2004 - 2005
ABSTRACT
CALIFORNIA LETTUCE RESEARCH BOARD
for the period
(April 1, 2004-March 31, 2005)
PROJECT TITLE: Lettuce Breeding, USDA-ARS
PROJECT INVESTIGATORS: R. C. Grube, R. Hayes, B. Mou, J. D. McCreight, USDA/ARS Crop Improvement and Protection Unit, Salinas, CA
SUMMARY:
Our objectives are to incorporate resistance to several diseases and insects into crisphead and mixed lettuce cultivars and breeding lines. In 2004, major efforts targeted resistance to lettuce big vein disease, downy mildew / Bremia, lettuce drop / Sclerotinia minor, Verticillium wilt, Fusarium root rot, lettuce mosaic virus, lettuce dieback/tombusviruses, bacterial leaf spot, corky root, leafminer, lettuce aphid, and multiple disease resistance. Minor programs addressed adaptation to low desert conditions, nutritional content, as well as resistance to Phoma basal rot and powdery mildew. In all programs, horticultural traits, adaptation, and resistance to tipbum are considered essential.
In 2004, we identified new candidate sources of resistance to big vein disease, Verticillium wilt, and lettuce drop. We confirmed resistance of previously identified germplasm to corky root, big vein disease, powdery mildew, and lettuce aphid. Additionally, cultivars, primitive lettuce, and wild Lactuca species were evaluated for nutritional content. Advanced breeding lines were evaluated for resistances to big vein disease, lettuce mosaic virus, corky root, downy mildew, powdery mildew, bacterial leaf spot, lettuce drop, Verticillium wilt (crisphead), dieback (romaine), and tip burn.
Genetic studies concurrent with breeding programs are being conducted to determine the inheritance of resistance to big vein disease, lettuce mosaic virus, lettuce drop, field resistance to DM, powdery mildew, Fusarium root rot, and Verticillium wilt. Publications during 2004-2005 included reports of research on nutritional content, and resistances to corky root, leafminer, yellow spot, big vein disease, and lettuce mosaic virus.
CALIFORNIA LETTUCE RESEARCH PROGRAM
April 1, 2004 to March 31, 2005 BREEDING CRISPHEAD LETTUCE
Richard W. Michelmore
Oswaldo E. Ochoa
The Genome Center and
The Department of Plant Sciences
University of California, Davis
rwmichelmore@ucdavis. edu
oeochoa@ucdavis.edu
SUMMARY
The program continues to emphasize the identification and incorporation of genes for disease resistance, particularly to downy mildew, lettuce mosaic virus, anthracnose, verticillium and corky root, into crisphead horticultural types suitable for California. Resistance for downy mildew is being introduced from several new sources and combined with resistance to LMV and corky root. Utilization of multiple new sources will minimize the chances that changes in the pathogen will render all cultivars susceptible simultaneously. Advanced lines are trialed at the USDA field station in Salinas. We have continued to monitor variation in the ability of the downy mildew pathogen to overcome resistance genes. There has been a continued increase in variation in the pathogen. Of the known resistance genes, only Dml 7 remained effective against all California isolates. We initiated a program for resistance to Verticillium.
CALIFORNIA LETTUCE RESEARCH PROGRAM April 1, 2004 to March 31, 2005 LEAF LETTUCE BREEDING
Richard W. Michelmore
Maria Jose Truco
Oswaldo E. Ochoa
The Genome Center and
The Department of Plant Sciences
University of California, Davis
rwmichelmore@ucdavis.edu
mitruco@vegmail.ucdavis.edu
oeochoa^ucdavis. edu
SUMMARY
Backcross programs are underway that emphasize the identification and introduction of genes for disease resistance, particularly to downy mildew, corky root, lettuce mosaic virus and anthracnose, into the four leaf lettuce types. Resistance for downy mildew is being introduced from eighteen new sources into cultivated genotypes suitable for California and will ultimately be combined with resistance to LMV, anthracnose and corky root. Utilization of multiple new sources and introduction of different resistances into the different types will probably increase the longevity of individual resistances and minimize the chances that changes in the pathogen will render cultivars of different lettuce types susceptible simultaneously. Genetic studies are in progress to determine the genetic basis for the resistance and to increase the efficiency of generating resistant varieties.
