Cultural, Morphological Characterization and Pathogenic Variations among Cercospora Isolates of Cotton from Major Cotton-Growing Regions of South India

Cercospora leaf spot is an important foliar disease that causes considerable yield loss in cotton. Studies of population structure and mechanisms of variation are primary factors in developing a disease management strategy.

One hundred samples of Cercospora leaf spot were collected from major cotton-growing states in southern India during the survey. The pathogen was isolated on PDA using the tissue segment method. The cultural characteristics of Cercospora gossypina isolates were studied on PDA. Conidial and conidiophore characters were examined. Pathogenicity and virulence of C. gossypina isolates were assessed on a 25-day-old susceptible cotton cultivar, LRA 5166, under glasshouse conditions. Isolates were classified as highly, moderately, or less virulent based on PDI.

51 isolates were obtained from samples collected in Karnataka (29) Andhra Pradesh (6), Telangana (7) and Tamil Nadu (9). Colony colour varied from dull to dark grey, ashy or greyish black, with irregular or smooth colony margins. Mycelial characters were recorded as flat, raised to medium raised, with cottony, villous to velvety and smooth textures. Conidiophores were short, unbranched, tufted, pale olivaceous brown and septate. They were fasciculate, pale towards the apex and straight or slightly curved. Conidia were thin-walled, hyaline and multi-septate. They were needle-shaped, slender and slightly curved. They appeared cylindrical, truncated to subtruncate at the base. The tips of the conidia were straight or variously curved. They were acicular with an acute base and the hila were thickened and darkened at the base. Conidia measured 55.72 -165.10 × 3.0 -5.0 μm, with 9 - 19 septa. PDI ranged from 11.0 to 34.0 among 29 Karnataka isolates. Of these, 3 were highly virulent, 13 moderately virulent and 13 less virulent. Telangana isolates showed PDI values from 14.0 to 23.5, with 3 moderately virulent and 4 less virulent. Andhra Pradesh isolates recorded PDI values from 12.0 to 21.5 for 6 isolates. Of these, 2 were moderately virulent and 4 were less virulent. Tamil Nadu isolates showed PDI ranging from 15.0 to 26.0 in 9 isolates. Of these, 5 were moderately virulent and 4 were less virulent.