The proposal for application notification has been unknowns, including who will be notified of a planned pesticide application, when and how will they be notified, and what products and application methods are regulated.
The California Department of Pesticide Regulation (CDPR) is in the process of developing a statewide pesticide application notification system. Roger Isom, president of Western Agricultural Processors Association, participated in a DPR focus group session in August and reported on this issue at the 2021 Crop Consultant Conference.
Input from the agriculture industry on this proposal to require notification of a pesticide application is vital, said Isom. He urged all growers and PCAs to voice their concerns about the draft proposal to CDPR. Public webinars on the proposal are being held this month. Isom said the draft regulation would be announced in spring 2022.
Unknown at this time are the questions of who will be notified of a planned pesticide application, when will they be notified, how do they get notified, what products are covered and what application methods are covered.
At this time, it is only a proposal, and details are not decided, but Isom warned that the governor has indicated that notification is a priority for him and $10 million has been directed to develop the proposal.
Isom said this action was triggered by Assembly Bill 617 and an air quality reading in Shafter in Kern County that measured 1,3-D; however, there was no known application within seven miles of the measurement. AB 617, signed by Governor Newsom in 2017, establishes a community-scale emissions abatement program and updates air quality standards for sources that contribute to poor air quality.
Isom pointed out that only three states have some type of notification system for pest control applications: Florida, Maine and Michigan. Monterey County has a program for fumigation application notifications for schools. The public is eligible for notification and it is done five days prior to a fumigation. Currently in Kern County, notification is only for restricted use materials and is only required to growers in surrounding areas of the planned application.
Isom noted that in some instances, anyone is able to sign up for notifications, and in the past, it was found that the majority of those requesting notification did not live in the area of the application. In Florida, those requesting notification must be on adjacent or contiguous property up to a half mile.
Isom said any form of this rule should be statewide, reasonable and justification for those asking to be notified of a pesticide application. Public webinars are being held this month. Written comments can be submitted to CDPR.
Potential for mold development in a walnut crop increases as harvest is delayed. Sunburnt nuts, windfalls on the ground for more than 15 days, nuts infested with navel orangeworm, nuts with large stem openings and larger nuts are subject to mold development (photo by C. Parsons.)
UC Davis Plant Pathologist Themis Michailides reported recent research findings on walnut mold and management at the 2021 Crop Consultant Conference.
The main pathogens of walnut mold are Alternaria, Fusarium species and Aspergillus niger. Botryosphaeria and/or Phomopsis canker and the bacterial disease walnut blight are also cause for mold in orchards.
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Brown Apical Necrosis is a special type of walnut mold that always starts from the stylar end of the nut and is sometimes associated with the bacterial disease walnut blight. Michailides confirmed that there is a strong correlation of fungi causing walnut mold with the same fungi attacking and decaying the hulls. The incidence of mold starting from the stylar end was significantly greater than that of mold starting from the stem end, suggesting possible stylar infection at bloom. In trials over two years, sprays at two to three weeks before hull split and early hull split reduced walnut mold in both early and late walnut cultivars.
In the U.S. standards for grades of walnuts, mold is listed as damage when attached to the kernel or when white or gray mold affects a portion of the surface of the kernel. Potential for mold development in a walnut crop increases as harvest is delayed. Sunburnt nuts, windfalls on the ground for more than 15 days, nuts infested with navel orangeworm, nuts with large stem openings and larger nuts are also subject to mold development.
Michailides said levels of mold reported in recent years included a Chandler sample from Tulare County showing 21% mold, grade sheets from 2020 from Butte County areas near the Sacramento River with 10% to 16% mold and, in 2019, reports of 10% to 20% mold in Butte and Glenn counties.
In his presentation, Michailides noted that previous studies on walnut mold showed that mid-May to mid-July sprays reduced Botryosphaeria and Phomopsis canker and blight, but they did not reduce molds caused by Alternaria or Fusarium. He said the fact that fungi-causing mold are the same as those that colonize the hulls led the decision to move towards sprays before hull split and early hull split.
To reduce incidence of mold in walnuts, Michailides said applying Merivon at three weeks prior to hull split reduces mold related to Botryosphaeria, Phomopsis and Alternaria. Adding Tebuconizole to the tank mix will increase efficacy against Phomopsis. To further increase efficacy, apply Rhyme at 20% to 30% hull split. If the high level of control is not needed, Rhyme can be applied at 20% to 30% hull split.
West Coast Nut magazine is offering California tree nut growers a rare opportunity to network across the industry at the second annual California Tree Nut Conference in Tulare Nov. 3. In addition to the usual continuing education seminars and industry trade show, growers attending this year’s free event will hear from California Agriculture Secretary Karen Ross, who will discuss priorities for the California Department of Food and Agriculture and how the agency will support nut growers in meeting those priorities.
In addition, growers will interact with commodity board leaders for the state’s top nut crops in a leadership panel titled “Where Are We Heading?” Panelists will include Michelle Connelly, executive director and CEO of the California Walnut Board and Commission, Richard Waycott, CEO of the Almond Board of California, Richard Matoian, CEO of American Pistachio Growers, and Mark Hendrixson, director of the California Pecan Growers Association.
“Given the many challenges nut growers have faced over the last year, we are using our conference to address big-
picture issues that impact the bottom line for nut growers in California,” said Jason Scott, Publisher of West Coast Nut magazine. “At the same time, we understand that continuing education and networking opportunities with industry suppliers are also important, and we have plenty of that as well.”
