Purple blotch disease of onion caused by Alternaria porri (Ellis) Cif. is one of the most destructive diseases of onion and thus has become a major constraint in onion cultivation. Among the various management practices, management through fungicide is the best option. The present investigations involving the bio efficacy of various new generation fungicides and efficacy of sequential sprays were undertaken. The bio efficacy studies involved in vitro efficacy of ten fungicides at various concentration levels and later field evaluation of promising fungicides against Alternaria porri, the causal agent of purple blotch of onion. Among the fungicides, azoxystrobin + tebuconazole was found to be the most effective chemical as it inhibited the growth of the mycelium completely at all levels of concentrations this was followed by the fungicides hexaconazole and difenoconazole. A study on the field evaluation of the fungicides that were found most effective against A. porri under in vitro conditions was carried out. A significant difference in per cent disease index among the treatments was noticed. The fungicide azoxystrobin + tebuconazole at 0.1 per cent concentration recorded the least disease severity (PDI 28.76) followed by hexaconazole (PDI 34.59) at 0.1 per cent. The treatment involving combi product pyraclostrobin + epoxiconazole at 0.1 per cent was least effective (PDI 41.25). The outcome on the management of purple blotch disease of onion using sequential sprays using the combination of 3 fungicides revealed that treatment involving spray of propiconazole followed by hexaconazole and then mancozeb at 15 days interval) was most effective in controlling the purple blotch of onion with least disease severity (PDI 32.08) and followed by the treatment involving spray of hexaconazole followed by propiconazole and then mancozeb at 15 days interval) with PDI of 36.25 per cent. The findings of this study might be useful in exploring new fungicides and also sequential spray pattern for the management of the purple blotch disease of onion.

Onion, Alternaria porri, fungicides and sequential spray

The purple blotch caused by Alternaria porri (Ellis) Cif. is the most destructive disease that causes severe damage to both bulb and seed production of the onion. Currently, the best management practice available is use of fungicides. Our studies involving use of fungicides and sequential sprays of fungicides revealed that combi product fungicide azoxystrobin + tebuconazole and other fungicide hexaconazole were found to be highly effective both under in vitro and in vivo conditions. The sequential spray comprising of the fungicides propiconazole followed by hexaconazole and then by mancozeb was not only effective in reducing disease severity but also contributed to high yield of onion bulbs.

Chandan M.J., Mushrif S. K., Devappa, V., Manjunatha Reddy T.B., Meenakshi Sood and Premalatha B.R. (2023). Bio Efficacy of Fungicides against Alternaria porri causing the Purple Blotch Disease of Onion. Biological Forum – An International Journal, 15(9): 623-630.