Evidences of leaf surface structure against cucumber mosaic virus resistance in watermelon

Twenty watermelon genotypes comprising seventeen land races (WM-5, WM-10, WM-14, WM-14*, WM-16, WM-18, WM-19, WM-22, WM-27, WM-30, WM-35, WM-40, WM-42, WM-43, WM-51, WM-52, Thar Manik), two exotic genotypes (Cal Sweet and Crimson Sweet) and one susceptible check (Sugar Baby) were screened under artificial and natural epiphytotic conditions to study the variability in leaf morphology conferring resistance against cucumber mosaic virus (CMV). The DAS-ELISA results confirmed the presence of CMV in watermelon genotypes. Phylloplane morphological characteristics were studied and correlated with per cent disease index (PDI) values. The stomatal size, stomatal density and stomatal index were positively and significantly correlated with the PDI. They were significantly higher in susceptible check (Sugar baby) as compared to resistant genotypes on both adaxial and abaxial phylloplanes. Trichome size and frequency were highest in resistant genotypes and lowest in susceptible check on both upper and lower phylloplanes. Higher trichome density and low stomatal aperture could act as structural barriers that provide physical hindrance against vector and therefore, reduce the chances of disease. Based on coefficient of infection (CI), leaf surface structures and their interaction among themselves, four genotypes viz., WM-5, WM-10, WM-30 and Thar Manik were found to be resistant against CMV both under artificial and natural conditions. Therefore, these leaf characteristics can be taken as possible morphological markers for screening and augmenting resistant breeding programmes.