A morning panel on “Irrigation Technology and Automating Monitoring Systems in Tree Nut Crops” will feature UC Davis irrigation experts Ken Shackel and Isaya Kisekka, as well as industry suppliers, growers and consultants. They will discuss existing and emerging technologies to determine soil and plant water status for data-driven irrigation management in nut crops.
After the trade show break and post-harvest nutrition demonstration, CEU talks related to nematode management and new technology for monitoring and managing nut pests will be featured. The California Tree Nut Conference will be held from 7 a.m. to 1 p.m. at the Tulare County Fairgrounds in Tulare. Registration is free and can be done online at WCNGG.com or by calling the JCS Marketing office at (559) 352-4456.
Consperse stinkbug along with southern green stinkbug have been found to damage processing tomato crops in the Central Valley. There is concern the invasive brown marmorated stinkbug may also move into tomato fields (photo courtesy Jack Kelly Clark, University of California Statewide IPM Program.)
Monitoring for stinkbugs and managing weeds to eliminate their overwintering habitat are two important pre-plant steps in processing tomato production.
The Pest Management Strategic Plan for California Processing Tomato Production, part of UC’s Statewide IPM Program, notes that stinkbug infestations are capable of reducing marketable tomato yields by up to 40%. The plan was prepared by Tunyalee Martin, Cassandra Swett, Amber Vinchesi-Vahl and Stephanie Parriera.
Stinkbugs cause the most damage in processing tomatoes in the southern growing region. They cause damage from late fruit set through fruit ripening. Consperse stinkbug is listed as the most damaging species, but southern green stinkbug has also been found to damage tomato crops. There is concern that infestations by the invasive brown marmorated stink bug will occur in the future due to this pest’s movement into other California crops.
Stink bug feeding causes calluses and discoloration of the fruit. Feeding on mature fruit can open the way for secondary infections and severe rot.
Thresholds for stink bug numbers are not used in pesticide application decisions due to this pest’s ability to cause damage even at low numbers. Pheromone lure traps in the fields can assist with early detections.
The strategic plan notes that weed management is an important component in control of stinkbugs. Removing weeds that are attractive to stink bugs, including little mallow, Russian thistle and mustards, is recommended.
Insecticides are used for management, but do not always provide a high level of control. UCCE trials conducted in Fresno County suggest that control is best when a neonicitinoid and pyrethroid are tank-mixed.
Coverage is critical but difficult as stink bugs can be at or below the soil surface during part of the day. Applications are reported to be more successful with an air-assisted sprayer.
In addition to pre-plant monitoring, the strategic plan recommends insecticide applications from planting to pre-bloom if stinkbugs are detected in the field. From bloom to early fruit set, monitoring with pheromone traps or beat trays will allow for early detection of infestations. An insecticide application is recommended if stink bugs are detected in the field.
Monitoring should continue from late fruit set to first red fruit.
Harvesting as soon as possible is a good strategy to avoid additional stinkbug damage and rot from yeasts, but is not always possible due to other factors, including predetermined arrangements with processors.
Freeze damage in oil olive trees occurs in the late fall or winter while trees are dormant. Initial damage includes tip dieback, lack of luster to the leaves and curling up of leaves as well as some necrotic or chlorotic lesions and leaf drop (photo by M. Nouri.)
Super-high-density oil olive plantings have increased in acreage in several growing regions in California while table olive acreage continues to decline. In San Joaquin County, nearly 5,500 oil olive acres are now in production. In their Field Notes publication, UCCE Advisor Mohammad Nouri and UC Kearney Plant Pathologist Florent Trouillas described the challenges of diagnosing symptoms with different causes.
Symptoms of the fungal disease Verticillium wilt and freeze or frost damage can be similar, Nouri noted.
The soilborne fungus that causes Verticillium wilt has both non-defoliating and defoliating types. Symptoms usually begin in the spring and slowly worsen in the summer. The non-defoliating type can cause rapid dieback and wilting, especially in young trees as the pathogen invades the tree’s vascular system. The defoliating type causes early drop of asymptomatic green leaves from individual twigs and branches.
Look inside the affected branches to tell the difference. With Verticillium wilt, compared to freeze or frost damage, obstructed vessels become dark and inner vascular tissues show dark streaking. Advanced stages include shoot wilting, dieback and foliar necrosis, which are also associated with freeze damage.
Nouri said olive trees affected by cold temperatures during the growing season have frost damage. Freeze damage occurs in late fall or winter while trees are dormant. Initial damage includes tip dieback, lack of luster to the leaves and curling up of leaves as well as some necrotic or chlorotic lesions and leaf drop.
His observations are that a dry fall may make freeze damage worse. Cutting back on irrigation in September and avoiding nitrogen applications can help slow growth and may help trees harden off before a sudden freeze event. If a freeze is forecast, having a moist soil surface can help store more heat. Symptoms of freeze damage will appear when the weather warms. Sufficient moisture helps trees recover. Pruning should be delayed until warmer weather begins to stress the trees to see where recovery is possible as new shoots grow. Branches and limbs damaged by freeze will be easily identified and can be removed. The soil profile should be wet to the depth of rooting when spring growth begins to stimulate root activity.
