Optimizing a Bacillus subtilis isolate for biological control of sugar beet cercospora leaf spot

Bacillus spp. have been used to control a number of leaf spot and post harvest diseases. Their capacity to form endospores facilitates long-term storage and relatively easy commercialization. This study focuses on optimizing a Bacillus subtilis isolate, BacB, for the control of sugar beet Cercospora leaf spot, caused by Cercospora beticola Sacc., by examining application timing, biocontrol agent (BCA) concentration, use of the selective nutrient substrate β-glucan, and the form of the BCA at time of application. A method for germinating endospores prior to spraying, without active aeration, is described. Examining the effects of varying β-glucan concentrations and levels of BacB at application demonstrated a complex interaction between β-glucan, BCA population, and disease control. In the 1998 field season, disease severity was significantly decreased, as compared to the control, at an application rate of 1×106 CFU/ml, or higher, with 0% β-glucan. In 1999, there was less disease pressure, and all treatments reduced disease severity. Growth chamber experiments demonstrated that applying the bacteria as vegetative cells instead of spores or applying the BCA 1–5 days before infection could significantly increase disease control. Laboratory experiments demonstrated the ability to induce germination and vegetative growth of BacB from a spore formulation, without shaking or fermentation equipment. This shows promise for optimizing Bacillus sp. for biological control. In field trials the vegetative cells did not perform better than the spore application, though the potential for β-glucan to increase disease was demonstrated.