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Microbial and Synthetic Products for Management of Botrytis Grey Mold in Tomato

Summary

A disease management field trial was conducted in a commercial tomato production greenhouse for comparison of several products with respect both to Botrytis incidence and severity and to tomato yield. The inoculum was naturally present in the greenhouse. Treatments were applied as a spray or drench twice before fruit set. Examination of leaves for signs of disease began at 14 weeks after seeding, and disease assessments continued weekly for 5 weeks. Tomatoes were harvested 3 times a week for 11 weeks. Analysis of the data showed that both chlorothalonil spray and Mycostop drench significantly reduced Botrytis infection compared to the control. Both Bac-Pack and Mycostop drenches increased the mean 11 week yield. Chlorothalonil, Actigard, and TopShield increased the number of plants that survived the season, while the Mycostop drench treatment significantly increased the yield per plant. These results suggest that commercial biological control products offer potential for disease control and increased yield in tomato production.

Introduction

The tomato production greenhouse is an ideal environment for fungal diseases. The dense plant population creates high relative humidity, with shaded regions in the canopy that are moist and dark. Training, pruning, and tomato picking cause both large and microscopic wounds (e. g. broken trichomes). The damaged leaves and stems are excellent infection sites for many kinds of pathogens. Although many fungal diseases can be managed by venting the greenhouse to reduce relative humidity, it can be very expensive. Another option relied upon heavily by many growers is fungicide application. Although this temporarily solved some disease problems for growers, pest resistance developed. A further drawback to reliance on fungicides is that very few products are labeled for use on greenhouse tomato.

Grey mold is one of the most ubiquitous and potentially serious fungal diseases of greenhouse tomato (Lycopersicon esculentum Mill.). The causal agent, Botrytis cinerea Pers.:Fr., has a very broad host range and survives saprophytically for extended periods in plant debris or as sclerotia. Resistance to grey mold has not been found in cultivated tomato. The protectant fungicides currently registered in New York for grey mold on greenhouse tomato inhibit germination of spores but cannot cure an infection in progress. Tomato flowers and leaves are highly susceptible to B. cinerea and are the first tissues to exhibit water-soaked lesions. In colonized plant tissue, the fungus can rest for weeks before sporulating, or it may immediately cause a necrotic lesion and generate thousands of spores. The two most important causes of economic damage from grey mold are from infections that girdle the stem and kill the plant before the fruit can be harvested, and from ghost spot of the fruit that renders tomatoes unsaleable.

Because B. cinerea has developed resistance to benomyl, vinclozolin, and other fungicides (Moorman and Lease, 1992), there is concern that selection for additional fungicide resistance in the pathogen will occur. Some products will not be registered for greenhouse use by the manufacturer because the development of pest resistance in a confined space is much more likely to occur, and it is to the manufacturer's benefit to preserve the effectiveness of the product as long as possible. In addition, many fungicides labeled for grey mold on greenhouse tomato are under EPA scrutiny as potential carcinogens. Future availability of fungicides for greenhouse use on vegetables may be limited because of concerns for the environment and human safety.

There has been a strong research and development throughst in the area of biological control of plant pathogens (Elad et al., 1996). Many microorganisms have been discovered that discourage the growth and survival of plant pathogens, and these antagonists are now arriving in the marketplace. The organisms have been tested for effectiveness, are now mass-produced, and are processed to ensure shelf life. Objective testing in commercial greenhouses is needed to provide answers to questions frequently asked by horticulturalists and to supply the data needed to increase producer confidence in fungicide alternatives.

In collaboration with a commercial greenhouse tomato grower, we developed a project to test the efficacy of microbials in the management of grey mold. Products were chosen that are commercially available or close to obtaining registration for use on greenhouse tomatoes. The product Mycostop is a formulation of the bacterium Streptomyces griseoviridis strain K61 originally isolated from peat in Finland (Kortema et al., 1997; Tahvonen, 1982), (AgBio Development Co., Westminster, CO). Trichoderma harzianum Rifai strain 1295-22 (BioWorks, Inc., Geneva, NY) has been shown to be effective on many vegetable and ornamental crops for root colonization and reduction of Pythium, Rhizoctonia, and Fusarium diseases. TopShield is a spray formulation of T. harzianum that is mixed in the tank with a patented surfactant (Lo et al. 1997). Bac-Pack is a suspension of beneficial bacteria including Pseudomonas and Bacillus species, prepared as a soil drench or root dip to increase plant productivity (Soil Technologies Corp., Fairfield, IA). Our objective was to compare several formulated biological control agents, horticultural oil, experimental products, and a fungicide for effects on gray mold disease incidence and severity, and yield.