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CALIFORNIA LETTUCE RESEARCH BOARD for the period (April 1, 2006-March 31, 2007) PROJECT TITLE: Lettuce Breeding, USDA-ARS PROJECT INVESTIGATORS: R. Hayes, I Simko, 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 2006, major efforts targeted resistance to lettuce big vein disease, lettuce drop / Sclerotinia species, Verticillium wilt, Fusarium root rot, lettuce dieback/tombusviruses, bacterial leaf spot, corky root, leafminer, lettuce aphid, tipburn and multiple disease resistance. Minor programs addressed adaptation to low desert conditions, nutritional content, as well as resistance to powdery mildew and yellow spot. In all programs, horticultural traits, adaptation, and resistance to tipburn are considered essential. In 2006, we identified new candidate sources of resistance to yellow spot and race 2 isolates of Verticillium dahliae. We confirmed resistance in previously identified germplasm to big vein disease, Verticillium wilt, and lettuce aphid. Selections were taken from breeding populations, and advanced breeding lines were evaluated, as part of breeding for resistance to big vein disease, lettuce drop, Verticillium wilt, powdery mildew, dieback, bacterial leaf spot, corky root, leafminer, tipburn, bolting, and for increased nutritional content. Three iceberg breeding lines with Verticillium wilt resistance were released as germplasm. Genetic studies concurrent with breeding programs are being conducted to determine the inheritance of resistance to bacterial leaf spot, big vein disease, lettuce mosaic virus, lettuce drop, leafminers, and Verticillium wilt. Publications during 2006-2007 included reports of original research on big vein disease, Verticillium wilt, and tipburn resistance, a germplasm release for verticillium wilt, and a book chapter on lettuce breeding. CALIFORNIA LETTUCE RESEARCH PROGRAM April 1, 2006 to March 31, 2007 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 lettuce mosaic virus 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 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 Dm17 remained effective against all California isolates. We have initiated a program for resistance to Verticillium and thermotolerance for seed germination. CALIFORNIA LETTUCE RESEARCH PROGRAM April 1, 2006 to March 31, 2007 LEAF LETTUCE BREEDING Richard W. Michelmore María José Truco Oswaldo E. Ochoa The Genome Center and The Department of Plant Sciences University of California, Davis rwmichelmore@ucdavis.edu mjtruco@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 twenty-one 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, 2006, to March 31, 2007 GENETIC VARIATION IN LETTUCE Richard W. Michelmore UC Davis Genome Center and The Department of Plant Sciences University of California, Davis rwmichelmore@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 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 have shown using post-transcriptional gene silencing that Dm14, Dm18, Dm16, and Ra are probably encoded by sequences similar to Dm3. More than 200,000 ESTs have been generated from lettuce and relatives representing over 35,000 unigenes and at least half of all genes in lettuce. Quantitative Trait Locus (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. PROJECT ABSTRACT CALIFORNIA LETTUCE RESEARCH BOARD April 1, 2006-March 31, 2007 Project Title: Molecular tools for the development of lettuce with immunity to big vein disease Investigator: William M. Wintermantel, USDA-ARS, 1636 E. Alisal St. Salinas, CA 93905, Ph: (831) 755-2824; Fax: (831) 755-2814 wwintermantel@pw.ars.usda.gov SUMMARY: Genetic resistance offers the most promising and economical method for control of lettuce big vein disease. The USDA-ARS in Salinas is currently the only publicly funded big vein resistance breeding program in the United States. Resistant germplasm has been released by the USDA-ARS, and new sources of resistance are currently under development. With the recent identification of Mirafiori lettuce big vein virus (MLBVV) as the causative agent for big vein, we have now demonstrated it is possible to use MLBVV accumulation as a tool to identify germplasm with the greatest potential for developing highly resistant cultivars. During preliminary research we conducted extensive studies on the prevalence of MLBVV and the associated virus, Lettuce big vein associated virus (LBVaV) in California and Arizona lettuce exhibiting symptoms of big vein disease, and determined the relationship of these viruses to those associated with big vein disease in other parts of the world. We developed a reverse transcription-polymerase