Nouri said that trees severely affected by freeze damage will have less total growth, which will reduce the nitrogen requirement, but will stimulate vigorous re-growth similar to heavy pruning. Fertilizer decisions for the growing season following a freeze should be based on current soil reports and leaf analysis.
On the upper surface of the leaves, rust appears as small yellow spots. On the lower surface, the spots take on a rusty red appearance when the spores in the lesions erupt (photo courtesy Jack Kelly Clark, University of California Statewide IPM Program.)
A sporadic disease in California almond growing regions, rust is linked to humid growing conditions and excessive levels of nitrogen.
Vigorous, higher-density plantings and microsprinkler irrigation with longer, more frequent irrigations can contribute to higher humidity and more accumulated leaf wetness hours resulting in more disease.
UC’s IPM Almond Pest Management Guidelines report that almond trees can be defoliated very rapidly when a rust infection becomes severe. Early defoliation deprives trees of needed nutrients and can reduce the following year’s bloom if not controlled. Rust can be a problem in non-bearing orchards where fungicides have not been applied.
This fungal disease survives from one season to the next in infected plant material.It is important to know the levels in the current and previous seasons as indicators for risk. This helps determine at what level the inoculum may or may not be present and disease progress can be monitored.
Rust appears as small yellow spots on the upper surface of leaves. On the lower surface of the leaf, spots take on a rusty red appearance when the rust-colored spores produced in the lesions erupt through the surface. These spores are spread by air movement and infect other leaves to continue the disease cycle. Young twigs may be infected, but twig lesions are not common.
In almond orchards with a history of rust, the guidelines recommend applying sulfur five weeks after petal fall and follow four to five weeks later with a quinone outside inhibitor (FRAC group number 11).
Two or three applications may be needed in orchards that have severe rust infections. To be effective, the fungicide application must be made before rust symptoms appear.
Sac Valley Orchard News recommends considering a foliar zinc sulfate fertilizer spray to get zinc into the trees as the leaves start to naturally drop. This will also hasten leaf fall and reduce infected leaf carry-over into next season. The application should not be done until late October or early November to allow leaves time to continue making photosynthate and build up energy storage in the trees after harvest.
Unchecked, the inoculum may build up, overwinter on the trees and infect leaves the following spring. In southern growing regions, leaves of some cultivars, such as Sonora, may remain attached over the winter and provide inoculum for new infections as leaves emerge the following spring.
Fungicides effective on rust can be found at the UC IPM guidelines or the Efficacy and Timing of Fungicides Publication.
Signs of powdery mildew infection include white, powdery or dusty areas on leaves (photos courtesy Jack Kelly Clark, University of California Statewide IPM Program.)
Sulfur applications in winegrape vineyards can kill powdery mildew spores that have not yet caused infection, kill incoming spores and cut down newly established colonies.
Sulfur has a long history of controlling powdery mildew, said UCCE Viticulture Advisor Larry Bettiga. In a UC Ag Expert webinar, Bettiga said sulfur was first used in 1850 to control powdery mildew. More sulfur is used in vineyards than any other major type of pesticide.
Powdery mildew has two spore types. Chasmothecia spores overwinter and cause initial spring infections. Conidia spores cause growing season infections. Powdery mildew begins on grapevine leaves as chlorotic spots on the upper leaf surface. Signs of the pathogen appear a short time later as white, webby mycelium on the lower leaf surface. As spores are produced, the infected areas have a white, powdery or dusty appearance. On fruit or rachises, the pathogen may colonize the entire berry surface.
Benefits of sulfur use in vineyards include good efficacy for powdery mildew control. There is low potential for resistance development by the pathogen. Compared to fungicides, sulfur applications cost less and they are acceptable in organic and biological production systems. Sulfur also helps with suppression of other pest populations.
Negatives of sulfur applications include increased potential for hydrogen sulfide production, drift from dust application and the potential for plant tissue toxicity, and it can be detrimental to some natural enemies. The potential for sulfite production during fermentation can be avoided if sulfur treatments end in the vineyard five weeks prior to harvest.
Sulfur applications are also less costly than synthetic fungicide applications. At a rate of 15 pounds per acre and adding the cost per acre for application, Bettiga said the total was less than $20 per acre. This was compared to Rally, Quintec and Pristine at $51.60, $53.92 and $74.60 per acre, respectively.
In an evaluation of sulfur and other fungicide tank mixes, after six applications, sulfur treatments alone resulted in 2.8 affected berries per cluster while tank mixes scored one or less.
Efficacy is influenced by the mode of action phase. As a contact, sulfur is not influenced by temperature. As a vapor, activity below 59 degrees F is very limited.
Density of foliage and canopy management will also affect efficacy of sulfur and other fungicides. Sprayer settings, including travel speed, volume and velocity of air, nozzle selection and droplet size, and nozzle orientation to canopy play a part in an effective application.
On fruit or rachises, the pathogen may colonize the entire berry surface (photo courtesy Jack Kelly Clark, University of California Statewide IPM Program.)
Carrot field under furrow irrigation system in the Imperial Valley (all photos by A. Montazar.)
Carrots are one of the 10 major commodities in Imperial County, with an average acreage of nearly 16,000 over the past decade. The farm gate value of fresh market and processing carrots was about $66 million in 2019. In the low desert region, fresh market and processing carrots are planted from September to December for harvest from January to May. Most carrots are typically sprinkler irrigated for stand establishment and subsequently furrow irrigated for the remainder of the growing season. However, there are fields that are irrigated by solid set sprinkler systems the entire crop season.