CALIFORNIA LETTUCE RESEARCH PROGRAM
April 1, 2004, to March 31, 2005
GENETIC VARIATION IN LETTUCE
Richard W. Michelmore
UC Davis Genome Center and
The Department of Plant Sciences
University of California, Davis
rwmichelmore(a),ucdavis. edu
SUMMARY
Novel methods for detecting, analyzing, and manipulating genetic variation are being applied to lettuce. The aim of this project is to ensure that lettuce does not lag behind other crops in benefiting from the application of genomic and biotechnological techniques. We have continued four projects: (i) Introduction of genes into lettuce using Agrobacterium tumefaciens and analysis of their expression, (ii) Cloning and characterization of disease resistance genes, (iii) Genetic mapping using a variety of molecular markers with the goal of locating most of the disease resistance genes known in lettuce, (iv) Comparative genomics to identify candidate genes controlling horticultural traits.
We are in the process of cloning functional homologs of Dm3 with different specificities from other genotypes with an initial focus on Dml8. We have shown using post-transcriptional gene silencing that Dm 18 and Dm 16 are probably encoded by homologs ofDm3. Over 66,000 ESTs have been generated representing over 19,000 unigenes and at least a third to a half of all genes in lettuce. QTL analysis has identified chromosomal regions controlling numerous horticulturally important traits. These are being mapped relative to candidate genes. We have developed and are curating a database for the Compositae (http://compositdb.ucdavis.edu/) that includes genetic, molecular marker, cultivar, and sequence data for lettuce as well as information on lettuce researchers. We are developing a new lettuce database, lettuce genetic map viewer, that will replace in time the old lettuce database version. The beta version is accessible at http://cgpdb.ucdavis.edu/database/genome viewer/viewer/.
PROJECT ABSTRACT CALIFORNIA LETTUCE RESEARCH BOARD
April 1, 2004-March 31, 2005
Project Title: Characterization and Management of Lettuce Dieback Disease and
Lettuce Tombusviruses
Investigator: William M. Wintermantel, USDA-ARS, 1636 E. Alisal St.
Salinas, CA 93905, Ph: (831) 755-2824; Fax: (831) 755-2814
SUMMARY
Lettuce dieback is responsible for losses in Romaine and leaf lettuce production in areas associated with rivers in California and Arizona. Losses vary, ranging from severe in some years to mild in others. The disease is tightly linked to the presence of river water, being found commonly in areas where flooding has recently occurred, where river water is used for irrigation of fields, and where dredge is deposited on fields. Characteristic symptoms include severe stunting, necrosis and dieback of lettuce plants. The disease is caused by isometric viruses in the genus Tombusvirus.
Field isolates from lettuce have been identified and characterized as Lettuce necrotic stunt virus (LNSV), and Tomato bushy stunt virus (TBSV). Studies conducted previously demonstrated that conditions within the soil environment, specifically soil salinity, influence lettuce dieback symptom development under greenhouse conditions. This suggested that poor drainage such as occur in areas near rivers may lead to high salt concentrations, and that these conditions may lead to increased incidence of infection. Microplots were developed in an attempt to replicate greenhouse salinity studies under controlled field conditions, however, results of two independent experiments yielded too few infected plants per plot to obtain a statistically valid study. Most likely additional factors are involved that could not be duplicated under microplot conditions. Recent studies have focused on ELISA-based detection of tombusviruses in lettuce, due to lower cost than RT-PCR based methods.
Variability among tombusviruses makes this method difficult for uniform detection, as the protein detected by ELISA is only 50 to 60 percent conserved among tombusviruses that infect lettuce. Current antisera also cross react with nonpathogenic tombusviruses, such as CNV. New knowledge of the molecular biology of lettuce-infecting tombusviruses is leading to the production of strain-specific antisera for tombusvirus detection. In order to more fully understand the causal agent of lettuce dieback, the complete sequence of a well-characterized LNSV isolate was determined. This information provides baseline information for future studies, and for comparison with other disease-causing isolates. Sections of divergent tombusvirus isolates have been cloned and sequenced, leading to identification of new types of tombusviruses associated with infected lettuce. Further characterization of these isolates may lead to improved methods for universal detection, and reduced interference with non-pathogenic isolates.
I. Abstract
Project Title: Getting a Handle on Lettuce Big-Vein and Development of Needed Tools and Information for LBV Control
Principal Investigator:
BryceW.Falk
Professor
Department of Plant Pathology
University of California
Davis, CA 95616
530-752-0302,752-5218
Email:bwfalk@ucdavis.edu
Summary:
Two viruses have been reported to be associated with Lettuce big-vein (LBV). These viruses are Mirafiori lettuce virus (MiLV) and Lettuce big-vein virus (LB W), and have been reported from Spain, northern Europe and Japan. We attempted to determine if these viruses are also associated with LBV in California. We also wished to develop the means to detect and differentiate these viruses from each other, and to perform preliminary analysis/comparison with those viruses from other parts of the world. We utilized nucleotide sequence information from public databases for European and Japanese isolates of MiLV and LBW. We compared these with each other and identified regions that were identical among all isolates described so far. These regions were used to design several oligonucleotide primers for each virus.