chain reaction (RT-PCR) test for detection of MLBVV and LBVaV in lettuce. RT-PCR testing of samples collected from throughout the Salinas Valley and from the Yuma production area demonstrated that MLBVV was present in all lettuce samples exhibiting big vein symptoms analyzed by our lab in both regions. LBVaV was also present in most of the samples from the Salinas Valley, but was less common in samples from Yuma. Results indicated isolates of each virus were highly similar throughout both the Yuma and Salinas production areas, and also had high levels of homology with isolates from Europe and Asia based on nucleotide sequence analysis (Hayes et al., 2006). This indicated that our detection methods should be effective in monitoring germplasm for accumulation of MLBVV or LBVaV in any field used for cultivar evaluation throughout the world. Our project objective for 2006-2007 was to develop Real-Time (quantitative) RT-PCR. We have developed and tested these primers in our laboratory, and can now reliably measure MLBVV concentration in L. sativa x L. virosa hybrids, L. virosa accessions, tolerant L. sativa cultivars, and susceptible L. sativa cultivars using Real-Time (quantitative) RT-PCR. Comparable tests with ELISA, using antiserum purchased from DSMZ (Germany) resulted in elevated background readings, making the quantitative RT-PCR the more reliable method. In our tests we saw no benefit to using the antiserum as it does not appear to be reliably detect MLBVV. New hybridization tests developed by our lab effectively screened a large collection of lettuce cultivars, hybrids and breeding lines for MLBVV accumulation using nucleic acid hybridization. This method did not quantify levels of virus, but provides an estimation of relative MLBVV concentration, and differentiates lines that accumulate MLBVV from those that do not. Lines with low MLBVV accumulation and desirable phenotypic traits are being selected for further development. Research Abstract to the California Lettuce Research Board for the period April 1, 2006 to March 31, 2007 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 Summary: This year there were three objectives to help growers manage symphylans (garden centipedes): there has been very little data available on how currently used insecticide management strategies impact symphylans and the damage they cause in lettuce on the Central Coast. The insecticides examined showed no difference compared to the untreated control in the same field. The second objective looked at the use of pre-plant bait stations to predict subsequent crop damage. We found that pre-plant counts did not have a relationship to counts later in the season or to plant weight. The final objective was to develop a bioassay technique to assess new or existing insecticides. A relatively easy and repeatable test was developed. The first two objectives were investigated at two grower-cooperator locations infested with symphylans. The first trial included Mustang 4.2fl oz/A and Permethrin at 8 oz (0.2lbs ai/A) knifed-in prior to planting, compared to an untreated control. The second trial tested three materials: Diazinon 15G @ 15lbs/A, Mustang @ 4.3 fl oz, or Warrior @ 3.2 fl oz were compared to an untreated control. These trials showed no important differences in bait station counts between treated areas and untreated areas. At harvest, there was no detectable pattern in plant weight or yield. A third grower cooperator suspected symphylans, but had springtails instead. This trial included Ambush 8 oz (0.2lbs ai/A) sprayed over top and irrigated prior to planting, as compared to an untreated control. This trial also showed no important differences between treated areas and untreated areas. This information is included because growers may want to know that plant damage due to springtails looks much like that by symphylans. Developing a bioassay was the final objective. A relatively quick, easy and repeatable test was developed. It involved standard materials readily available and challenged symphylans in an environment where they were forced to walk on and potentially eat treated lettuce leaves. Preliminary data show that no material currently registered for lettuce provided enough mortality in the lab to be a good candidate for further testing in the field. Tillage, crop rotation, or cropping around peak symphylans activity in the Spring appear to be the current best options for fields with symphylans infestations. 