Source: UNIVERSITY OF ARIZONA submitted to
BIOLOGICAL CONTROL OF LETTUCE DROP: AN ESSENTIAL ELEMENT IN DEVELOPING AN IPM PROGRAM FOR DESERT-GROWN WINTER LETTUCE
 
PROJECT DIRECTOR: Pryor, B. M. Matheron, M. E.
 
PERFORMING ORGANIZATION
PLANT SCIENCE
UNIVERSITY OF ARIZONA
TUCSON,AZ 85721
 
NON TECHNICAL SUMMARY: The fungal disease lettuce drop, caused by Sclerotinia spp., is a key factor limiting production in desert winter lettuce. Current fungicides provide only modest control and significant losses occur annually. The purpose of this study is to evaluate biocontrol strategies for disease management and the effects of sclerotium density and irrigation methods on biocontrol efficacy.
 
OBJECTIVES: OBJECTIVES Most research on lettuce drop has been conducted in production areas of coastal California, Canada, and Europe. The unique aspects of desert agriculture suggest that many findings obtained in other research may not translate to desert ecosystems. As such, these findings must either be verified under desert agricultural conditions or new epidemiological data be established. The importance of winter lettuce production to the lettuce industry of Arizona and California, and to the US consumer necessitates that this work be performed. The overall goal of this research is to develop an integrated strategy for controlling lettuce drop on crisphead, romaine, and leaf lettuce in desert agroecosystems through reduction in soil sclerotium density. A key aspect of these studies is the development of biological control strategies that specifically target sclerotium survival. To augment this effort, irrigation methods that increase sclerotium mortality and/or enhance survival and efficacy of specific biocontrol agents will be evaluated. Specific objective are: 1. Determine sclerotium density in typical winter lettuce production fields (generally 20-40 acres) with a history of lettuce drop, and establish sampling protocols to accurately estimate sclerotium density for future sampling efforts. 2. Establish relationships between sclerotium density, incidence of lettuce drop, and yield for crisphead, romaine, and leaf lettuce under desert soil and climatic conditions and under different irrigation methods. 3. Compare efficacy of C. minitans with other chemical and/or biological agents and combinations of these products in controlling lettuce drop and improving yield for crisphead, romaine, leaf lettuce under desert soil and climatic conditions at different sclerotium densities and under different irrigation methods.
 
APPROACH: APPROACH AND PROCEDURES OBJECTIVE 1. In the first year, 4 fields (20 acres each) will be selected within Yuma and Imperial counties (2 fields each). Within each field, 8 plots (10 X 20 m) will be selected from portions of the field known to be infested with Sclerotinia, and delimited for high density sampling. Each sampling location will be geo-referenced using DGPS. Using GIS software ArcView, sampling location will be over-laid on the geo-referenced boundaries of the field. Within each subplot, 22 individual soil cores will be taken at 2.0 m intervals in an X pattern and the sample position within the pattern recorded. Soil samples will be wet-sieved for recovery of sclerotia. The number of recovered sclerotia from each sample will be referenced against sample position to establish relationships necessary for characterizing spatial dimensions of aggregation. The variance in sclerotia numbers recovered among samples per plot will be calculated and used to determine the minimum number of random samples (n) necessary to estimate sclerotium density per plot within 90% of the true mean. OBJECTIVE 2. Experimental plots will be established at the UA Yuma Agricultural Center and the UC Desert Agricultural Research Center. In years 1 and 2, experiments will be established on a 1.5 acre sites in a Split-Split Plot Design with 3 blocks. Irrigation (fallow and sprinkler) will be the main plot, lettuce type (crisphead, romaine, and leaf) as the subplot, and sclerotium density (0, 2, 10, 40, and 100 sclerotia/m bed) as the sub-subplot. Each treatment plot will consist of 4 beds (42 in.) of 10 m length with 1 m buffer between different plots within a row. Prior to planting, sclerotia of S. sclerotiorum will be broadcast on the tops of each bed. The experiment will be planted using standard planting equipment and seed densities. At harvest, disease incidence, average head weight, and yield will be recorded from the center two rows of each plot. Data will be analyzed using PROC GLM (SAS Institute) to perform ANOVA. OBJECTIVE 3. In years 1 and 2, experiments will be established on 2.5 acre sites at the Yuma and Desert Research Centers in a SSPD with 3 blocks. Irrigation (fallow and sprinkler) will be the main plot, sclerotium density (0, 10, and 100 sclerotia/m2 bed) as the subplot, and product application (biological, chemical, and/or combinations) will be the sub-subplots. Plots will be planted with the crisphead cultivar Winterhaven. To assess sclerotium mortality and C. minitans survival, soil samples will be collected midseason and at harvest from each plot. Samples will be assayed for sclerotium density, and 10% of recovered sclerotia will be plated on agar medium to assess sclerotia viability. In addition, samples will be assayed for C. minitans (cfu/g soil). At harvest, disease incidence, average head weight, and yield will be recorded from the center two rows of each plot. These data, in addition to results of sclerotia recovery, viability, and C. minitans population, will be analyzed using PROC GLM (SAS Institute) to perform ANOVA.
 
