Sympatric occurrence of sibling Phytophthora species associated with foot rot disease of black pepper in India.

Foot rot disease caused by Phytophthora capsici is a serious threat to black pepper cultivation in India and globally. High diversity exists among the Phytophthora isolates of black pepper and hence detailed investigations of their morphology and phylogenetic taxonomy were carried out in the present study. In order to resolve the diversity, 182 isolates of Phytophthora, collected from different black pepper-growing tracts of South India during 1998-2013 and maintained in the National Repository of Phytophthora at ICAR-Indian Institute of Spices Research, Kozhikode, were subjected to morphological, molecular and phylogenetic characterization. Morphologically all the isolates were long pedicellate with umbellate/simple sympodial sporangiophores and papillate sporangia with l/b ranging from 1.63 to 2.55 µm. Maximum temperature for the growth was ~ 34 °C. Chlamydospores were observed in "tropicalis" group, whereas they were absent in "capsici" group. Initial molecular studies using internal transcribed spacer (ITS) marker gene showed two clear cut lineages-"capsici-like" and "tropicalis-like" groups among them. Representative isolates from each group were subjected to host differential test, multilocus sequence typing (MLST) and phylogeny studies. MLST analysis of seven nuclear genes (60S ribosomal protein L10, beta-tubulin, elongation factor 1 alpha, enolase, heat shock protein 90, 28S ribosomal DNA and TigA gene fusion protein) clearly delineated black pepper Phytophthora isolates into two distinct species-P. capsici and P. tropicalis. On comparing with type strains from ATCC, it was found that the type strains of P. capsici and P. tropicalis differed from black pepper isolates in their infectivity on black pepper. The high degree of genetic polymorphism observed in black pepper Phytophthora isolates is an indication of the selection pressure they are subjected to in the complex habitat which ultimately may lead to speciation. So based on the extensive analysis, it is unambiguously proved that the foot rot disease of black pepper in India is predominantly caused by two species of Phytophthora, viz. P. capsici and P. tropicalis. Presence of multiple species of Phytophthora in the black pepper agro-ecosystem warrants a revisit to the control strategy being adopted for managing this serious disease. The silent molecular evolution taking place in such an ecological niche needs to be critically studied for the sustainable management of foot rot disease.