We then attempted to develop a reverse transcription polymerase chain reaction (RT-PCR) procedure which could provide rapid and specific identification of each virus. Our experiments were successful. We confirmed that MiLV and LBW were present in California LBV-affected lettuce. Initial sequence comparisons also showed that for the genomic regions analyzed, Salinas isolates of both MiLV and LBW are closely related to European isolates of the respective viruses. In addition to qualitative differentiation of these viruses, our approach has great applicability for quantitative differentiation and for use in comparisons of lettuce breeding lines/cultivars for resistance to quantitative and qualitative resistance against LBV.
Research Abstract to the
California Lettuce Research Board's Research Program April 2004 to March 2005
Project Title:
Project Investigators:
Collaborators:
Management of Phoma basal rot disease of lettuce
Steven T. Koike
University of California Cooperative Extension Monterey County
Krishna Subbarao
Department of Plant Pathology University of California at Davis
Becky Grube, Ed Ryder, Gerhard Verkley, Tim O'Neill, Diana Fogle
Summary:
Phoma basal rot disease of romaine lettuce, first found in California in 2000, is now apparently established in various parts of coastal California. The pathogen is a fungus in the genus Phoma; the species is likely Phoma exigua. The Phoma group includes a large number of soil-borne species, some of which are weak pathogens that usually attack plant stems. Experiments showed that romaine isolates were able to consistently cause disease on romaine and crisphead cultivars, and crisphead isolates consistently caused disease on crisphead and romaine cultivars. The disease has not yet been found in Ventura County, the Huron area, or in Yuma, Arizona.
When applied shortly after thinning, the products Quadris, Endura, and Switch were very effective in preventing Phoma basal rot from developing. However, timing of sprays may be critical; in a different 2003 trial, our first spray was applied seven days later than scheduled. For this location, the treatments (which consisted of various rates and timings of Quadris and Endura) did not perform well.
For a number of years, there has been an interesting correlation between Phoma outbreaks on lettuce and previous strawberry crops. For many romaine crops, significant disease has occurred when the lettuce is the first crop following strawberries from the previous season. We therefore conducted an analysis of strawberry transplants and field-planted strawberries to see if the Phoma pathogen was present on such plant material. For each sample, roots were rinsed and then directly plated onto semi-selective medium. Another set of roots was placed into sterile distilled water and incubated on a rotary shaker; the resulting solution was filtered through cheesecloth and then dilution plated onto semi-selective medium. For all samples, no Phoma suspects were observed in any of the plates. The source of this pathogen, therefore, remains unknown.
CALIFORNIA LETTUCE RESEARCH BOARD
FINAL REPORT MAY 1,2005
Project Title: DEVELOPMENT OF TOOLS TO SPEED DEVELOPMENT OF LETTUCE LINES RESISTANT TO BACTERIAL LEAF SPOT OF LETTUCE
Principle investigator: Cooperators:
Carolee Bull, USDA/ARS
Polly Goldman, USDA/ARS Ryan Hayes, USDA/ARS Ed Ryder, USDA/ARS
Summary:
We previously developed a rating system to quantify to bacterial leaf spot of lettuce (BLS) caused by Xanthomonas campestris pv. vitians. This rating system considers severity, incidence and location of symptoms as they relate to economic impact. Using this system we identified several sources of resistance to BLS. We are using the identified sources of resistance to improve resistance in other head types. During this funding cycle we continued evaluating progeny from a cross of Salinas x (Salad Crisp X Iceberg) in field experiments in Spring of 2004. Four progeny with promising head types were identified. Additionally we evaluated 15 F3 lines of Salinas 88 x (Salad Crisp x Iceberg) for resistance to BLS. Ten individual plants were selected for type and resistance. Low disease severity due to unseasonable weather hampered these experiments to some degree. Additionally, we evaluated 25 commercially available iceberg-types for BLS resistance; however, none of the evaluated cultivars showed promising levels of resistance.
Previously we developed an assay for the quantification of resistance to bacterial leaf spot among lettuce cultivars. Although the assay can be used to evaluate established cultivars and lines when adequate seed is present, it is not useful in determining whether individuals in a population are resistant or susceptible to BLS. In order to increase the speed of the screening of germplasm an assay is needed from which resistant individuals can be selected. We tested three strategies for meeting this goal and have found one that appears to be able to identify resistant individuals in a mixture of genotypes. This method relies on high planting density with continual misting and multiple inoculations during the course of the experiment. Additionally, a resistant control, Little Gem, is planted with the mixture and is used to standardize the threshold level of disease permitted in individuals identified as resistant.
