Biofungicides as Alternatives to Copper for Management of Powdery Mildew in Organic Tomato High Tunnel Systems

Biofungicide products are a rapidly expanding sector of the plant protection market. Powdery mildew of tomato (Solanum lycopersicum), caused by Erysiphe neolycopersici, can result in significant yield loss in high tunnel (HT) tomato production. Copper-based fungicides are heavily used in HTs, especially those in certified organic production, to control powdery mildew and other fungal diseases. Reliance on copper can lead to its overuse, subsequent resistance development in pathogens, and accumulation of high amounts of copper in the soil. In this study, we evaluated two bio-product alternatives to copper for efficacy against powdery mildew in an organic HT tomato production system over three growing seasons. These alternatives were a commercial biofungicide containing Bacillus subtilis GB03 and a filtered and unfiltered microbial fermentation product (F-MFP and UF-MFP, respectively). UF-MFP was a proprietary blend of yeast cell walls and inactive fermentation media, whereas F-MFP was processed to remove any particles larger than 0.2 μm. The HT-grown tomato plants were inoculated with E. neolycopersici (104 conidia/mL) and three to five foliar applications of biofungicide were made per season. Powdery mildew severity was lower with MFPs compared to the water-treated inoculated treatment and B. subtilis, although this was not always statistically significant (p = 0.05). At assessment dates where statistically significant difference among treatments were observed, the MFP treatments were generally equivalent to the cuprous oxide standard. These results suggest that MFP may be a suitable alternative or alternation partner to copper-based products currently in use in HT tomato systems affected by powdery mildew.