Carrot is a cool-season crop that demands specific growing conditions and effective use of nitrogen (N) and water applications for successful commercial production. N and water management in carrot is crucial for increasing crop productivity and decreasing costs and nitrate leaching losses. The N needs of carrots for optimum storage root yield depends on the climate, soil texture and conditions, residual soil N from the previous season and irrigation management. There is not enough research on N management to free local growers of the worry associated with being short on the amounts applied, which may cause a loss in yield and profitability. The industry needs reliable information on N and consumptive water use of carrots to optimize irrigation and N management, enhance water and nitrogen use efficiency and achieve full economic gains in a sustainable soil and water quality approach.
This study aimed to quantify optimal N and water applications under current management practices and to fill knowledge gaps for N and water management in carrots through conducting experimental trials in the low desert of California. This article presents some of the information developed for desert fresh market carrots.
Field Trials and Measurements
Field trials were conducted on fresh market carrot cultivars at the UC Desert Research and Extension Center (DREC) and four commercial fields in the low desert region during the 2019-20 and 2020-21 seasons (Table 1). The sites represent various aspects of nitrogen applied (N applications ranged between 176 and 272 lbs ac-1), irrigation water applied (varied from 1.6 to 2.9 ac-feet/ac), irrigation systems (three fields under sprinkler irrigation and two fields under furrow irrigation) and soil types (sandy loam to silty clay loam).
Table 1: General information for the experimental sites. Plants were established using sprinkler irrigation at all sites.
The DREC trials consisted of two irrigation regimes and three nitrogen scenarios (Fig. 2). At the commercial sites, due to logistical limitations, the measurements were carried out from five sub-areas selected (50 feet x 50 feet) in an experimental assigned plot (400 feet x 400 feet) with a homogeneous soil type, which was the dominant soil at the site. These areas represented common irrigation and N fertilizer management practices followed by growers.
Figure 2: The DREC trials consisted of two irrigation regimes and three nitrogen scenarios.
The actual consumptive water use (actual crop evapotranspiration (ET)) was measured using the residual of the energy balance method with a combination of surface renewal and eddy covariance equipment (fully automated ET tower, Fig. 3). As an affordable tool to estimate actual crop ET, Tule Technology sensors were also set up at all experimental sites. The Tule ET data were verified using the ET estimates from the fully automated ET tower. Canopy images were taken on weekly to a 15-day basis utilizing an infrared camera (NDVI digital camera) to quantify crop canopy coverage over the crop season. Actual soil nitrate content (NO3-N) at the crop root zone (one to five feet) and the total N percentage in tops and roots were determined pre-seeding, post-harvest and monthly over the season. Plant measurements were carried out on 40-plant samples collected randomly per replication of each treatment/sub-area, and determinations were made on marketable yield and biomass accumulation. Fresh weight and dry weight of roots and foliage were measured on a regular basis.
Figure 3: Monitoring stations in one of the commercial experimental sites.
Findings and Recommendations
Irrigation Management
The common irrigation practice in carrot stand establishment in the low desert is to irrigate the field every other day using sprinkler systems during the first two weeks after seeding. Carrots germinate slowly, and hence, the beds need to be kept moist to prevent crusting. A comparison between the averages of applied water and actual consumptive water use for a 30-day period after seeding suggested that carrots are typically over-irrigated during plant establishment. An average of 3.8 inches was measured as actual consumptive water use for this period across the experimental sites (Fig. 4), while the applied water varied from two to three times of this amount.
Figure 4: Cumulative actual crop water consumption (actual ET) at each of the experimental sites. Surface renewal actual daily ET is reported here.
The results clearly demonstrated that the carrot sites had variable actual consumptive water uses depending upon early/late planting, irrigation practice, length of crop season, soil type and weather conditions. For instance, site C-4 was a sprinkler irrigated field with a dominant soil texture of sandy clay loam where the carrots were harvested very late 193 days after seeding (DAS). The seasonal consumptive water use was 19.2 inches at this site (Fig. 4). Our results show that the seasonal crop water use of fresh market carrots is nearly 16.0 inches for a typical crop season of 160 days with planting in October. Approximately 50% of crop water needs occurred during the first 100 days after seeding and the other 50% during the last 60 days before harvest. Crop canopy model developed in this study demonstrated that fresh market carrots reach 85% canopy coverage by 100 days after seeding.
The amount of water that needs to be applied in an individual field depends on crop water requirements and the efficiency of the irrigation system. Assuming an average irrigation efficiency of 70%, the approximate gross irrigation water needs of carrot fields in the low desert would be 2.0 ac-feet/ac (pre-irrigation is not included) for a 160-day crop season. Pre-irrigation along with proper irrigation scheduling over the season may effectively maintain crop water needs and salinity in carrots.
Figure 5: Canopy development curve for the low desert fresh market carrots over the growing season.
Water stress should be avoided throughout the carrot growing cycle. The critical period for irrigation is between fruit set and harvest. Sprinkler irrigation may be considered as a more effective irrigation tool when compared with furrow irrigation. More frequent and light irrigation events are possible by sprinkler irrigation. Over-irrigation of carrot fields increases the incidence of hairy roots, and severe drying and wetting cycles result in significant splitting of roots. Sprinklers also reduce salinity issues which is important since carrots are very sensitive to salt accumulation.