CALIFORNIA LETTUCE RESEARCH BOARD MAY 1, 2005 FINAL REPORT
Project Title:
DEVELOPMENT OF TOOLS FOR MANAGEMENT OF CORKY ROOT OF
LETTUCE
Principle investigator: Cooperators:
Carolee Bull, USDA/ARS
Polly Goldman, USDA/ARS Beiquan Mou, USDA/ARS Steven T. Koike, UCCE
Summary:
We conducted two field trials aimed at evaluating the effectiveness of various inoculation methods of lettuce seeds and seedlings to develop plots providing consistent levels of disease and to help evaluate the process by which the pathogen, Rhizomonas suberifaciens infects roots. In these assays, the treated plants did not differ from the untreated controls for disease incidence or severity. Although the background inoculum in these plots appears to be sufficient for evaluating lettuce germplasm, the lack of increase in disease in inoculated plots makes the task of studying the infection process in the field more difficult.
We have modified a corky root assay for use to evaluate germplasms and control measures in greenhouse studies. We have improved the assay during the course of the last year and have acquired new strains of the pathogen and improved our handling of these strains to maintain virulence.
Using in vitro assays we have identified a number of promising biological control agents. In particular myxobacteria which prey on other bacteria by producing compounds that break-open the bacterial cells of the pathogen, may be useful. We have described a strain specific interaction between myxobacteria and Rhizomonas suberifaciens and cell cycle and motility factors for myxobacteria which are important in predation of/?, suberifaciens. These data will be important in helping us develop these organisms for use under field conditions.
CALIFORNIA ICEBERG LETTUCE RESEARCH PROGRAM
April 1, 2004 - March 31, 2005
EPIDEMIOLOGY AND CONTROL OF LETTUCE DROP CAUSED BY SCLEROTINIA
SPECIES Krishna V. Subbarao
Department of Plant Pathology University of California, Davis
SUMMARY
One research objective for the current year was to continue to determine the efficacy of biocontrol agents in the Imperial Valley fcr lettuce drop control. In addition to this objective, we also completed the fungicide degradation study that was pending from previous years. As in the previous year, the biocontrol treatments evaluated on both Sclerotinia sclerotiorum and S. minor included, Coniothyrium minitans (Contans), Trichoderma harzianum (Plant Shield), and Bacillus subtilis (Companion). The experiment also included the standard Rovral treatment along with uninfested and infested, unsprayed controls.
The lettuce crop in the different treatments was scored for lettuce drop incidence and the soil from each treatment plot was assayed for the number of sclerotia of the corresponding species. Lettuce drop caused by S. sclerotiorum was not observed in any of the treatments this year and thus, the success with Contans observed during the previous year could not be duplicated. Despite the lack of treatment differences on lettuce drop incidence caused by S. sclerotiorum, Contans significantly reduced the number of sclerotia in the soil. Consistent with the previous year's results, none of the treatments were effective on lettuce drop caused by S. minor nor did they reduce soilborne sclerotia of S. minor.
This experiment is being repeated in a new area in 2005. From 1999-2002, both registered and experimental fungicides were evaluated for efficacy on lettuce drop caused by S. minor. Implicit in this study was to test if efficacy was related to the fraction of the fungicide remaining after the two applications. Less than 30% of Rovral and Botran remained within 2 wk after application in contrast to the nearly 70% of BAS 510 (Endura) and Switch that remained through the end of the season. This partially explained why both BAS 510 and Switch were more efficacious than Rovral and Botran.
CALIFORNIA LETTUCE RESEARCH PROGRAM
April 1, 2004 - March 31, 2005 BIOLOGY AND EPIDEMIOLOGY OF VERTICILLIUM WILT OF LETTUCE
Krishna V. Subbarao
Department of Plant Pathology University of California, Davis
SUMMARY
The research objectives this year included: (1) continued monitoring of Verticillium wilt and soil inoculum density in coastal California; (2) to evaluate incidence of Verticillium wilt on lettuce following strawberry in plots treated with chloropicrin, telone+chloropicrin, and methyl bromide+chloropicrin (MBC); (3) to determine the efficacy of fumigation at different soil depths and follow the recolonization of soil by Verticillium dahliae; (4) to determine the effects of infested seed on soil infestation by Verticillium dahliae and on the incidence of Verticillium wilt of lettuce; (5) to continue the breeding program to identify and develop sources of resistance to Verticillium wilt in crisphead, leaf, and other lettuce types; and (6) to determine potential relationships between days to maturity and susceptibility to Verticillium wilt in crisphead, leaf, butterhead and romaine lettuce types in the greenhouse and field.
