EPIDEMIOLOGY AND MANAGEMENT OF PURPLE BLOTCH OF ONION CAUSED BY Alternaria porri (Ellis) Cif.
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Date
2017-12
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University of Horticultural Sciences, Bagalkot
Abstract
The present investigation on the epidemiology and management of purple
blotch of onion was conducted at Collage of Horticulture, Bagalkot during 2015-16.
The survey carried out in major onion growing districts of northern Karnataka during
2015-16, revealed that the highest incidence of the disease was in Haveri district
(37.00 %) and least in Koppal district (19.00 %). Over all disease severity was higher
in irrigated crop than rain fed crop. Among the eight isolates collected from different
locations during the survey, the isolate API-7 has found to have bigger size of conidia
with more number of septa. Among the eight solid and liquid media tested, pathogen
showed the maximum growth and sporulation on potato dextrose broth. A temperature
of 35 oC, pH of 6.0, RH of 90 per cent and incubation of 12 days on potato dextrose
broth were found best for the growth and sporulation of A. porri.
The experiment on transplanting at different days revealed that PDI (36.00 %)
was lowest when onion was transplanted on 2nd July with highest yield of 23.91 t/ha.
The epidemiological studies revealed that per cent disease index was progressing at
linear rate throughout the crop growth and it was negatively correlated with relative
humidity (morning and evening) and maximum temperature. It was positively
correlated with minimum temperature, while non significant with rain fall. The PDI at
all stages were highly and positively correlated with thrips incidence and wind speed.
Multiple linear regression analysis revealed that weather parameters influenced to the
extent of 35 to 88 % on per cent disease index. Fifty five genotypes were screened to
identify the resistance source, among which the genotype DOA-24 was found
moderately resistant to purple blotch of onion.
Among the 12 different fungicides tested in vitro, tebuconazole +
trifloxystrobin completely inhibited (100 %) the mycilial growth of the fungus at 0.15
per cent, followed by propiconazole @ 0.15% (90.37 %). In the field experiment two
sprays of tebuconazole + trifloxystrobin @ 0.1 per cent at 15 days interval recorded
maximum disease control with highest yield and net returns.