Effect of passage on the development of thiophanate methyl resistance inAlternaria alternata (Fries) Keissler causing brown spot of tobacco

This study was conducted to investigate the in vivo development of thiophanate-methyl resistance in Alternaria alternata(Fries) Keissler, the causal agent of brown spot disease in tobacco. The experiment evaluated the effects of repeatedfungicide exposure through three different passage strategies: continuous, alternate and mixture. A sensitive isolate of A.alternata obtained from Jainyal village, Kolhapur, Maharashtra, was subjected to eight successive passages with thiophanate-methyl (200 ppm). Continuous exposure led to a significant increase in both fungicide resistance and disease infection. Incontrast, alternate and mixture passage effectively suppressed the development of resistance and reduced infection rates.Alternating thiophanate-methyl with chlorothalonil and copper oxychloride (200 ppm each) resulted in inhibition ofpathogen growth by the 2nd passage, reducing the disease infection and showed only 10% and 20% infection, respectively.Similarly, alternating thiophanate-methyl with carbendazim and mancozeb (200 ppm each) achieved a notable reductionin infection by the 1st passage. Mixtures of thiophanate-methyl with carbendazim and copper oxychloride separately, alsosuppressed pathogen growth in the 2nd passage, with corresponding infection levels of 10% and 20%. Notably, combiningthiophanate-methyl with chlorothalonil and mancozeb (200 ppm each) led to complete inhibition of both pathogen growthand infection at the 1st passage. These findings highlight the effectiveness of fungicides used alternately and in mixedform, in decreasing resistance development and managing A. alternata infections in tobacco.