Late blight (Phytophthora infestans) is a major problem for tomato production in the Southeast. It is by far the biggest problem with the production of organic tomatoes in Western North Carolina. This area is known for its tomatoes because the warm days and the cool nights allow fruit set to proceed through the summer when other areas are too hot to reliably set fruit. With the cool nights come foggy mornings and ideal conditions for fungus growth. Late blight can defoliate a tomato plant within a week. Conventional growers minimize this problem with a weekly or more frequent spray program. An organic spray (copper sulfate) can serve as a protectant and is fairly effective on late blight but it is water soluble and must be applied after each rain. Recoating all leaf surfaces after each rain is impractical, not to mention the problem of copper buildup in the soil. Without some late blight treatment, organic tomato production is possible only about one year in five in an area known for its conventional tomatoes.
This project was conducted to test the hypothesis that foliar applications of compost tea can suppress late blight on several varieties of tomatoes. While laboratory results showed strong suppression by a variety of organisms that were aerobically extracted from compost, in the field experiments late blight was present in the first year when compost tea was used. Conditions were so dry during the second and third years of the project that late blight was not a problem.
Our recommendation is for organic growers to consider compost tea for disease suppression for diseases that are less aggressive than late blight. We encourage growers to explore disease resistant varieties of tomatoes to help manage this disease on organic farms.