After a lull in 2003, Verticillium wilt on lettuce returned causing severe losses in many new fields in Watsonville and Salinas. The incidence ranged between 30 to 90%. As in previous years, Verticillium wilt reappeared in two fields previously fumigated with MBC. Soil assays from all fields in which the disease appeared had high levels of microsclerotia g"1 soil. The lettuce crops following strawberries after fumigation with Chloropicrin, Telone+Chloropicrin, and MBC were evaluated for incidence of lettuce drop, crown rot and Verticillium wilt. While no crown rot was observed this year in any of the plots, Verticillium wilt developed for the first time in all treatments (7-18%) and the number of microsclerotia varied between 63-99 g"1 soil. In a field that had very high levels of Verticillium wilt in 2002, we began long-term studies to determine the efficacy of fumigation with MBC to reduce microsclerotia of V. dahliae at different soil depths. Another implicit objective of this study was to determine the effects of various cropping patterns on the microsclerotia at different soil depths. Following fumigation and strawberry planting, radicchio was planted in these fields. Soil at the four sites was again sampled in 2-inch increments to a depth of 24 inches.
At all sites, there was a general decline in the number of microsclerotia and yet, the numbers were again high near the surface and progressively declined. Radicchio did not develop Verticillium wilt. Again no tangible data for objective 4 were available this year. We had a very successful year evaluating a large number of commercial cultivars of each lettuce type in both greenhouse and the field. Limited screening of leaf and butterhead lettuce types along with a large number of crisphead types over many years had lead to the conclusion that the former types are more resistant than crisphead types. This year's results clearly showed that there is a range of reactions within each lettuce type and resistance or susceptibility to Verticillium wilt is not determined by the type of lettuce. A number of Lactuca virosa accessions were also evaluated in the field and lines with very high levels of resistance were identified. There was no relationship between days to maturity and resistance to Verticillium wilt.
Abstract
Project title: Fusarium wilt of lettuce: management through detection, avoidance and disease resistance
Principal investigator:
Thomas R. Gordon Department of Plant Pathology University of California Davis, CA 95616
Layman's summary
Fusarium wilt of lettuce (also known as Fusariun root rot) is caused by the soilbome fungus Fusarium oxysporum f. sp. lactucae. Extensive damage is done to the roots of susceptible varieties, resulting in stunting and possibly death of the affected plant. Management of Fusarium wilt requires practices that minimize levels of the pathogen in soil and the use of lettuce cultivars with resistance to the disease. Previous work has shown considerable variation in the susceptibility of currently grown cultivars, based on seedling root-dip inoculations. One objective of the current study was to determine if cultivars judged to be resistant in the root-dip test would also appear resistant when infected by soilbome inoculum, as would typically occur under field conditions. Our results showed that plants grown in infested soil generally responded in the same manner as those inoculated by the root-dip procedure.
The exception was one iceberg type (Kahuna) that was highly resistant in the root-dip assay but was clearly susceptible when grown in infested soil. This suggests that most cultivars manifesting resistance as seedlings are likely to suffer relatively little damage under field conditions. However, it is also apparent that some mechanisms of resistance that are operative at the seedling stage may not confer resistance under field conditions. Tests of isolates from various locations in California confirmed that only race 1 is present. Preliminary studies indicate that resistance to Fusarium wilt in the cultivar 'Salinas' is unlikely to be determined by a single gene. However, it remains possible that a major component of resistance is conferred by one or a small number of genes. Among warm weather crops that might be grown in rotation with lettuce, melon supported the highest levels of the pathogen. Tomato, cotton and pepper had significantly lower rates of infection.
Research Abstract to the California Lettuce Research Board
for the period April 1,2004 to March 31,2005
TITLE: Insect Management for Central Coast Lettuce
PRINCIPLE
INVESTIGATOR: William E. Chaney
Farm Advisor - Entomology
U.C. Cooperative Extension - Monterey County
1432 Abbott Street
Salinas, CA 93901
The management of soil pests is a key issue of increasing concern for many growers. The key soil arthropods of concern in lettuce are garden centipedes (symphylans), springtails and bulb mites (in that order of importance). The soil pests are especially difficult to research due to the difficulty in both sampling and predicting to occurrence of these pests. During that past season's work, we were able to establish several important facts, some useful to growers and PCAs now, others important to us in furthering this work.
Some key findings in regard to symphylans are that the predictability of damaging populations, even in the same location, from year to year is more difficult than previously assumed. Some factors that contribute to this appear to be soil moisture, previous crop and tillage practices. It also appears that the tilth of the soil is key; both to sampling using the baiting procedure and to the effectiveness of any of the control options. An easily friable soil with good tilth is ideal for sampling and managing symphylans.