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 Fusarium root rot) is caused by the soilborne fungus Fusarium oxysporum f. sp. lactucae. Crop rotation can be used to reduce soil inoculum levels of this pathogen and thereby lower the risk of economic damage. To determine how much time is required for the pathogen population to decline by attrition, we have monitored survival in fallow soil in the field and in the greenhouse. In both cases, a reduction of approximately 90% was evident within one year. However, the rate of decline appears to be much slower thereafter. Previous work has shown commonly-grown lettuce cultivars to differ in susceptibility to Fusarium wilt. To confirm the results of greenhouse seedling tests, representative cultivars were grown in an infested field and rated for the extent of disease development. The results showed that both Salinas and Grand Max were more resistant than most iceberg types but they were clearly susceptible in our field tests. On the other hand, the romaine and leaf cultivars that were tested showed little or no damage in our field trials. This experiment was conducted three times during the summer and fall of 2006. Disease was most severe in the first experiment, which was planted in June and corresponded to the highest ambient temperatures. Weather was progressively cooler following subsequent plantings in July and September. Disease developed most slowly in the September trial when temperatures were the lowest. Thus our results are consistent with previous findings and grower observations that Fusarium wilt is most damaging during the warmest planting windows. CALIFORNIA LETTUCE RESEARCH BOARD April 1, 2006-March 31, 2007 Project Title: Development and use of tools to assist breeding for resistance to and management of bacterial leaf spot and corky root of lettuce Principle investigator: Carolee Bull, USDA/ARS, Salinas, CA. cbull@pw.ars.usda.gov 831.755.2889 Cooperators: Polly Goldman, Ryan Hayes, and Beiquan Mou, USDA/ARS; Steven T. Koike, UCCE Summary: We continued accessing progeny resistance to bacterial leaf spot caused by Xanthomonas campestris pv. vitians in greenhouse and field based experiments. However, the majority of our work concentrated on corky root of lettuce caused by Sphingomonas suberifaciens and other Sphingomonas species. Allelism studies indicated that resistance in two additional PI lines appears to be conferred by the same gene as in Montello, but that other gene(s) may be involved. In PI491239 and PI273597c these differences may result in increased resistance to more virulent strains of the pathogen. We demonstrated that resistant cultivars become severely diseased in soil from a field in Watsonville, California. We began characterizing the pathogens in this soil. PI lines PI491239 and PI273597c performed better in this soil than the commercial resistant cultivars tested. Progress was made toward developing tools for studying the ecology of the pathogen and for detection and quantification of the pathogen in field samples. We procured and isolated plasmids that confer green fluorescent protein to bacteria. We are in the process of transforming cells with these plasmids. This will allow us to visualize the infection process. Additionally, we have designed primers that appear to be specific for DNA from Sphingomonas suberifaciens. Further research with these primers is needed to develop methods for rapid detection and quantification of the pathogen from field samples. Additional primers will be needed for other species that cause corky root. Additional research from this project also indicates that lettuce seedlings remain susceptible even up to 7 weeks after planting. California Lettuce Research Board Research Report April 1, 2006 to March 31, 2007 Project Title: Evaluation of Paenibacillus polymyxa as a biocontrol agent against Sclerotinia minor in lettuce Project Investigator (s): Barry Pryor, Assoc. Professor, Division of Plant Pathology and Microbiology, Department of Plant Sciences, University of Arizona Summary Lettuce drop, caused by two closely related fungi Sclerotinia minor and S. sclerotiorum, is perhaps the single most important disease affecting lettuce production in California and Arizona. As such, considerable amount of effort is directed towards disease management and yield loss prevention. Current strategies rely heavily on chemical applications. However, novel strategies are needed to supplement chemical strategies in an effort to prolong their efficiencies and reduce environmental impacts. Biocontrol is one such strategy. There are several biocontrol options available for the control of lettuce drop caused by Sclerotinia sclerotiorum. However, there are no or few effective options available for the control of lettuce drop caused by S. minor. Of the organisms to appear in recent years in published