Studies on Leaf Blight of Bt Cotton Caused by Alternaria spp.
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Date
2013
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UAS, Dharwad
Abstract
Cotton is a major fibre crop of global importance and has high commercial value next
to food grains. Among the various diseases, Alternaria leaf blight disease is gaining
importance due to its increased incidence. A survey was carried out during kharif 2012 in
North Karnataka. The maximum disease was recorded in Raichur, Belgaum and Bellary
districts. Morphological variability test showed that conidia of different isolates were septated
by 1-7 vertical and 3-9 horizontal septe, the isolates from Shaktinagar and Raladoddi showed
maximum size of 34.60 × 7.40 and 33.55 × 9.10 μm, respectively. The least size of the
conidia was recorded in Dharwad (11.29 × 2.56 μm) followed by Agsikoppa (14.45 × 3.62
μm). Among the twelve isolates, eight isolates showed resemblance with conidial
measurements of Alternaria macrospora descried by Ellis M. B. and remaining four isolates
were found to be different from Alternaria macrospora morphologically. Maximum width of
mycelia was observed in Tagalladoddi (1.99 μm) followed by Navali (1.85 μm) isolates on
potato dextrose agar. Five isolates showed irregular and seven isolates showed smooth colony
margin. Chicknasbi, Kadagammanadoddi and Tagalladoddi isolates showed excellent
sporulation on PDA. All the isolates inoculated on healthy cotton plant showed symptoms.
More variability was observed between Tagalladoddi and Raladoddi isolates and less
variability was observed between Annigeri and Dharwad through molecular variability test.
Among the 199 cotton genotypes screened, 50 genotypes showed moderate resistant, 69
genotypes showed moderate susceptible and 80 genotypes were found highly susceptible and
among 14 commercial Bt cotton hybrids screened, nine hybrid were found moderately
susceptible and the remaining five hybrids were highly susceptible to Alternaria leaf blight
disease. In field evaluation of nine different fungicides, propineb (0.2%) showed better
control of the disease with maximum yield.