Diversity of cassiicolin profiles and culture filtrate toxicity of Corynespora cassiicola isolates from South Indian rubber plantations

This study aimed to determine the cassiicolin profile (presence/absence and sequence polymorphism) of Corynespora cassiicola (Berk. & Curt.) Wei., the causative agent of Corynespora leaf fall disease (CLFD) in Hevea brasiliensis (Willd. ex A. Juss.) Muell. Arg. and the toxicity of their culture filtrates towards both susceptible and tolerant rubber varieties in India. Polymerase Chain Reaction with primer targeting the various cassiicolin genes produced a single amplicon in one isolate only (out of 30). This amplicon was later confirmed by sequencing to be the Cas 2 gene. All other isolates were classified as Cas 0 (without cassiicolin gene). The Cas 2 gene detected from the Indian C. cassiicola isolate (HV) was found identical or closely related to the Cas 2 genes from Brazilian and Chinese C. cassiicola isolates. Analysis of the culture filtrates toxicity on leaves from three rubber cultivars revealed that three isolates, including the Cas 2-containing isolate, induced a strong wilting effect whatever the cultivar. For the other isolates, various levels of toxicity were measured on GT 1 and RRII 105, while the cultivar FX 516 was mostly insensitive. Virulence of the three isolates showing filtrate toxicity was confirmed by controlled inoculation with mycelium plugs, on all three rubber cultivars. C. cassiicola isolate HV, obtained from a heavily Corynespora leaf fall disease-infected rubber plantation in India, exhibits a close phylogenetic relationship of Cas 2-containing isolates with those from China and Brazil. The link between cassiicolin isoform Cas 2 and the virulence is still unknown, but the fact that both Cas 0 and Cas 2 isolates show similar levels of filtrate toxicity on the same cultivars indicates that toxic factors other than cassiicolin remain to be discovered.