\u3cem\u3eBeauveria bassiana\u3c/em\u3e, A Cotton Endophyte With Biocontrol Activity Against Seedling Disease

Beauveria bassiana isolate 11-98 is entomopathogenic, endophytic in tomato, and has biocontrol activity against Rhizoctonia solani on tomato. The objectives of this study were to determine 1) if B. bassiana is endophytic in cotton, following seed treatment; 2) if B. bassiana can protect cotton against seedling pathogens; 3) if different conidial rates, applied to seed, alter effectiveness of B. bassiana; and 4) mechanisms of biocontrol used by B. bassiana against plant pathogens. Cotton ‘Delta Pine 436’ seed was inoculated with isolate 11-98 conidia, sown in a gnotobiotic system, and then isolated from surfacesterilized seedlings on selective medium. Using scanning electron microscopy, hyphal penetration of epithelial cells was observed. Using ITS primers, polymerase chain reaction (PCR), and electrophoresis of PCR products, 11-98 was detected (single 421-bp band) in a dilution series of fungal and plant DNA, and from cotton seedlings endophytically colonized by 11-98. Biocontrol of B. bassiana against R. solani, Pythium myriotylum, and Thielaviopsis basicola was examined using several seed treatment rates (log 5 to log 11 CFU). Disease was suppressed and plant growth was increased in potting mix with B. bassiana at log 7 and log 9 CFU/seed, and in sandy loam soil at log 7 CFU/seed. With low disease pressure from P. myriotylum, seed treated with 11-98 or BotaniGard isolate GHA increased plant growth. Assays with T. basicola were inconclusive. Beauveria bassiana was evaluated for induced systemic resistance in cotton against Xanthomonas axonopodis pv. malvacearum. Root drench treatments were B. bassiana at log 5, log 7, and log 9 CFU/ml, untreated, 2,6-dichloro-isonicotinic acid (INA), and untreated without Xanthomonas. After 13 days, primary leaves were wounded and challenge-inoculated with Xanthomonas. Treatment with B. bassiana (log 7 CFU/seed) had less disease than untreated controls and was as effective as INA. In antibiosis assays, Beauveria bassiana out-competed T. basicola on cotton agar, however no clear zone of inhibition was observed; B. bassiana was outcompeted by R. solani and P. myriotylum, however it maintained its original colony diameter. Beauveria bassiana hyphae coiled around P. myriotylum hyphae in parasitism assays; no coiling was observed with R. solani; results for T. basicola were inconclusive