It appears that some of the findings from symphylans do not hold for springtails. Because the Board has identified this work as less critical to most growers, this work will continue when the opportunity arises, to help insure that some options and information are available when needed.
Thrips work is difficult for many reasons, the two most important are that damaging populations are often fleeting and arise when nearby crop or non-crop areas provide a source of adult thrips that move into lettuce. This makes finding and scheduling trial work difficult. We are using the assumption that the greatest chance of problems will be in fields located near bulb onions near harvest or artichokes near cut¬back. A reliable sampling method is also difficult to develop. In-field counts are not acceptable and some sort of washing or shaking technique will be necessary. This was independently confirmed by Dr. John Palumbo of University of Arizona, Yuma in his thrips trial work.
Work in these areas continues as well as new work in bio-control of lettuce aphid funded by Region 9 EPA and supported by this Board.
Research Abstract to the California Lettuce Research Board's Research Program
April 1, 2004 to March 31, 2005
TITLE:
Thrips Management for Low Desert Lettuce
PRINCIPAL INVESTIGATOR:
Eric T. Natwick
Farm Advisor - Entomology
University of California Cooperative Extension - Imperial County
1050 East Holton Road
Holtville, CA 92250
SUMMARY:
Several registered and experimental insecticides were evaluated for their efficacy western flower thrips, Frankliniella occidentalism in Romaine leaf lettuce in Imperial Valley, CA. Bean thrips, Caliothrips phaseoli, were not found in the leaf lettuce experiment in 2004/05. Bean thrips and western flower thrips are the two primary thrips pests of lettuce in southern California. Insecticide resistance in western flower thrips has been well documented worldwide. Bean thrips has only recently become a common pest on lettuce in the Imperial Valley, but the continued reliance of a limited arsenal of insecticides to control this pest could lead to insecticide resistance problems.
Insecticides included in the experiment for thrips control were Admire 2F, Platinum 2 SC, Success 2 SC, Warrior 1 CS, Lannate LV, MustangMax and Assail 70 WP. The best treatments for thrips control were Platinum 2 SC in-furrow at planting at 0.125 lb AI per acre followed by foliar treatments with Warrior 1 CS at 0.03 lb AI per acre, foliar treatments of Lannate LV at 0.675 lb AI per acre plus Warrior 1 CS at 0.03 lb AI per acre followed by Success 2 SC at 0.094 lb AI per acre plus Warrior 1 CS at 0.03 lb AI per acre, MustangMax at 0.03 lb AI per acre plus Lannate LV at 0.675 lb AI per acre, and foliar treatments with Success 2 SC at 0.094 lb AI per acre.
Harvest data did not reveal any differences among the treatments for marketable heads for cartons of #36's, #42's, and #48's, for percentages of marketable fruit for the domestic market, nor were there any differences among the treatments for pounds of marketable heads or culls due to insect contamination. However, there were significant differences and mean separations for percentages of insect free heads that could go to the export market. The best treatment for percentages of insect free heads were Admire 2F at 0.25 lb AI per acre followed by Success 2 SC at 0.094 lb AI per acre plus Warrior 1CS at 0.03 lb AI per acre, Platinum 2 SC at 0.125 lb AI per acre followed by Warrior 1 CS at 0.03 lb AI per acre, and Warrior 1 CS at 0.03 lb AI per acre with 84.5 percent, 83.3 percent and 80.3 percent, respectively.
CALIFORNIA LETTUCE RESEARCH BOARD
April 1, 2004 - March 31, 2005 WEED MANAGEMENT SYSTEMS FOR LETTUCE
Steven A. Fennimore
Dept. of Plant Sciences
University of California, Davis
Salinas, CA
Richard F. Smith
University of California Cooperative Extension
Monterey County
Salinas, CA
Jose L. Aguiar
University of California Cooperative Extension
Riverside County
Indio, CA
SUMMARY
Field and lab studies were undertaken to evaluate four aspects of lettuce weed management. In the first series, field studies were conducted to evaluate the safety of herbicides on lettuce germplasm from the University of Idaho (ED-BR1). The conventionally bred ID-BR1 lettuce germplasm was reported to be tolerant to sulfonylurea herbicides. Preemergence applications of sulfonylurea herbicides Maverick and Upbeet, and postemergence applications of Harmony were found to be safe on the ID-BR1 germplasm. In a second experiment, we evaluated treatments to improve nutsedge control in fallow ground prior to lettuce planting. Nutsedge control in lettuce was better in plots previously treated with sequential applications of Telone C35 followed by Eptam, compared to plots treated with Eptam alone.
