Rainfall and crop residue effects on soil dispersion and Beauveria bassiana spread to corn

The entomopathogenic fungus Beauveria bassiana infects a wide range of insects and survives as a soil saprophyte and a plant endophyte. The objective of our study was to determine the role of rainfall in dispersing B. bassiana (Balsamo) Vuillemin to the surface of corn (Zea mays) from soil with different levels of crop residue. Laboratory studies which simulated field tillage systems, had levels of crop residue which covered 0.53 (control), 34, 59 and 84% of the soil surface. Simulated rainfall in a raindrop tower at an intensity of 73 mm/h caused a significant increase in the mean number of B. bassiana colony forming units (CFU) isolated from the surface of corn plants. Plants receiving rain had a mean (+/-S.E.M.) of 8.8 +/- 2.8 CFU per plant; controls had a mean of 0.03 +/- 0.01 CFU per plant. The mg of soil collected from the surface of plants also was significantly influenced by rainfall. Plants receiving rain had a mean of 15.7 +/- 1.7 mg of soil per plant while controls had a mean of 3.4 +/- 0.4 mg of soil per plant. Linear regression revealed highly significant negative relationships between the mean mg of soil and the mean number of CFU per plant, over the four levels of crop residue. The amount of soil and number of CFU per plant decreased significantly with increasing levels of crop residue. In field studies with conservation- and no-till systems, results were similar to those recorded in the raindrop tower. The mean number of CFU and mg of soil per plant were both higher in conservation-till plots than in no-till plots, where surface residue averaged 45 and 95%, respectively. Rainfall plays an active role in the dispersal of B. bassiana from the soil environment to the surface of whorl-stage corn. Increased levels of crop residue reduce the amount of soil and fungal transfer to the surface of young corn. (C) 2002 Elsevier Science B.V. All rights reserved.