Development of purple blotch incited by Alternaria porri and its losses in seed crop of onion (Allium cepa)

Purple blotch of onion (Allium cepa L.) caused by Alternaria porri (Ellis) Cif. is serious bottleneck in onion seed production This disease reaches epidemic proportion every year during March. The purple spots/blotches were observed supemumerary on older leaves and scapes when fortnightly dew fall is >1.0 mm, mean maximum relative humidity >75% and mean maximum temperature 20-30 degrees C with >18 hr favourable temperature (10-30 degrees C) duration. Exposure of leaf and/or scape wetness for 8 hr was pre-requisite for conidial germination. With the increase in intensity of infection, every yield component was adversely affected. Severe most infection reduced number of scapes/ plant, height of scape, number of umblets/umbel, number of seeds/umblet, 1 000-grain weight, number of seeds/plant and seed yield/plant to the tune of 28.7, 74.5, 89.9, 41.7, 35.7, 95.7 and 97.3% respectively compared to healthy plants.