Nitrogen Management
The results demonstrated that a wide range of N accumulated both in roots and tops at harvest (Fig. 6). For instance, a total N content of 312.9 lbs ac-1 was observed in a fresh market carrot field with a long growing season of 193 days, including 202.9 and 110.0 lbs N ac-1 in roots and tops, respectively. The total N accumulated in plants (roots + tops) was less than 265 lbs ac-1 in the other sites.
A linear regression model was found for the total N uptake in roots after 60 to 73 DAS without declining near harvest (Fig. 6). Small, gradual increases in N contents of roots were observed until about 65 DAS. This suggested that N begins to accumulate at a rapid rate between 65 and 80 DAS; however, the period of rapid increase could vary depending on early (September) or late (November) plantings. N uptake in tops increased gradually following a quadratic regression, and in most sites levelled off or declined slightly late in the season. Although the N accumulated in tops appeared to drop down or level off in most sites beyond 120 to 145 DAS, the N content decline occurred after DAS 155 at site C-4 with a longer growing season.
Figure 6: N accumulation trends in storage roots, tops and total (plants) over the growing season at the experimental sites.
These findings suggest that a total N accumulation of 260 lbs ac-1 occurred by 160 DAS, with 145 lbs ac-1 in roots and 115 lbs ac-1 in tops. Across all sites, nearly 28% of seasonal N accumulation occurred by 80 DAS (Fig. 6) when the canopy cover reached an average of 67% (Fig. 5). The large proportion of this N content was taken up during a 30-day period (50 to 80 DAS). The results also suggest that nearly 50% of the total N was taken up during a 50-day period (80 to 130 DAS). This 50-day period appears to be the most critical period for N uptake, particularly in the storage roots, when carrots developed the large canopy and the extensive rooting system. The majority of N is taken up during the months of December to February, and, hence, proper N fertility in the effective crop root zone is essential during this period. For a 160-day crop season, 22% of N uptake could be accomplished over the last 30 days before harvest.
Carrots have a deep rooting system that allows for improved capture of N from deep in the soil profile. The fibrous roots were present at the depth of five feet below the soil surface at site DREC-2 (Fig. 7). There is a risk of leaching soil residual N due to heavy pre-irrigation (a common practice for salinity management in the low desert) in late summer prior to land preparation. N is likely accumulated at the deeper depths by the beginning of the growing season, and consequently, there is a potential N contribution from the soil for carrots when the roots are fully developed. Since residual soil N contribution can be considerable in carrots, pre-plant soil nitrate-N assessment down to 60 cm depth could be a tool enabling farmers to improve N management and maximize yield and quality while minimizing economic and environmental costs.
Figure 7: Carrot storage and fibrous roots system at site DREC-2.
Careful management of N applications in the low desert carrots is crucial because fertilizers are the main source of N, particularly due to low organic matter content of the soils and very low nitrate level of the Colorado River water. Knowing this fact, the soil NO3-N contents pre-seeding and over the growing season at different sites revealed that none of the sites had N deficiency during the crop season, and consequently, the practice of splitting N applications, as done by the farmers (applying 9% to 15% of total seasonal N as pre-plant and the remainders through irrigation events over the season), was likely effective in most cases. It appears that the practice of 15% to 30% seasonal N applications though irrigation events 45 to 70 DAS has similar effectiveness to sidedress N applications.
Within the range of N application rates examined at the experimental sites, there were no significant relationships between carrot fresh root yield and N application rate, although the results suggested a positive effect of N application on carrot yield. Sufficient N availability in the crop root zone over the growing season and the lack of significant yield response to N applications demonstrate that N optimal rates could be likely less than the applied amounts in most sites. Adequate nitrogen and water applications reduce costs and help prevent leaching, while excess N may lead to excessive N storage in the roots, which may be a concern for processing carrots. Integrated optimal N and water management needs to be approached to accomplish greater N and water efficiency, and consequently keep lower rates beneficial to overall profitability.
Funding for this study was provided by California Department of Food and Agriculture (CDFA) Fertilizer Research and Education Program (FREP) and California Fresh Carrots Advisory Board.
Maintaining AMF colonization in grapevine roots is particularly important for vineyards in Oregon’s Willamette Valley where red-hill soils are most commonly found.
Nitrogen (N) is one of the most managed nutrients in vineyards, since it strongly affects vine growth and fruit development. Although numerous studies have been conducted to understand how N fertilization influences vine productivity and fruit composition, the impacts of N on vine nutrient status and soil microbes receive less attention.
Among a wide range of soil microbes that play vital roles in soil health and vine productivity, arbuscular mycorrhizal fungi (AMF) are unique due to their symbiotic association with grapevines and their contribution to vine nutrient acquisition. Arbuscular mycorrhizal fungi obtain nutrients from the soil, especially for phosphorus (P) and other poorly mobile nutrients, and transfer those nutrients to the plant. In turn, plants provide sucrose and fatty acids to AMF to support fungal functions and growth.