research on potential biocontrol agents, Paenibacillus polymyxa is one of the most exciting. Our lab has recently isolated two strains of P. polymyxa from parasitized sclerotia recovered from soil. Preliminary results showed that these strains may be promising candidates for future development as biocontrol agents against S. minor in lettuce. The main goal of this study was to develop a biocontrol strategy for control of Sclerotinia minor in lettuce. A specific objectives are to 1) Further define optimal growth parameters (media, temperature) for Paenibacillus polymyxa for inoculum production and in vitro inhibition studies against S. minor. Continue inhibition studies against other soil borne pathogens of lettuce. 2) Determine the efficacy of two Paenibacillus polymyxa isolates as biological agents against S. minor and S. sclerotiorum in lettuce under field conditions. Included with this objective for comparative purposes were treatments consisting of high rates of Contans, a commercially available fungal parasite with proven efficacy against S. sclerotiorum, but not S. minor. The results from the lab studies revealed that L medium and 30 C were the most favorable medium and temperature for both Paenibacillus isolates. Both isolates inhibited the growth of both lettuce drop pathogens and also other important soil borne pathogens of lettuce. The width of inhibition zone of P. polymyxa-011 against Sclerotinia spp. in in vitro studies was significantly higher than that of P. polymyxa-095. The results from the field studies revealed that the two applications of the highest rate of Contans (10 lb/acre) was the most effective treatment for the control of lettuce drop caused by S. minor. Three applications of P. polymyxa-011 was the next best biocontrol treatment for the control of lettuce drop caused by S. minor. In addition to disease suppression, P. polymyxa treatment also has definite growth enhancing effect on lettuce and increased the lettuce head weight and yield significantly. In contrast, both rates of Contans (2 lb and 10 lb/acre) were equally effective and were the best for the control of lettuce drop caused by S. sclerotiorum. CALIFORNIA ICEBERG LETTUCE RESEARCH PROGRAM April 1, 2006 - March 31, 2007 EPIDEMIOLOGY AND CONTROL OF LETTUCE DROP CAUSED BY SCLEROTINIA Krishna V. Subbarao Department of Plant Pathology, University of California, Davis SUMMARY The four objectives during the current funding cycle were: a) to compare pre- and post-Endura populations of Sclerotinia minor for potential resistance against Endura; b) to continue the evaluation of breeding lines for Sclerotinia minor resistance in the field and greenhouse; and c) compare the effects of 40-inch and 80-inch bed configurations on the inoculum dynamics and incidence of lettuce drop caused by S. minor and S. sclerotiorum. The pre- and post-Endura populations of Sclerotinia minor were compared for potential resistance to Endura. Five isolates representing the post-Endura S. minor population were compared with five isolates from the pre-Endura population in a culture medium amended with various concentrations of Endura. Neither the pre- nor post-Endura isolates grew on media containing Endura. The response of the two populations to concentrations of Endura was not significantly different. Additional post-Endura isolates are required to conclusively prove whether or not resistance to Endura is responsible for control failures observed in some fields recently. We are continuing our collaborations with Ryan Hayes’ to develop sources of resistance in lettuce to S. minor. Resistance so far identified in the germplasm is linked to early flowering but transferring this resistance to horticulturally superior lettuce backgrounds is difficult. Additionally, the germplasm evaluation and screening of the breeding material in field so far relied on inoculum from a single isolate with moderate virulence. Since our evaluation of the virulence among isolates of S. minor revealed significant variation, we altered the inoculum to include isolates representing this variation this year. All inoculum for field screening was supplied and the results from this year’s trial will be in the report by Ryan Hayes. We previously reported on the effects of 40 vs 80” bed widths and irrigation frequency on lettuce drop caused by S. minor and S. sclerotiorum. Because of the increased drop incidence in 80” beds with the biweekly irrigation frequency, we had postulated that the soil under this treatment will add greater numbers of sclerotia to soil. Soil assays over two years clearly demonstrated that the 80” beds return significantly greater number of