Differential fungicidal sensitivity of Pythium species associated with yellowing and wilting of black pepper (Piper nigrum L.)

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Yellowing and wilting, which become prevalent with the advent of the post-monsoon season, are a serious threat to black pepper cultivation. Detailed investigations into the aetiology revealed an association of three Pythium species: P. deliense, P. cucurbitacearum, and P. catenulatum, with the syndrome. As a preliminary phase to develop a promising strategy to manage the yellowing and wilting, new generation fungicides, namely famoxadone-cymoxanil, cymoxanil-mancozeb, strobilurin fungicides [RIL-070/FI (72 FP) and kresoxim-methyl], propiconazole, propineb, chlorothalonil, iprovalicarb-propineb, and fenamidone-mancozeb were screened in vitro against the pathogens and compared the efficacy with commonly used copper oxychloride and metalaxyl-mancozeb at recommended dosages. Initially, metalaxyl-mancozeb (1250 ppm), propiconazole (2000 ppm), fenamidone-mancozeb (500 ppm), and RIL-070/FI (400 ppm) were found to be promising; however, the pathogens exhibited resurgence of growth, except for P. cucurbitacearum. The LD₅₀ and LD₉₀ values, as well as mycelial degradation, which occurred upon exposure to the fungicides, were also determined. In the Alamar Blue dye assay, a change of colour from blue to pink indicated resistance of the pathogens to the tested fungicides. The in planta assay showed typical wilting and root rot with propiconazole, while no visible symptoms were observed with metalaxyl-mancozeb and fenamidone-mancozeb. The results of the present study indicated the multiple fungicide tolerance of Pythium spp. associated with the yellowing and wilting of black pepper, which warrants further evaluation of the fungicides under field conditions in endemic regions.