Study of pathogenicity, morphological and cultural variability of alternaria spp. on bt cotton

The isolates of Aternaria were collected from Bt cotton growing areas and their identification was confirmed by depositing the culture at Agharkar Research Institute, Pune. Among seventeen isolates, eight were identified as Alternaria dianthi, five as Alternaria alternata, and one as Alternaria chlamydospora while three isolates remained unidentified up to species level. Morphological studies of the isolates revealed that, on an average the colony diameter ranged from 60 to 84 mm and mycelial width from 2.53 to 4.76 μm with either poor to excellent sporulation. The spores were with 1–4 vertical and 3–9 horizontal septa. The conidia of Kalmala isolate were large in size (43.98–76.89 x 8.43–18.70 μm) in comparison to the smallest size (17.30–25.06 x 4.388.49μm) observed in Bijapur isolate, while maximum beak length of 16.30 μm was observed in Annigeri isolate. Studies on growth of isolates on different solid media viz., PDA, PCA, RSA, CDA and OMA revealed that Annigeri isolate recorded highest radial growth (86.25 mm), while OMA proved as an excellent growth media recording highest radial growth of 89.22 mm. Highest dry mycelial weight was recorded by Gabbur isolate (260.20 mg), followed by Hanumanamatti (255.10 mg) and Dharwad (250.50 mg) isolates on liquid media. It was maximum (311.47 mg) when the isolates were grown on in CDB medium.