Biochemical Constituents in Malformed Tissues of Pearl Millet Cultivars Caused by Aggressive Pathotype of Sclerospora graminicola Causing Downy Mildew Disease

Downy mildew (DM) of pearl millet [Pennisetum glaucum (L.) R. Br.] caused by Sclerospora graminicola is the most widespread and destructive disease. In DM affected plants disease symptoms appear suddenly with the emergence of green ear, which exhibits all possible degrees of proliferations and malformation of the panicle. The pathogen population at Jodhpur, India is more virulent among other prevalent pathotypes as highly resistant pearl millet lines turned susceptible at this location. Virulence of pathotype rapidly changes host physiology producing varied symptoms in leaves and ear heads. Biochemical components including carbohydrates, phenols, free proline, photosynthetic pigments and enzymes like polyphenol oxidase (PPO), peroxidase (POX), IAA oxidase (IAAO) and catalase were found considerably deranged in malformed tissues. Results indicated that in two highly susceptible cultivars (Nokha local and Eknath) high soluble sugars were recorded in DM necrotic/chlorotic leaves and malformed ear heads, whereas starch contents were reduced in infected ear heads. Total and O-dihydroxy phenols were higher in DM infected leaves as well as in the malformed ear heads. Free proline contents were increased manifold in DM infected leaves and in proliferated panicles. Total chlorophyll contents reduced drastically in DM infected leaves. In ear heads showing tufting and complete malformation, total chlorophyll and carotenoids were low when compared to healthy and diseased leaves. Activities of PPO, POX, IAAO and catalase were higher in DM affected leaves and suppressed and completely malformed ear heads in comparison to their healthy counterparts. The study suggests that accumulation of total phenols caused the hyperphenolicity in infected host tissues despite increased activities of POX, PPO, catalase and IAA oxidase.