Varietal Breakdown to Red Rot in Sugarcane Revealed by Comparing Two Colletotrichum falcatum Inoculation Methods

Abstract

Colletotrichum falcatum Went causing red rot is a major constraint to cane production and productivity across sugarcane growing countries in Asia. The fungal pathogen exhibits enormous variation under field conditions; the pathogenic variants emerge regularly in tune with deployment of new host varieties for cultivation making the resistant to susceptible referred as ‘varietal breakdown’. Although such phenomena occurred regularly, how the varieties succumb to the C. falcatum pathotypes is not clearly established, and hence, longevity of red rot-resistant varieties become unpredictable under field conditions. The soil-borne inoculum surviving as crop debris is the known source of C. falcatum (Cf) inoculum to cause infection in the field, probably after repeated attempts in a variety which was hitherto resistant. Hence, a detailed study was conducted with a set of Cf pathotypes varying in their virulence and ten varieties with their disease reactions vary from moderately resistant (MR) to highly susceptible (HS) by inoculating the pathogen by the plug method on standing canes and applying Cf inoculum to the soil, under field conditions. The three seasons study clearly indicated that disease reactions in sugarcane varieties to different Cf pathotypes in the plug method did not vary among the seasons, whereas the sugarcane varieties behaved differently for disease development from soil-borne inoculum. When disease development from the two inoculation methods was compared with respect to their known disease reaction, MR and HS varieties showed a similar pattern. However, four of the five moderately susceptible (MS) varieties showed a deviation for higher disease development from soil inoculum to certain pathotypes. Such disease development in the trial from soil inoculum is reflected by a similar behaviour in disease endemic locations, where they succumbed to the pathogen. Overall, the study explains susceptibility of sugarcane varieties for Cf infection from the inoculum surviving in the soil in due course in the field, although their host reactions are MR or MS. Also these findings provide an evidence for varietal breakdown to C. falcatum in sugarcane from soil-borne inoculum under field conditions, for the first time.