In some soils, high levels of irrigation can wash Kerb herbicide from the upper soil layer resulting in poor weed control. Some have suggested that intermittent irrigation improves weed control with Kerb compared to continuous irrigation. In a laboratory soil column experiment, we found that Kerb mfoved whether intermittent or continuous irrigation was used on soils from Yuma, AZ. Kerb did not move from surface soil layers regardless of irrigation pattern in Chualar series soils from Salinas, CA. In the fourth trial series, Treflan applied post plant preemergence was evaluated in lettuce as a potential tank mix partner for Kerb. While the Treflan plus Kerb tankmix slightly improves weed control compared to Kerb alone, the risk for Treflan to injure lettuce outweighs the weed control benefits.
CALIFORNIA LETTUCE RESEARCH BOARD
April 1, 2004 - March 31, 2005
Title: Mustard Cover Crops to Optimize Crop Rotations for Lettuce Production
Project Investigators: Richard Smith, Steve Koike and Tiffany Bensen, UCCE,
Monterey County
Krishna Subbarao and Steve Fennimore, University of California
SUMMARY
Effective rotations are an essential part of lettuce production. Unfortunately, given economic pressures such as high land rents and lower returns for rotational crops, effective rotations are not always possible. As a result, Sclerotinia minor, causal agent of lettuce drop, is the key soilborne disease in the Salinas Valley. Mustard cover crops have been researched for a number of years in other parts of the world as a means of suppressing soilborne diseases and weeds. The beneficial effect of mustard cover crops on soilborne pests may be due to toxic chemicals that they release upon incorporation into the soil or due to soil microbiological effects. These studies were undertaken to determine if mustard cover crops are suppressive to lettuce drop and weeds in lettuce production.
To evaluate the impact of mustard cover crops we established short-term and long-term trials. A total of seven short-term studies were conducted with cooperating growers in which the mustard cover crop blend Caliente 119 (Sinapis alba and Brassica juncea) was grown in comparison with Merced Rye or a bare fallow treatment, hi addition, a long-term rotational plot was also established in which fall cover crops were grown and followed by two lettuce crops planted the following spring and summer; data herein reflect three years of these field studies. Lettuce Drop Studies: A preliminary survey of three cover crop fields indicated that mustard cover crops are susceptible to S. minor and may increase the amount of soilborne inoculum. The question arises, does the susceptibility of mustard cover crops to S. minor override the suppressive effect upon incorporation? hi the short term trials, there was no significant reduction of infected heads in mustard cover cropped over bare fallow plots in individual trials.
However, a summary analysis indicates a slight but significant reduction in the level of lettuce drop infection of lettuce at harvest in the mustard cover cropped versus bare fallow plots. The metam sodium equivalent content of mustard cover crops was measured. Mustards contain low amounts (1.7 -2.3 gal/A) as compared to label rates (37.5 - 75.0 gal/A) of metam sodium. Rye, and white and Indian mustard cover crops increased the yield of the first lettuce crop following incorporation of the cover crop in one trial this year. Weed Studies: Total weed emergence was reduced in two of six short-term cover crop trials. However in the long-term trial no effect on weed emergence at thinning was observed and no reductions in the soil weed seed bank were observed. Other evaluations: Nitrate leaching: Nitrate leaching was significantly reduced over the winter of 2003-04 by these fall-grown cover crops in the long-term trial. However nitrate leaching was not reduced in the 2004-05 evaluation and the difference could be due to higher rainfall in the winter of 2004-05. Soil characteristics: After one cover crop cycle in the long-term cover crop trial there was a significant increase in soil carbon and sodium in the cover cropped plots.
California Lettuce Research Board Annual Report 2004 - 2005
Salinity Effects on Quality and Yield of Drip Irrigated Lettuce
Michael Cahn Husein Ajwa
Irrigation and Water Resources Advisor Cool Season Vegetable Specialist
University California, Cooperative Extension Vegetable Crops Dept.
1432 Abbott St. University of California
Salinas, CA 93901 1636 East Alisal
Salinas, CA 93905
SUMMARY
Published thresholds of salinity tolerance for lettuce may not be reliable for the weather regime, water quality, cultivation practices, and varieties of the central coast. Because lettuce growers in the Salinas and the Pajaro Valleys face multiple challenges in managing salinity, including sea water intrusion, salinity TMDLs, and increased use of reclaimed water, there is a pressing need to establish salinity thresholds for currently grown lettuce varieties. We conducted drip-irrigated field trials to measure the effect of varying levels of salinity and sodium adsorption ratios (SAR) on the quality and yield of head lettuce at the USDA Spence Research Field Station during the 2003 and 2004 seasons. The trials followed a split-plot, randomized complete block design with 4 replications, where variety (Salinas and Sniper) was the main plot and salinity was the sub-plot. The salinity levels of the irrigation water treatments, measured by the electrical conductivity, ranged from 0.6 to 8.5 dS/m and the SAR levels ranged from 3 to 10.