To sustain intensive nutrient exchange between two partners, AMF colonize individual cortical cells of fine roots and form arbuscules, which are tree-like fungal structures that greatly increase surface area contact between plants and AMF (Fig. 1). Grapevines are considered a “super” host of AMF. The percentage of fine roots colonized by AMF is generally above 60% in field and greenhouse conditions. Such high colonization rates also reflect the great dependency of grapevines on AMF. Indeed, non-mycorrhizal vines are stunted in low P soils, while mycorrhizal vines could acquire adequate P from the soil, overcome P limitation and grow normally.
Figure 1: Grapevine roots colonized by arbuscular mycorrhizal fungi (AMF). Red arrow indicates arbuscules, “tree-like” structures that vastly increase surface area contact between the host plant and the fungus (photo courtesy Dr. R. Paul Schreiner, USDA-ARS.)
Willamette Valley Trials
Maintaining AMF colonization in grapevine roots is particularly important for vineyards in Oregon’s Willamette Valley where red-hill soils are most commonly found. Since those highly weathered acid soils have low P availability, grapevines rely on AMF for ample P acquisition. In other crops, N fertilization was shown to reduce root colonization by AMF, but it is unclear whether N applied at moderate rates would decrease mycorrhizal colonization in grapevines. If N applications would suppress AMF and impair vine P uptake, this negative effect should be accounted for when developing fertilization management plans for vineyards. This article summarizes part of my Ph.D. research conducted in Oregon, which explored how vineyard N applications affect vine nutrient status, root growth and AMF.
Experiments were conducted in a Chardonnay vineyard and a Pinot noir vineyard over three years in Willamette Valley. Both vineyards are somewhat limited by N but have varying levels of soil P. At each site, we evaluated three treatments, including no N application (No N), N applied to the soil (soil N) and N applied to the foliage (foliar N). Each treatment was replicated four times. The soil N vines were fertilized two or three times between bud break and veraison using UAN-32 at the rate of 40 to 60 lbs N/acre/year. The foliar N vines received three urea sprays to the canopy from fruit set to two weeks post veraison at the rate of 19 to 23 lbs N/acre/year.
All treatments were evaluated across three years in both vineyards with the exception that foliar N treatment was assessed only in Year 2 and 3 in Chardonnay. In each season, leaf blades and petioles were sampled at bloom and veraison for nutrient analysis. Due to the late initiation of foliar N treatment in Chardonnay, bloom leaf samples were collected only in Year 3 for this specific treatment. Soils and roots were sampled three times a year when berries were pea-size, near veraison and about a month after harvest.
Effects of Soil N Applications in Chardonnay
As expected, soil N applications increased vine N status starting from Year 1 (Fig. 2). For simplicity, only petiole nutrient data at bloom are presented here. Changes of nutrients in corresponding leaf blades followed a similar trend. Previous work on Pinot noir in the Willamette Valley proposed 0.7% as the critical value of petiole N concentration at bloom to ensure sufficient yield (2.5 to 3.5 U.S. ton/acre) and adequate fruit N.
Figure 2: Effect of nitrogen (N) applications to the soil and the foliage (soil N and foliar N) on petiole N and phosphorus (P) concentration at bloom in Chardonnay. Vines that received no N application (no N) served as the control. Means followed by different letters indicate significant differences between treatments in each experimental year based on a t-test or Tukey HSD test at 95% confidence.
Since Chardonnay vines in this region generally carry heavier crop load (four to five U.S. ton/acre) and develop larger canopies compared to Pinot noir vines, Chardonnay vines may have a higher N requirement. With a petiole N concentration of 0.6% at bloom, Chardonnay vines that received no N applications clearly experienced some N limitation in this study. Soil N applications improved bloom petiole N concentration by 15% to 30%. In response to greater vine N status, the soil N vines had 30% higher yield and about 35% more pruning mass as compared to the no N vines.
Soil N applications decreased vine P status in Chardonnay in Year 2 and 3, where petiole P concentration at bloom was about 30% lower in the soil N vines than no N vines (Fig. 2). The negative effect of soil N fertilization on vine P status became more evident in late season. The concentration of petiole P at veraison decreased 50% in the soil N vines in the last two years of the experiment.
Why did soil N applications reduce vine P status? The most straightforward answer would be the dilution of P in leaves due to N stimulated canopy growth. However, this is unlikely the sole reason. Soil N applications increased veraison leaf area by 10% to 19% in Year 2 and 3, while the corresponding leaf blade P decreased to a larger extent (19% to 29%). The second possible explanation for the decreased leaf P under increased soil N supply is that fertilization altered P allocation within the plant and less P was translated above ground. Indeed, because soil N fertilization increased root growth (Table 1), more P can be retained belowground to support new root development. Yet, this assumption is not supported by our observation in the greenhouse or previous studies where soil N fertilization generally increases the proportion of P allocated to aboveground tissues. Unfortunately, we did not sample roots for nutrient analysis in this study, and thus effects of soil N on root P concentration cannot be further examined. In addition to the two explanations presented above, we suspect that soil N applications might lower AMF colonization in roots and therefore decrease vine P uptake.
