Inhibition ofPhytophthora vignaeand Stem and Root Rot of Cowpea by Soil Bacteria

Bacteria isolated from Sri Lankan cowpea fields significantly reduced in vitro mycelial growth and sporangial production of Phytophthora vignae by producing agar-diffusable and volatile inhibitors. Bacterial volatile inhibitors were evolved when the bacteria were grown on tryptic soy agar, nutrient agar, or King's B medium, although relatively less on the latter two media. The pH of agar exposed to the volatiles rapidly increased, suggesting that ammonia was involved in the inhibition. Volatile inhibitors were produced by the bacteria in soil amended with tryptic soy broth or cowpea seed extract, but not in unamended soil. Oospore—induced disease by P. vignae was inhibited by Brevibacterium strain DF-3101 in soil. Soil or seed treatment with three Sri Lankan bacteria, Brevibacterium linens (DF-3101); Bacillus thuringiensis (DF-7107), and Bacillus pumilus (DF-1481) in greenhouse studies suppressed the disease on cowpea caused by P. vignae. None of the bacteria tested functioned as plant growth-promoting rhizobacteria (PGPR) on cowpea. Colonization of roots by strain DF- 3101-R, a rifampicin-resistant mutant of Brevibacterium linens DF-3101 was initially lower on resistant ‘Mississippi Purple’ (6 log cfu/g root) than on susceptible ‘California Blackeye’ (8 log cfu/g root) but, after two weeks, the population level on both varieties was equal at 6 log cfu/g root.