sclerotia to the soil. By every metric measured under experimental conditions, it is clear that the establishment of airborne phase of S. sclerotiorum in the Salinas Valley is a distinct possibility. Data from surveys of commercial fields over the past five years suggest that the number of lettuce fields with the airborne S. sclerotiorum infections in the lower half of the Salinas Valley is increasing incrementally. CALIFORNIA LETTUCE RESEARCH PROGRAM April 1, 2006 - March 31, 2007 BIOLOGY AND EPIDEMIOLOGY OF VERTICILLIUM WILT OF LETTUCE Krishna V. Subbarao Department of Plant Pathology University of California, Davis SUMMARY There were five objectives during the current funding cycle and included: a) to continue monitoring of Verticillium wilt and soil inoculum density in coastal California; b) to determine the efficacy of fumigation at different soil depths and follow the recolonization of soil by Verticillium dahliae; c) to determine the horizontal spread of Verticillium dahliae microsclerotia after fumigation; d) to continue identification and development of resistance in crisphead, leaf, and other lettuce types; and e) to determine the potential of Acibenzolar-S-methyl (Blockade) to control Verticillium wilt in lettuce. Surveys revealed that it was a very active Verticillium wilt year with the identification of nine new fields where Verticillium wilt incidence varied between 30 and 80%. Another five fields were identified after-the-fact and thus no incidence data could be collected. For the first time this year, Verticillium wilt on lettuce appeared in a field east of Hwy 101 and another in King City. This obviously is a worrying development. Soil assays from the nine fields revealed between 50 and 150 microsclerotia per gram of dry soil. Studies to determine the efficacy of fumigation with methyl bromide + chloropicrin on the microsclerotia of V. dahliae at different soil depths was completed. The post-fumigation recolonization of soil by V. dahliae and the number of microsclerotia across the different depths depended on the crops grown. Lettuce crops increased the numbers of microsclerotia at different soil profiles. In contrast, cabbage crops reduced microsclerotia as the lettuce strain is non-pathogenic on cabbage. Crops such as Radicchio had minimal effect on the soil inoculum. Soil sampling to determine the horizontal spread of V. dahliae post-fumigation continued and the data suggest that the site that had the highest disease incidence prior to fumigation also had the highest microsclerotia in 2006. The most significant discovery over the past year and a half has been the presence of two distinct races in V. dahliae from lettuce. Sources of resistance to race 1 are available and resistance is governed by a single dominant gene. Resistance to race 2 is currently not available and efforts to identify sources of resistance are currently underway. We continued both greenhouse and field screening of breeding lines, commercial cultivars and lines previously identified as resistant to race 1 this year. Five new lines with resistance to race 1 were identified. Finally, greenhouse studies were conducted to evaluate two rates of Blockade on two lettuce cultivars, Salinas and Pacific. Verticillium wilt incidence or severity on Blockade sprayed plants was not significantly different from control plants. We are currently evaluating earlier and more frequent sprays for efficacy. CALIFORNIA LETTUCE RESEARCH BOARD April 1, 2006 – March 31, 2007 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 Michael Cahn University of California Cooperative Extension Monterey County Salinas, CA SUMMARY Field and lab studies were undertaken to evaluate three aspects of lettuce weed management. In the first series, field studies were conducted to evaluate the performance of Kerb herbicide used in combination with full-season drip irrigation. Kerb was applied by conventional spray application and by chemigation through surface or buried drip irrigation line. We found that weed control with Kerb applied by spray or chemigation both worked well in combination with drip irrigation. In a laboratory soil experiment, we found that Kerb is somewhat mobile in soils from Yuma, AZ. We are attempting to identify the soil factors that contribute towards this mobility in Yuma soils, to determine in which soils Kerb application should be delayed after lettuce planting. Studies are being conducted to identify new lettuce germplasm that is tolerant to herbicides that currently cannot be used to treat lettuce. CALIFORNIA LETTUCE RESEARCH BOARD April 1, 2006 – March 31, 2007 Title: Mustard Cover Crops to Optimize Crop Rotations