The percentage of fine roots colonized by AMF (fungal hyphae, arbuscules, vesicles and spores) decreased with soil N supply in Year 2 and 3 (Table 1), in accordance with reduced vine P status in Chardonnay. The percentage of roots colonized by arbuscules also reduced in the soil N vines in Year 2, but not in Year 3. The greater suppression of arbuscular colonization in the soil N vines in Year 2 was likely attributed to the fact that more N (20 lbs N/acre) was applied in Year 2 than Year 3. Clearly, increased root growth played a role in the decrease of AMF in the soil N vines because root colonization usually lags behind root growth. Soil N fertilization might affect AMF through other mechanisms as well. For example, N fertilization could reduce the amount of carbon translocated from vines to AMF and, in turn, decrease P delivered by the fungus. Or soil N supply reduced N translocated from AMF to vines, resulting in a decrease of mycorrhizal colonization.
Table 1: Effects of N applications to the soil and foliage (soil N and foliar N) on root density and colonized by arbuscular mycorrhizal fungi (AMF) in Chardonnay. Vines that received no N application (no N) served as the control. Means followed by different letters indicate significant difference between treatments in each experimental year based on a Tukey HSD test at 95% confidence.
Effect of Soil N Applications in Pinot Noir
Similar to what we observed in Chardonnay, soil N applications improved vine N status in Pinot noir across three years (Fig. 3). Soil N fertilization also increased root growth and decreased mycorrhizal colonization in Pinot noir, although the effects were less evident as compared to Chardonnay (Table 2). The percentage of roots colonized by AMF was lower in the soil N vines than no N vines in two of three years, while the percentage of roots colonized by arbuscules reduced in the soil N vines only in one year.
Even though soil N altered mycorrhizal colonization, it had no influence on leaf blade or petiole P concentration at bloom or veraison in any year, except petiole P concentration at bloom was lower in the soil N vines than no N vines in Year 2 (Fig. 3). Even so, P concentration of corresponding leaf blades was not affected by soil N supply, suggesting an overall small impact of N fertilization on vine P status in Pinot noir. The difference in how vine nutrition and mycorrhizal colonization responded to soil N application between Chardonnay and Pinot noir can be attributed to the difference in soil N and P availability.
Compared to the Pinot noir vineyard, the Chardonnay vineyard has lower soil N and higher P concentration. It seems soil N fertilization would suppress AMF colonization and decrease vine P status to a greater extent in vineyards with lower N and higher P availability.
However, since the Chardonnay and Pinot noir vineyards differ in many other aspects, such as canopy size, irrigation and cropping level, the comparison between these two varieties are not straightforward. Thus, upon the completion of field experiments, we conducted a series of greenhouse experiments to further examine how N and P regulate vine nutrient status and mycorrhizal colonization under a more controlled environment. The negative effect of soil N applications on AMF was observed again in vines supplied with N at a high rate in the greenhouse.
Figure 3: Effect of N applications to the soil and the foliage (soil N and foliar N) on petiole N and phosphorus (P) concentration at bloom in Pinot noir. Vines that received no N application (no N) served as the control. Means followed by different letters indicate significant difference between treatments in each experimental year based on a t-test or Tukey HSD test at 95% confidence.
Effect of Foliar N in Chardonnay and Pinot Noir
Foliar N applications had minor influence on vine N status, vine P status, root growth, and mycorrhizal colonization in both varieties (Figs. 2 and 3, Tables 1 and 2). This is somewhat expected, since a large amount of N applied to the foliage appeared to be transferred to the fruit rather than other plant organs.
Table 2: Effects of nitrogen fertilization to the soil and foliage (soil N and foliar N) on root density and colonized by arbuscular mycorrhizal fungi (AMF) in Pinot noir. Vines that received no N application (no N) served as the control. Means followed by different letters indicate significant difference between treatments in each experimental year based on a Tukey HSD test at 95% confidence.
Conclusions
The evidence obtained from the field experiments indicates that soil N fertilization at moderate rates can negatively influence mycorrhizal colonization and reduces the benefits conveyed by this symbiotic relationship. Foliar N applications, on the other hand, had no impact on AMF or vine P status. The negative effect of soil N applications on mycorrhizal association provides another justification for being judicious with N fertilization in vineyards.
This project was funded by Oregon Wine Board and USDA-ARS. The author would like to thank Erath winery and Results Partner Inc. for their help and support.
Runoff and wind erosion from agricultural fields introduces excess phosphorus and nitrogen to freshwater and marine environments. Nutrient loading causes algal blooms and eutrophication, killing off fish and destabilizing the entire ecosystem.
Phosphorus (P) availability limited food production and human population until the Green Revolution. After the second World War, mineral fertilizers and powerful new pesticides drove record yields and exponential world population growth. While nitrogen fertilizer usually takes all the credit, phosphorus is a close second, and even the major limiting element under some conditions. Mining P-rich ore introduced more P to the biosphere than ever before. Instead of relying on biological P cycling, mineral fertilizer now keeps our fields productive through years of back-to-back planting. P fertilizer provides undeniable improvements to yield and crop quality, but leaks in the system destabilize surrounding ecology, causing a cascade of effects that shift global P cycling. With fertilizer prices on the rise and environmental impact mounting, everyone can benefit from improving P management.
All living organisms require P. It constitutes about 9% of our DNA, it is the backbone of the phospholipid fatty acids giving our cells their structure, and P is a critical component of Adenosine Triphosphate (ATP), the powerhouse of the cell. Plants take up most of their P via the roots as the anion phosphate (P2O43-). Crops require lots of P early in development to support rapidly growing cells. P is required at every stage of growth, first to support DNA transcription and translation, then to build the cellular structure and to supply energy needed to carry out all the activity. Ensuring early access to enough plant-available P drives vigorous root growth and sets the crop up for success.
