Management of late leaf spot of groundnut (Arachis hypogaea) with chlorothalonil-tolerant isolates of Pseudomonas aeruginosa

Fifteen groundnut-associated bacterial isolates that inhibited by > 90% the in vitro conidial germination of Phaeoisariopsis personata, causal agent of late leaf spot disease of groundnut, were applied as a prophylactic spray (10(8) cfu mL(-1)) and tested for control of the disease in the glasshouse. Two groundnut seed-associated bacterial isolates, GSE 18 and GSE 19, identified as Pseudomonas aeruginosa, reduced the lesion frequency (LF) by up to 70%. A 90-day-old peat-based formulation of P. aeruginosa GSE 18 reduced LF measured 15 days postinoculation by up to 60%. Both P. aeruginosa GSE 18 and GSE 19 were tolerant to chlorothalonil (Kavach (R)) up to 2000 mu g mL(-1) in LB broth. In glasshouse trials, GSE 18 and GSE 19 tested in combination with reduced concentrations of chlorothalonil were highly efficient in management of the disease. The disease was completely controlled by chlorothalonil (> 250 mu g mL(-1)), and in the presence of GSE 18 or GSE 19, 100 mu g mL(-1) chlorothalonil was equally effective. Application of rifamycin-resistant mutants of GSE 18 or GSE 19 together with chlorothalonil significantly increased the survival of these isolates in the groundnut phylloplane. In the field, a combination of GSE 18 and 500 mu g mL(-1) chlorothalonil reduced disease severity comparable to 2000 mu g mL(-1) chlorothalonil alone. Use of chlorothalonil-tolerant pseudomonads together with a quarter concentration of the recommended field dose of chlorothalonil doubled pod yield compared with the untreated unsprayed control.