Variability analysis of Phytophthora meadii - A major causal agent of capsule rot (Azhukal) disease in small cardamom

Capsule rot disease, caused by Phytophthora spp., pose a significant challenge to the cultivation of cardamom and other spices and plantation crops, leading to considerable yield losses. In the present study, the impact of P. meadii on small cardamom was assessed in the primary cardamom-growing regions of Tamil Nadu and Kerala through a roving survey conducted during 2023–2024. The survey revealed varying levels of disease severity, ranging from 8.23% to 52.80%. The highest incidence was recorded in the Udumbanchola region (52.80%), while the lowest was observed in Thandikudi (8.32%). A total of eight isolates of Phytophthora (designated as PHY-1 to PHY-8) were collected from diseased samples and purified using the single hyphal tip method. Pathogenicity studies were conducted to evaluate the virulence of these isolates through two different methods: a detached capsule assay and an in-planta assay. Among the 8 pathogenic isolates, PHY-4 exhibited the highest level of virulence (75%) and presented typical symptoms of capsule rot disease. The isolates were analyzed for cultural and morphological variability. All eight isolates displayed distinct variations in growth patterns and sporangial morphology. Optimal growth and development of P. meadii were observed at temperatures ranging from 25–30ºC. Scanning electron microscopy (SEM) analyses revealed specific morphological characteristics of the isolates, including hyaline, coenocytic mycelium and distinct sporangial structures.