Control of charcoal rot disease of okra plants using certain chemical plant resistance inducers

Abstract

This study was aimed to determine the efficacy of certain chemical inducers on incidence of charcoal rot disease caused by Macrophomina phaseolina (Tassi) in okra okra (Abelmoschus esculentus (L.) under greenhouse and field conditions. Pathogenicity of 12 isolates of Macrophomina phaseolina was carried out on okra under greenhouse conditions. These isolates capable to infect okra plants caused charcoal rot on the basil stem with various degrees of diseases severity. Isolate No. 3 caused the highest charcoal rot severity (70.15%) followed by isolate No. 6 (58.69%). The positive effect of four inducer chemicals, i.e salicylic and ascorbic acids at concentrations 50, 100, 200 ppm; Benzothiadiazole, Bion at 200, 400, 800 ppm and humic acid at 500, 1000, 2000 ppm  on the induction of systemic resistance in okra against charcoal rot disease and its effect on growth parameters and green fruit yield components were studied. In vitro, all tested chemical inducers able to suppress growth of M. phaseolina.  The highest decrease in linear growth of M. phaseolina was noticed with Bion at concentration 800 ppm (66.96%). Under greenhouse conditions, all the tested chemical inducers significantly decreased charcoal rot severity at all concentrations compared with control.  The reduction of charcoal rot in okra was enhanced by increasing chemical inducers concentration. Ascorbic acid was the most effective chemical inducers as they greatly retarded charcoal rot caused by M. phaseolina especially at the higher concentration (200 ppm). While, salicylic acid at 50 ppm recorded the lowest protection of charcoal rot severity. Under field conditions, the percentages of charcoal rot severity were significantly reduced due to soaking the seeds before sowing in any of chemical inducers in both trial seasons (2013 and 2014). The most effective inducer was humic acid  at 2000 ppm (84.44 and 85.65% reduction of charcoal rot severity), followed by ascorbic acid at 200 ppm (79.79 and 79.28% reduction of charcoal rot severity) in the first and the second growing seasons, respectively. Also, these treatments significantly increased growth parameters i.e. plant height and number of branches plant^-1 as well as, green fruit yield parameters i.e. number of green fruit plant^-1; weight of green fruit plant^-1 (gm); and total green fruits (ton/acre). Generally, humic acid gave the best results in all growth and green fruit yield parameters under field conditions especially at higher concentration (2000 ppm).