Pathogen virulence in sugarcane red rot pathogen versus varieties in cultivation: Classical case of loss in virulence in the pathotype CF06 (Cf671)

Abstract

In India, red rot caused by Colletotrichum falcatum exhibits enormous variation in pathogenicity on sugarcane. The new cultivars released for commercial cultivation succumb to new pathotypes of the pathogen after some years in disease endemic regions. Due to the variability in C. falcatum, screening for red rot resistance in sugarcane progenies is being taken up with designated pathotype(s) separately for each agroclimatic regions in the country. The predominant C. falcatum pathotype of tropical India Cf671 (CF06) isolated from the then ruling cultivar CoC 671 is being used in tropical regions. However, recently the pathotype has exhibited a reduced virulence on new cultivars and that has resulted in inconsistencies in disease reaction between artificial testing and field reaction under natural conditions. Hence, a detailed study was conducted during the past seven seasons on its pathogenicity on a set of 32 cultivars varying in red rot resistance in comparison to a new pathotype Cf94012 isolated from the cultivar Co 94012. The resistant cultivars such as BO 91, CoS 8436 and Co 98010 exhibited resistance reaction to both the pathotypes. However, other resistant cultivars such as Co 94008, Co 99006, Co 2001-15, Co 0238 and CoV 92102 showed a shift in their reaction to the new pathotype Cf94012 as moderately susceptible (MS) or susceptible (S) from resistant (R) /moderately resistant (MR) reactions in case of Cf671. Further, disease reaction on eight cultivars with MS behaviour and 11 susceptible cultivars clearly revealed a higher virulence of Cf94012 as compared to Cf671. Overall, the pathogenic behavior of the two pathotypes during seven seasons revealed a higher virulence of Cf94012 as compared to Cf671 on resistant, moderately susceptible and susceptible cultivars. Although the pathotype Cf671, remained virulent on its host cultivar CoC 671, it could not exhibit such virulence on the cultivars developed in the recent years. Further, the study revealed that once a popular cultivar is removed from cultivation as in the case of CoC 671, the pathotype adapted to it no longer maintain its virulence whereas the recently originated pathotype exhibits a higher virulence on the new cultivars of the host. This study also suggests adaptation of C. falcatum to the host cultivars