Emergence of new pathogenic variants in Colletotrichum falcatum, stalk infecting ascomycete in sugarcane: role of host varieties

Abstract

Red rot caused by Colletotrichum falcatum is more than a century old disease in sugarcane, responsible for several epiphytotics in India. Due to breakdown of many elite hybrid varieties to the pathogen they were moved from cultivation in different decades. Overall, disease resistance in sugarcane varieties was found to be short-lived. Since emergence of new variants is regular under our conditions a 10 year (2007-2017) detailed field study was taken to assess behavior of 12 isolates including four designated pathotypes of the tropical region on 20 popular varieties varying in disease resistance under field conditions. Virulence behavior of C. falcatum isolates was grouped under virulent, moderately virulent and less virulent categories. The results revealed that the isolates always exhibited virulence on the susceptible varieties but not on the resistant and moderately susceptible varieties. The isolates originated from susceptible and moderately susceptible varieties did not show any distinction in their virulence. Virulence frequencies of the isolates on the moderately susceptible varieties ranged of 21.3 - 40% as compared to 62.9 - 97.9% on the susceptible varieties. One of the eight resistant varieties succumbed to the pathogen virulence on a few occasions which indicated that the pathogen is able to breach the barriers of resistance and caused disease on the variety. The isolates of more than 30 years such as Cf419, Cf671 and Cf997 exhibited less virulence with more than 50% less occurrence comparatively to the nine isolates originated after 2000 except Cf95020. Overall, our study revealed that C. falcatum isolates exhibited a year to year variation in their virulence on the host varieties, attempting to overpower host resistance, leading to emergence of new variants