To keep the pathogen from spreading to infected fields, Bolda recommended minimizing movement of soil from field to field on shoes and equipment (photo courtesy Joji Muramoto.)
Wilt disease in strawberries caused by the pathogen Fusarium oxysporum forma speciales fragariae is expanding quickly and merits attention from strawberry growers.
Mark Bolda, UCCE strawberry and caneberry farm advisor in Santa Cruz County, emphasized that fumigation operations in infected fields are worth the cost even when using plants resistant to this pathogen. Speaking at the 2021 Crop Consultant Conference, Bolda also noted that crop termination works to improve yields in resistant varieties through the season and in the first half of the season in susceptible varieties.
“Crop termination very much looks like it enhances chloropicrin efficacy in susceptible varieties,” Bolda said.
The majority of the genus Fusarium oxysporum is not pathogenic and is common in soil and around roots. Individual types are specific to host plants. This pathogen does grow on other field crops, but does not thrive, except on strawberries. Infected plants become noticeable from May to June as crowns split and become discolored. Bolda said 80% of the disease load is in the crown of the plant.
Bolda said Fusarium in strawberry could possibly coincide with the reduction and prohibition of methyl bromide. When many plants are diseased at once in a field, that does not necessarily mean the disease came with the transplants. This specific pathogen does not show symptoms at low levels and can augment populations over several successive crops of strawberries. Management of this pathogen consists of sanitation, fumigation, crop rotation and possibly fungicide applications.
He advised minimizing movement of soil from field to field on shoes and equipment to keep from spreading the pathogen to uninfected fields. Crop rotation is at least 18 months before returning to strawberries. The longer the better, Bolda said.
Bolda’s field trial set out to test efficacy of fumigant treatments and crop rotations against the strawberry-specific Fusarium pathogen. Treatments tested were KPAM at 20 gallons per acre and crop termination, Dominus at 20 gallons per acres, Tri Clor 80 at 350 pounds per acre and KPAM drip at 47 gallons per acre. Varieties were Fusarium-resistant San Andreas, Fusarium-susceptible Monterey and Fusarium-resistant Fronteras. Dominus, Bolda noted, is close to being registered in California.
All of the treatments showed significantly higher crop yields in Monterey from April through August compared to the untreated control. The treatment of KPAM plus crop termination then Tri Clor 80 had the highest yields.
The invasive vine mealybug is the most problematic of all mealybug species and causes the most damage in California vineyards (photo by Jack Kelly Clark, UC Statewide IPM program.)
New insecticides, new combinations of insecticides, biological controls and improved mating disruption to control vine mealybug (VMB) are all under study, but costs and necessary control levels are issues, said UCCE Assistant Specialist Kent Daane.
The invasive vine mealybug is the most problematic of all mealybug species and causes the most damage in California vineyards. Damage by the vine mealybug is similar to that of other grape-infesting mealybugs in that it produces honeydew that drops onto the bunches and other vine parts and serves as a substrate for black sooty mold. Mealybugs also will infest grape clusters. Vine mealybug can vector grapevine leafroll associated viruses which have been associated with sudden vine death.
During dormancy and early spring, VMB are found primarily on the trunk, canes and roots, but this can vary by vineyard location. In the spring, VMB moves onto canes and later onto leaves. In the summer, VMB moves into grape clusters. The trunk and the bark of canes provide refuge from insecticides, including systemics and natural enemies.
Ants can be a sign of a VMB infestation. Daane said that ants give refuge to VMB and also improve their habitat by removing honeydew. This tending by ants, increases VMB populations.
Bark wetness is a sign of a mealybug infestation as honeydew production increases. Leaf drop in June can also be sign of an infestation. If a VMB infestation is suspected, crowns and trunks can be inspected in the spring for adult females and crawlers. Starting at bloom, cordons, canes and basal leaves should be inspected. When fruit is present, clusters can be scouted for signs of VMB. Pheromone traps in vineyards from August to October can give record of VMB numbers and indicate numbers for next season. Daane warned that it is common to have high trap counts with little actual crop damage.
Increasing resistance to chemistries used in the past to control VMB make it important to rotate those chemistries which are still effective. Daane said there are both conventional and OMRI-approved insecticides to control VMB, but none provide 100% control.
“Movento is the best, but we are seeing cracks in control,” he warned.
Use of mating disruption devices in the vineyard can help with control. They work best with low VMB pressure. Daane said the effect is better in the second or third year. Ants also must be controlled.
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.
