Diversity of Colletotrichum falcatum population in India: Comparative virulence at two different agro-climatic regions

The fungal pathogen Colletotrichum falcatum causing red rot in sugarcane exhibits enormous variation in pathogenicity. Detailed studies were conducted to assess comparative virulence of about 117 C. falcatum isolates collected from different states in India at two different climatic conditions viz., tropical (Coimbatore) and sub-tropical (Karnal) conditions for five years under field conditions on a highly susceptible cv CoC 671. Pathogenic reactions of the isolates were categorized into highly virulent (HV), virulent (V), moderately virulent (MV), less virulent (LV) and least virulent (LeV) based on disease reactions in the stalks. The isolates showed a year to year variation in their behaviour for the disease reaction at both the locations. Although pooled HV and V reactions from both locations were more than 60%, tropical location recorded 23.21% HV and 42.32% V categories, whereas at sub-tropical location both the reactions were almost equal. At sub-tropical location, MV reactions ranged from 5.22 to 21.15% with a mean of 11.94% whereas, at tropical location showed a mean of 12.73% with limited year to year variation. Pooled LV and LeV reactions of the isolates were 21.74% at tropical and 25.62% at sub-tropical locations, respectively indicating a higher number low virulence scores at sub-tropical location. Testing of more than 100 C. falcatum isolates clearly revealed a huge diversity in their pathogenicity at both the locations. Stable and unstable behavior in their virulence could be attributed to source of host variety, place of testing and prevailing environment. This is the first report of assessing pathogenic variation in a diverse collection of C. falcatum isolates and further studies are required to identify influence of host genomic background which favour emergence of virulent isolates and specific environment that promotes expression of pathogenic virulence.