CRIS NUMBER: 0200545 SUBFILE: CRIS
PROJECT NUMBER: ARZT-340480-G-25-546 SPONSOR AGENCY: CSREES
PROJECT TYPE: SPECIAL GRANT PROJECT STATUS: TERMINATED MULTI-STATE PROJECT NUMBER: (N/A)
START DATE: Aug 1, 2004 TERMINATION DATE: Jul 31, 2006

GRANT PROGRAM: INTEGRATED PEST MANAGEMENT/BIO-CONTROL
GRANT PROGRAM AREA: Special Research Grant

CLASSIFICATION
Knowledge Area (KA)Subject (S)Science (F)Objective (G)Percent
212143011604.220%
212402011024.220%
215143011604.220%
215402011024.220%
216143011604.220%

CLASSIFICATION HEADINGS
KA212 - Pathogens and Nematodes Affecting Plants
KA215 - Biological Control of Pests Affecting Plants
KA216 - Integrated Pest Management Systems
S4020 - Fungi
S1430 - Greens and leafy vegetables
F1102 - Mycology
F1160 - Pathology
G4.2 - Reduce Number and Severity of Pest and Disease Outbreaks


RESEARCH EFFORT CATEGORIES
BASIC 30%
APPLIED 70%
DEVELOPMENTAL (N/A)%

KEYWORDS: sclerotinia sclerotiorum; sclerotinia minor; irrigation; fungus diseases (plants); plant disease control; biological control (diseases); lettuce; integrated pest management; deserts; arizona; winter; plant ecosystems; epidemiology; ecosystems; crop production; soil fungi; soil microbiology; crop yields; disease incidence; microbial competition; deuteromycetes

PROGRESS: Aug 1, 2004 TO Jul 31, 2006
Lettuce drop is one of the most common and destructive diseases of lettuce in the deserts of Arizona and California, as well as most other lettuce-growing regions of the world. The disease is caused by two related fungi, Sclerotinia sclerotiorum and S. minor, however, S. sclerotiorum is the most prevalent in desert areas of AZ and CA. Both fungi produce hard, durable survival structures known as sclerotia, which enable these pathogens to persist in soil for many years during unfavorable conditions, and then function as disease inoculum in subsequent lettuce crops. Present disease management strategies rely primarily on fungicide applications, but these provide only a moderate level of control. Previous studies have revealed that the commercially available biofungicide ContansTM (Coniothyrium minitans) is effective against Sclerotinia species under certain conditions. This mycoparasitic fungus specifically parasitizes soilborne sclerotia of pathogenic fungi and reduces their ability to cause disease in numerous crops. The objective of this project was to document natural soil sclerotial density in commercial lettuce production fields in AZ, determine the effect of artificially established sclerotial density on the incidence of disease in 3 lettuce types, and to conduct a series of trials to assess the efficacy of Contans in controlling lettuce drop at different application rates and frequencies, and under different disease pressures and irrigation methods. Trials were conducted in AZ and CA in both 2004 and 2005. A survey of 4 fields (96 samples/field) revealed sclerotia densities per sample of 0-36 sclerotia/100 g soil. Average density per field ranged from 0.9 to 3.6 sclerotia/100 g soil. In artificially inoculated romaine lettuce plots, 2, 10, 40, and 100 sclerotia/m2 bed resulted in 0, 9, 13, and 25% incidence of disease, respectively. In crisphead lettuce plots, 2, 10, 40, and 100 sclerotia/m2 bed resulted in 10, 15, 33, and 57% incidence of disease, respectively. In leaf lettuce plots, 2, 10, 40, and 100 sclerotia/m2 bed resulted in 13, 30, 65, and 72% incidence of disease, respectively. In product efficacy trials, the best treatments in low inoculum plots (20 sclerotia/m2 bed) were1 or 2 applications of Contans (2 or 4 lb/acre) or Contans in combination with Rovral (1 lb a.i./acre), which resulted in disease incidence of <5%. In high inoculum plots (200 sclerotia/m2 bed), the best treatments were 2 applications of Contans (either rate) or those in combination with Rovral, which resulted in disease incidence of <10%. Treatments using only Rovral resulted in disease incidence of 50-75%. There was no significant difference in the efficacy of Contans when used under sprinkler or furrow irrigation. Trials conducted in AZ and CA and in 2004 and 2005 revealed similar findings. Interestingly, Contans provided little or no control against lettuce drop caused by S. minor. Additional studies are currently underway to confirm findings from the described trials, to elucidate mechanisms responsible for differential parasitism by C. minitans against Sclerotinia spp., and to evaluate other biocontrol agents for efficacy against S. minor.

IMPACT: 2004-08-01 TO 2006-07-31 The objectives of this study are to provide lettuce producers with a safe and effective alternative to fungicides for controlling lettuce drop in desert production areas. Factors relating to the efficacy of biocontrol as a management strategy will be evaluated and optimized to insure uniform and consistent results from its use. Results of these studies will be disseminated to stakeholders through print and through web-based information systems.

PUBLICATION INFORMATION: 2004-08-01 TO 2006-07-31
Chitrampalam, P., Figuli, P., and Pryor, B. M. 2006. Spatial patterns of sclerotia of Sclerotinia sclerotiorum in commercial lettuce fields of southern Arizona. Phytopathology 96:S24

PROJECT CONTACT INFORMATION
NAME: Pryor, B. M.
PHONE: 520-626-5312
FAX: 520-621-9290