Developing integrated approaches for managing vegetable and oomycete diseases

Summary

Production of vegetable crops in Georgia is seriously affected by a number of troublesome diseases including Phytophthora blight, Fusarium wilt, and southern blight.In this project, effective fungicides, cover crops, and biocontrol agents were identified to significantly reduce the impact of the diseases. In addition, findings in the project provide useful information for developing more effective disease management programs through identification of resistant genetic sources, determination of pathogen diversity, and analysis of volatile signatures associated with disease development.

Situation

Georgia is a major producer of vegetable crops in the nation with a farm gate value over $1 billion annually. The sustainable and profitable production of vegetables is impacted by a number of destructive diseases. Fusarium wilt incited by Fusarium oxysporum f. sp. niveum (FON) causes increasing damage on both seedless and seeded watermelon in the southeastern United States. Strategies for managing Fusarium wilt of watermelon are limited. Only one fungicide has been registered for the disease and risk for the pathogen to develop resistance to fungicides is high. Monitoring fungicide resistance development in the pathogen populations and developing effective alternatives for management of the disease are desperately needed. Phytophthora blight caused by Phytophthora capsici is a major limiting factor in the production of peppers and cucurbits. Lack of resistant crop cultivars, development of resistance in pathogen populations to fungicides, and unpredictable disease outbreaks are big challenges in management of Phytophthora blight. Other diseases that cause significant loss include southern blight (Sclerotium rolfsii) and black shank (Phytophthora nicotianae), and development of integrated approaches for effective management of the diseases is highly desirable.

Response

Funding received from USDA-NIFA, Georgia and national commodity commissions, and private chemical industries was used to conduct a series of lab, greenhouse and field studies to combat the diseases. Field experiments were conducted to evaluate registered and experimental fungicides, systemic resistance inducers, biocontrol agents, and different application methods for managing Fusarium wilt of watermelon. FON isolates were collected from commercial watermelon fields at different locations in Georgia to monitor potential resistance development to prothioconazole. Cover crops were planted in winter 2015 and incorporated as soil amendments in spring 2016 to evaluate the efficacy in disease reduction. Soil microbial community structures in the field plots with different cover crops are being analyzed. Cucumber accessions with potential resistance to P. capsici were screened in repeated experiments to identify genetic sources of resistance and mechanisms associated with disease resistance. To predict disease development volatile organic compounds produced by squash fruit infected by P. capsici at different infection stages were analyzed by GC-MS. Studies were conducted to isolate and identify Phytophthora species from irrigation ponds and determine their pathogenicity on vegetable crops. Sensitivity of Phytophthora nicotianae isolates from tobacco in Georgia to selected fungicides was determined. Experiments were conducted to isolate and screen biocontrol bacteria for Fusarium wilt of watermelon and southern blight of tomato, and develop integrated programs incorporating host resistance, biologicals, chemicals and cover crops for managing Fusarium wilt, Phytophthora blight and southern blight.

Impact

Prothioconazole, azoxystrobin and some experimental products were effective in reduction of Fusarium wilt of watermelon in field studies, with prothioconazole and an experimental product being more effective than azoxystrobin. Integrated use of prothioconazole, acibenzolar-S-methyl, and an experimental product reduced disease significantly. FON isolates resistant to prothioconazole were not identified. Disease incidence was monitored in field plots with different cover crops incorporated as soil amendments. Fusarium wilt incidence was the lowest in the treatments where hairy vetch or rye/hairy vetch mixture was used for soil amendment. Populations of fungi in the soil such as Trichoderma spp. were not significantly altered but bacterial populations including Pseudomonas spp. were increased significantly following incorporation of some cover crops. Genetic diversity of FON isolates and relationship to races, fungicide resistance, and geographic origins are being analyzed. Repeated experiments to screen accessions of cucumber for resistance to P. capsici identified two accessions with good resistance to the pathogen. Interactions between P. capsici and the accessions, including protein profiles, disease resistance related enzymes, colonization and multiplication of the pathogen in plant tissues, are being studied. Analysis of volatile signatures from squash fruit infected by P. capcisi identified that 4-ethyl phenol and 4-ethyl-2-metoxy phenol were produced by squash fruit infected by P. capsici but not by non-inoculated control fruit. Studies to isolate oomycetes from irrigation ponds identified a few new species of Phytophthora that are pathogenic on vegetables. Determination of sensitivity of 106 P. nicotianae isolates from tobacco in Georgia indicated that they were sensitive to some newer fungicides with lower EC50 for oxathiapiprolin than for fluopicolide and mandipropamid. A series of studies were conducted to screen biocontrol bacteria for southern blight of tomato and Fusarium wilt of watermelon, and promising biocontrol agents of Bacillus and Pseudomonas species were identified. Studies are in progress to develop integrated programs incorporating effective planting dates, resistant cultivars, selected fungicides, cover crops and biocontrol agents to reduce losses caused by these major soilborne diseases.

State Issue

Agricultural Profitability and Sustainability

Details

• Year: 2016
• Geographic Scope: National
• County: Tift
• Program Areas:
  • Agriculture & Natural Resources