Organic Amendment and Transitional Cropping System Effects on Crop Diseases

Organic inputs have been suggested to have either suppressive or conducive effects on levels of plant diseases. In this study, we compared effects of organic matter amendments and transitional cropping systems on plant diseases during the transition from conventional to organic agricultural production and 2 yr following. During the 3‐yr transition, organic amendments included raw dairy manure, composted dairy manure, and non‐amended treatment superimposed on three cropping systems (perennial pasture, cash grain crops, and vegetable crops). Two years of post‐transition crops included tomato (Lycopersicon esculentumMill.) and bell pepper (Capsicum annuumL. var. annuum) in the first year and soybean [Glycine max(L.) Merr.] in the second. During transition, raw manure amendments resulted in higher levels of diseases caused by some biotrophic pathogens, including rusts in corn (Zea maysL.) and orchardgrass (Dactylis glomerataL.). However, raw manure applications were also found to suppress some diseases caused by necrotrophic pathogens, including anthracnose and Septoria leaf spot in tomato. Manure applications promoted higher soybean yields. Powdery mildew of cucurbits (Curbubita moschataDuchesne) was suppressed by composted manure. After transition, tomato plants grown in plots previously under the vegetable system had the highest levels of anthracnose, and soybean grown in plots previously in the pasture system had the highest levels of bacterial pustule, despite the fact that these same plots produced the highest yields in soybean. Yield benefits resulting from applications of raw manure to a pasture transition system may be attractive for farmers, but disease management strategies will need to be considered to prevent increased disease levels.