Effect of 2,6-dichloroisonicotinic acid or benzothiadiazole on Alternaria leaf spot, bacterial blight and Verticillium wilt in cotton under field conditions

In field experiments, cotton (Gossypium hirsutum/G. barbadense) plants were treated with foliar applications of the synthetic activators 2, 6-dichloroisonicotinic acid (INA) or benzo [1, 2, 3] thiadiazole-7-carbothioic acid S-methyl ester (BTH) and the severity of natural infection with Alternaria macrospora, Xanthomonas campestris pv. malvacearum and Verticillium dahliae was assessed. Percentage leaf area infected with A. macrospora was significantly (P=0.05) lower in plants following one application of INA or BTH in the 95/96 and 97/98 seasons, respectively, compared with untreated plants. There was also significantly (P=0.05) less defoliation in the treated plants. Plants treated with one application of BTH had significantly (P=0.05) lower leaf areas infected with X. campestris pv. malvacearum and decreased defoliation compared with untreated plants in the 97/98 season. The severity of foliar symptoms of Verticillium wilt was significantly (P=0.05) decreased in plants treated with one application of INA or multiple applications of BTH compared with untreated plants in four experiments conducted over four field seasons. The reduced susceptibility of the cotton plants to Alternaria leaf spot, bacterial blight and Verticillium wilt is attributed to systemic acquired resistance following application of INA or BTH.