Activity and Availability
P bioavailability limits many natural ecosystems, and plants have evolved several ways to gain access to the essential element. P is immobile in soil, and most of it is tied up in mineral pools with very little available as phosphate in soil solution. Some P minerals are very stable and resist dissolution. Others readily dissolve with slight adjustments in pH or enzyme activity. Plants and fungi take advantage of the labile mineral pool by excreting phosphatase and lowering the pH in their direct vicinity. Mycorrhizal fungi bond with plant roots, extending their hyphae far past where the plant can reach to bring back phosphorus and water in exchange for photosynthate. Soil organic matter also stores P, and microbial activity releases phosphate into solution according to population dynamics and access to carbon and nutrients.
Most P fertilizer recommendations are based on observed crop response to fertilization at different soil P concentrations. Many studies in the western region show that crop yield and quality increases when fertilizer is applied to soil with less than 40 ppm P measured by the Olsen test. Soil containing 40 ppm P holds roughly 180 lbs plant available P2O5 in the upper six inches. Celery takes up about 100 lbs of P2O5 per acre, and about 70% of the P is removed from the field with the harvested crop. Soils with 100 ppm P have 460 lbs P2O5 per acre down to six inches, providing more than five times the P demand of most vegetable crops. Most crops send roots below six inches, gaining access to even more P.
Phosphorus fertilizer gives crops immediate access to P, circumventing the slower biological cycling. Phosphoric acid, monoammonium phosphate and other sources initially spike soil solution phosphate, but the effect does not last. In calcareous and high-pH soils, P eventually disappears from the plant available pool as it binds with calcium to form the mineral apatite. Under acidic conditions, phosphate precipitates with iron and aluminum hydroxides. The P fixation rate depends on many factors, including pH, temperature, moisture and the concentration of other compounds in soil solution. Growers apply more P fertilizer every year to meet immediate crop needs, even though total soil P levels continue rising.
Likelihood of crop response to phosphorus fertilizer (adapted from Geisseler, Daniel. (2015). California Fertilization Guidelines. Fertilizer Research and Education Program. http://geisseler.ucdavis.edu/Guidelines/Home.html.)
Environmental Impacts
While adsorbed P might not be accessible to the crop, the extra nutrition disproportionately impacts freshwater and marine environments when it escapes the farm via runoff or wind erosion. Relatively small increases in P concentration in lakes, streams and ocean water cause major ecological shifts. High N and P levels induce eutrophication by triggering algal blooms that block sunlight from penetrating the water’s surface layer. Unable to photosynthesize, aquatic plants die and sink to the bottom, introducing an overabundant food supply to microorganisms. Aerobic metabolism depletes the water’s oxygen concentration as microbes decompose the plant material. Oxygen diffusion down to lower depths can’t keep pace with the consumption rate. Hypoxic zones drive away or kill off fish, and the aquatic ecosystem unravels, leading to permanent dead zones under the worst conditions.
Researchers point to organic matter as the solution to almost every soil quality challenge, and phosphorus is no exception. Increasing soil organic matter and microbial activity helps prevent erosion and increases the bioavailability of P already in the soil. Microbial metabolism releases carbon dioxide, dissolving calcium phosphate minerals. Enzymes and organic acids also liberate phosphate, while mycorrhizal networks mine phosphorus from parts of the soil profile that plant roots cannot reach. Meanwhile, microbial activity and organic matter build soil structure, forming stable aggregates that resist erosion from water and wind. One major windstorm can blow away an inch of topsoil carrying away valuable phosphate fertilizer. Soil with 100 ppm P concentration holds almost 80 pounds of plant-available phosphate in the upper inch of soil. Many fields have accumulated P to well over 200 ppm, doubling or tripling the cost of eroded P.
Phosphorus fertilizer is a valuable tool and a mainstay of almost every fertilizer regimen. Increased yields and reasonably priced fertilizer keep growers applying mineral P every season. While a heavy P application may have significantly increased yield the first couple of years, continued applications at the same rate may have little effect. Soil tests can help determine baseline P content and the soil’s adsorption capacity. Water quality, soil pH and calcium content affect how quickly P fertilizer precipitates out of the plant-available pool. Management practices like splitting P into several applications or applying it with an organic amendment can help keep a steady supply of P in plant-available form. Like most agricultural challenges, the best solutions are multipronged, with many little adjustments adding up to dramatically improve the big picture. Better P management will protect our freshwater and marine environments, enhance crop quality and even save growers some money.
References
Brady, Nile C. and Weil, Ray R. (2008). The Nature and Properties of Soils. Fourteenth Edition. Pearson Prentice Hall.
Filippelli, Gabriel. (2008). The Global Phosphorus Cycle: Past, Present, and Future. Elements. 4. 89-95. 10.2113/GSELEMENTS.4.2.89.
Liu, Guodong, Li, yuncong, & Gazula, Aparna. (2019). Conversion of Parts Per Million on Soil Test Reports to Pounds Per Acre. University of Florida Extension. https://edis.ifas.ufl.edu/publication/hs1229
Wyant, Karl A., Corman, Jessica R., & Elser, James J. (Eds.). (2013). Phosphorus, Food, and our Future. Oxford University Press.
