Compatibility of the biocontrol fungus Fusarium oxysporum strain CS-20 with selected fungicides

Fusarium oxysporum strain CS-20 reduces incidence of Fusarium wilt in tomato and other crops. To be integrated into most production systems, strain CS-20 must be compatible with other management practices. We assessed the compatibility of strain CS-20 with seven fungicides recommended for tomato in Maryland. Radial growth of strain CS-20 was recorded on agar medium amended with various concentrations of the fungicides. Fungicides tested did not kill strain CS-20 at the concentrations tested in the in vitro experiment. Azoxystrobin (Quadris) and chlorothalonil (Bravo) were most toxic to strain CS-20 and significantly reduced growth rate and final colony size at 10 ppm a.i. or greater concentrations compared to growth on unamended medium. Thiram (thiram) significantly reduced final colony size at 30 ppm or greater. Mefenoxam + chlorothalonil (Ridomil Gold Bravo) significantly reduced final colony size at 50 ppm or greater. Mancozeb (Manzate) and mancozeb + copper (Mankocide) reduced final colony size only at 100 ppm, while mefenoxam (Ridomil Gold) and mefenoxam + copper (Ridomil Gold Copper) did not affect growth of strain CS-20. In greenhouse tests, tomatoes were drenched with strain CS-20 at seeding and just before transplanting into field soil infested with the pathogen. Plants were treated with fungicides at the highest label rate. Mancozeb is labeled as both a seed treatment and a spray, and was applied each of these ways as separate treatments. Disease incidence of plants from seeds treated with thiram and strain CS-20 was not different from those in the pathogen only control, indicating that thiram was toxic to strain CS-20. Other fungicides toxic in vitro, were less toxic in greenhouse tests, probably because they are applied as sprays to the above-ground portions of the plant.