Dynamics of pathogenic variation in Colletotrichum falcatum, red rot pathogen of sugarcane

Abstract

Red rot caused by Colletotrichum falcatum was reported more than 120 years ago in India and still it continues to be a major threat to sugarcane cultivation in the country. Continuous evolution of new variants and their adaptation to the new selected varieties contributed to the appearance of new pathogenic variants of C. falcatum. Earlier the variants were categorized into ‘light’ and ‘dark’ isolates and the former was considered as highly sporulating and virulent and the latter as less sporulating and less virulent. Later differential host varieties were identified to assess pathogenic variation in the vast C. falcatum population and more than 10 pathotypes were found in the country. The newly identified pathotypes were found to be virulent and represent the prevailing pathogenic population in the ecosystem like the recently identified pathotype CF12 which was found to be highly virulent over the erstwhile predominant pathotype CF06 in the tropical region. Further, the pathogenic population exhibited a very clear cut with a tropical and subtropical grouping of their pathogenic behaviour. Characterizing a large population of C. falcatum isolates from tropical and subtropical regions by documenting their pathogenic behaviour at tropical and subtropical locations revealed that the isolates exhibit comparatively low virulence in the subtropical conditions. Recent studies on the behaviour of 12 isolates of tropical region on 20 popular varieties revealed that the isolates more than 30 years old were lesser in their virulence than the isolates originating after 2000. Our studies clearly evidenced a huge diversity for virulence and year to year variation indicating an instability in the pathogenic behaviour of C. falcatum. Probable stable and unstable behavior in the pathogenic virulence of the isolates could be attributed to their host variety and prevailing environment during host-pathogenic interaction. Further, continuous evaluation of the isolates for many seasons revealed their stability in their behaviour and gain/ loss of virulence and their attempt to suppress host resistance, possibly leading to emergence of new variants