DEVELOPMENT OF INVIVO ASSAYS FOR PRESCREENING ANTAGONISTS OF RHIZOCTONIA-SOLANI ON COTTON

Prescreens of bacteria for biological control of soilborne diseases traditionally consist of tests for antibiosis in vitro. Two in vivo assays were developed as possible alternative prescreens for selecting candidate biological control agents for preemergence and postemergence damping-off diseases of cotton caused by Rhizoctonia solani. In the "radicle assay," radicles from surface-disinfested seed were inoculated with R. solani. In the "hypocotyl assay," excised surface-disinfested hypocotyls were inoculated with the pathogen. Inoculated radicles and hypocotyls were rated daily for symptom development for the next 6-7 days using a scale of 0-7 (0 = no symptoms; 7 = complete necrosis). The effects of growth medium for the fungal inoculum, form of inoculum, and age of host tissue on the development of necrosis were determined for both assays. The assays were "optimized" by selecting those experimental parameters that allowed a progressive development of symptoms. When tested in the optimized radicle and hypocotyl assays, the previously reported biological control rhizobacteria strains Pf-5 (Pseudomonas fluorescens) and GBO-3 (Bacillus subtilis) significantly reduced symptom development. Pf-5 populations increased during the evaluation time from 0.5 to 4 log units on radicles and from 3 to 7 log units on hypocotyls, depending on the inoculum density. Populations of GBO-3 on radicles increased 3 log units with low inoculum density but decreased 1.4 log units with high inoculum density, whereas, on hypocotyls, populations increased 1.4 log units with low inoculum density and decreased 3.5 log units with high inoculum density.