for Lettuce Production Project Investigators: Richard Smith, and Steve Koike, 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, Lettuce Drop caused by Sclerotinia minor, has become the key soilborne disease in the Salinas Valley. Growers in the Salinas Valley and other areas plant mustard cover crops (Brassica and Sinapis spp.) as a rotational crop because they contain glucosinolates that, upon incorporation into the soil, breakdown to isothiocyanates which biofumigate the soil. Our evaluations over the past three years indicate that mustard cover crops provide limited control of some weed species (Smith 2006) but have little to no impact on soil sclerotia abundance or on S. minor infection of subsequent lettuce crops (Smith et al 2006). The lack of definitive impact on mustard cover crops on S. minor may be due to the small amount of isothiocyanates that they contain. However, based on positive results from studies of other mustard species in Australia , we changed the focus of the mustard cover crop evaluations for this funding cycle and initiated examinations of the impact of the mustard variety, BQMulch, on soilborne pests of lettuce. According to the Australian work, BQMulch (B. napus and B. rapa) has high percentage of glucosinolate in root tissue. It was hypothesized that the glucosinolates in the roots may be more efficiently incorporated into the soil and less subject to volatilization. In our studies however, BQMulch did not reduce Sclerotinia infection on two on-farm trial or in a trial conducted at the Hartnell East Campus facility. The comparison variety, Caliente 99 had lower soil sclerotial counts on one sample date. BQMulch had lower purslane but higher malva counts on one evaluation during the subsequent lettuce crop. Caliente 99 had higher shepherd’s purse emerge in soil seed bank studies. Some of the lack of impact of mustard cover crops on infection from S. minor and the inconsistency that we observed in their impact on weeds can probably be explained by the low amounts of biofumigant (glucosinolate) that is contained in the mustards. They in general contained only a small fraction of the equivalent amount of fumigant that is contained in a commercial application of the fumigant Vapam. California Lettuce Research Board Annual Report 2006 – 2007 Cultural Strategies for Germinating Lettuce with Drip Irrigation Principal Investigator: Michael Cahn University California, Cooperative Extension 1432 Abbott St. Salinas, CA 93901 Co-Investigators (Cooperating Personnel): Richard Smith University California, Cooperative Extension 1432 Abbott St. Salinas, CA 93901 Steve Fennimore Plant Science Dept. University of California 1636 East Alisal Salinas, CA 93905 SUMMARY The acreage of lettuce under drip irrigation in the Salinas Valley has rapidly expanded during the past 10 years; however, overhead sprinklers are primarily used for stand establishment. Although some growers use drip irrigation to germinate lettuce, emergence rates can be low at some locations within a field if moisture does not adequately move from the drip tape to the seed line. This project evaluated the effect of cultural strategies on horizontal movement of moisture and germination under drip irrigation. Factors evaluated included depth of the drip tape, spacing of emitters, discharge rate of tape, and compaction of bed tops through rolling. The project also compared water use, nitrate leaching, and germination between sprinkler and drip-germinated lettuce in commercial fields. In addition, the project evaluated practices for optimizing weed control for drip-germinated lettuce. Results of a replicated trial on a Chualar loam soil demonstrated that for the initial irrigation, soil moisture in the seed line was highest when the drip tape was buried shallowly, bed tops were compacted by rolling, and the discharge rate of the tape was low. Also, depth of tape significantly affected soil moisture during subsequent irrigation. The cultural practices did not have a significant effect on germination rate. A replicated trial comparing drip with sprinkler germinated lettuce resulted in higher germination under both buried and surface drip compared to sprinklers using similar amount of applied water (3 inches). In commercial fields, water-use for germination ranged from 1.5 to 8.3 inches for drip and 5.3 to 8.1 inches for overhead sprinklers. Average losses of the initial level of soil nitrate were 44% for sprinklers and 35% for drip during germination. Germination rates of the drip irrigated blocks were higher or equal to the germination rates measured in the sprinkler irrigated blocks at 4 of the 5 commercial sites. |