartemink 2023).
The influence of soil pH on bioavailability is indirect at best through the competition with cations for dissolved ligands or surface functional groups and through breakdown of minerals by the protons which may enhance the bioavailability of some metals. There is also a direct effect of acidity on plant roots and on soil microorganisms (Sposito 1989), and pH at the root surface may differ from that of the bulk soil . Some recent research highlighted the importance of root-induced changes in the rhizosphere pH. In soils with pH-dependent charge (e.g., ultisols, oxisols), pH increases tend to increase the P concentration in solution and its availability to plants, whereas in soils with permanent charge it is typically the other way around (Hartemink and Barrow 2023).
Truog believed the soil pH-nutrient availability diagram presented a fairly reliable picture, but he stressed it was generalized and tentative and partly based on assumptions as data were lacking. The 1946 paper “Soil reaction influence on availability of plant nutrients” provided no data and no references. The diagram has never received further investigation but ended up in many textbooks and popular soil books and continues to be used in textbooks, encyclopedias, extension bulletins and numerous papers. The diagram has many more usages, often without citation, which suggests it has been accepted as common knowledge. It has become a defining principle in soil fertility and plant nutrition.
Since the 1950s, a large amount of research work has been done on the solubility of nutrients, the biological transformations of nutrients in soils and the effect of soil pH on adsorption and plant uptake. None of that can possibly be summarized in a simple diagram. The relationship between soil pH and nutrient availability remains of interest as nutrient availability in acid and alkaline soils is unique for each soil, crop and climatic region.
References
Barrow, N.J., 2017. The effects of pH on phosphate uptake from the soil. Plant and Soil, 410(1): 401-410.
Barrow, N.J. and Hartemink, A.E., 2023. The effects of pH on nutrient availability depend on both soils and plants. Plant and Soil, 487(1-2): 21-37.
Blamey, F.P.C., 2005. Comments on a figure in “Australian Soils and Landscapes: An Illustrated Compendium” ASSSI Newsletter, 142.
Blamey, F.P.C. and Chapman, J., 1982. Soil amelioration effects on peanut growth, yield and quality. Plant and Soil, 65(3): 319-334.
Hartemink, A.E. and Barrow, N.J., 2023. Soil pH-nutrient relationships: the diagram. Plant and Soil, 486(1-2): 209-215.
Penn, C.J. and Camberato, J.J., 2019. A Critical Review on Soil Chemical Processes that Control How Soil pH Affects Phosphorus Availability to Plants. Agriculture, 9(6): 120.
Pettinger, N.A., 1936. A useful chart for teaching the relation of soil reaction to the availability of plant nutrients to crops. Virginia Agricultural and Mechanical College and Polytechnic Institute and the United States Department of Agriculture, Cooperating, Blacksburg.
Sposito, G., 1989. The chemistry of soils. Oxford University Press, New York.
Truog, E., 1937a. Availability of essential soil elements – a relative matter. Soil Sci. Soc. Am. Proc.(1): 135-142.
Truog, E., 1937b. A new soil acidity test for field purposes. Soil Science Society of America Proceedings, 1: 155-159.
Truog, E., 1946. Soil reaction influence on availability of plant nutrients. Soil Science Society of America Proceedings, 11: 305-308.
