Infection processes of Sclerospora graminicola on Pennisetum glaucum lines resistant and susceptible to downy mildew

The host-pathogen interaction between pearl miller seedlings and Sclerospora graminicola, causing downy mildew disease, was studied. Two-day-old seedlings of pearl millet, with high resistance (0% of the plants systemically infected), resistance (1-10%), susceptibility (11-25%) and high susceptibility (> 25%) to downy mildew disease were inoculated with zoospores of the pathogen. Infected tissues were macerated in NaOH and stained with cotton blue in lactophenol to observe the differential behaviour in germination and colonization of zoospores in tissues of the coleoptile, mesocotyl and root regions of different lines of pearl millet. The formation of the infection structures by the pathogen such as appressoria, vesicles, infection hyphae, haustorial mother cells and haustoria, as well as the formation of papillae in the host cell walls was observed in the inoculated tissues. The percentage of host cells colonized by the pathogen, and the number of haustoria formed per 100 infected host cells were highest in the coleoptile, followed by the mesocotyl and root. The percentage of infected cells and the number of haustoria formed were least in highly resistant and highest in highly susceptible lines. The number of papillae, however, was maximum in highly resistant lines, the number reducing with increasing susceptibility of the host to the disease. These responses may be helpful in developing a rapid screening method to determine the resistance of pearl millet lines to downy mildew