Salt toxicity symptoms were not observed on the leaves, except for the most saline treatment during the 2nd year of the trial. However, moderate levels of salinity in the irrigation water caused a build up of salts in the soil and reduced yield of 24-sized heads, marketable biomass, and head weight, and increased culled heads for both varieties of lettuce. The salinity treatments had similar effects on both varieties of head lettuce during the first year of the trial, but during the second year, when salinity levels had built up in the soil profile, Sniper demonstrated more tolerance to salts than Salinas. In addition, the injection of gypsum minimized yield loss of the Sniper variety more than the Salinas.
Using regression analysis, we determined that a 10% yield loss of 24-size heads occurred when the salinity level of water was in the range of 1.1 to 2.4 dS/m. The exact threshold depends on the existing level of salinity in the soil and the lettuce variety. A salinity level greater than 1.8 dS/m in the upper 12-inches of the soil was estimated to cause a 10% loss in yield of marketable biomass. v
The threshold at which salinity was found to cause yield loss in head lettuce is in the range of levels encountered in soil and irrigation water on the central coast. Producers using water with elevated levels of salts may have significant yield loss without observing salt toxicity symptoms. Growers can avoid yield losses due to salinity by: 1. monitoring salinity levels in the soil profile, 2. applying gypsum, 3. maximizing leaching of salts during the winter, and 4. planting salt-tolerant varieties.
California Lettuce Research Board Summary Abstract 2004-05
Assessment of Indicator and Pathogenic Bacteria in Lettuce Production Environments.
Project Leader: Trevor Suslow, UC Davis Dept. Plant Sciences Objectives:
1. To assess the presence of nonpathogenic and pathogenic E. coli in irrigation and run-off water.
2. To assess the presence of nonpathogenic indicator E. coli on lettuce
3. To determine the frequency and taxonomic identity of "pathogenic E. coli false-positive'
detections in water, soil, and on lettuce using available commercial test kits.
Background:
Outbreaks of pathogenic E. coli O157:H7 were linked to consumption of Romaine lettuce in 2002 and to mixed Crisphead and Romaine and Romaine consumption in two separate events in 2003. Although definitive proof has not been established, public health officials have evidence supporting the possibility of on-farm contamination. To provide guidance in Best Management Practices to growers and the industry as a whole, the purpose of this project was to begin to develop documentation of the regional presence/absence of E. coli O157:H7 in on-farm Salinas Valley water and irrigation run-off specific to head lettuce and leaf lettuce.
Outcomes:
At this point in the project, over the course of a spring to winter cycle, we have found no evidence of viable E. coli O157:H7 in any sample taken in conjunction with environmental or crop production block evaluations, within the limits of sampling, detection, and recovery methodologies used and their inherent limits of detection. Given the positive detection and recovery of "inoculated control" E. coli O157:H7 in water and soil samples, one may extrapolate with a reasonable degree of confidence that E. coli O157:H7 contamination of the environment and farmed ground would have been detectable if their population densities exceed 10 CFU/100 ml of water and 10 CFU/g of soil, at the specific sampling locations. Frequent "false positive" detections and detection of single or dual virulence markers were observed in all sample types. Isolates of Citrobacter were common interfering bacteria.
During the Fall reservoir sampling survey, nonpathogenic E. coli populations were recovered at levels generally higher than summer levels, averaging 2.72 log CFU/100 ml. This level is above the single-point maximum of 2.48 log CFU/100 ml and individual reservoir mean value of 0.81 log CFU/lOOml determined in our CLRB funded survey conducted between 2001 and 2003.
Evidence from the low levels of recovery of nonpathogenic indicator E. coli in the cropped soil strongly • suggests that these populations become rapidly non-viable or non-recoverable. No operations were identified as widely introducing new populations or stimulating re-growth of E. coli once soil populations had declined. The nature and characteristics of the soil amendments had a stronger influence on soil indicator coliform populations than crop presence alone.
During this project period, nonpathogenic indicator E. coli population levels on head and romaine lettuce plants from three geographically separate operations were below the limit of detection (0.31 log CFU/25g) for most replicated samples. However, we believe it is potentially significant that, as in field surveys conducted during 2002 and 2003, individual plants were infrequently observed to have nonpathogenic E. coli levels of several hundred per 25g sub-sample. We feel it is important to the industry to, eventually, determine the likely sources of these indicator bacteria and to determine whether their presence on these specific plants can be correlated with the presence of enteric pathogens.