Studies on conidial germination and initial growth of the grapevine powdery mildew Uncinula necator on artificial substrates

Abstract

The requirements for conidial germination and formation of infection structures of the grapevine parasites Uncinula necator were studied on different substrates and supports. Direct contact with water or agar surfaces frequently caused bursting of conidia or inhibited formation of infection structures. Normal germination and initial growth depended on the presence of a dry surface covering a liquid substrate which could be reached by the penetration pegs. The water necessary for germination must be provided by a high air humidity whose optimal value (99.8%) was determined using closed slide microchambers in which air humidity was controlled by salt solutions. Even 99.6% humidity led to significant growth reduction, while at higher values deleterious condensation droplets formed around the spores. If liquid substrates are covered with thin (0.5 μm), waterproof polyacrylic membranes, germination appears normal. Formation of appressoria, successful penetration of the membrane by infection pegs and the development of functional haustoria indicate a lack of host specifity during the initial growth stage. although on water and on glucose media growth stopped after 1–2 weeks, this method seems to indicate a way to axenically culture this obligate parasite. The lack of host specificity at the level of host surface or tissue anatomy was corroborated by a successful culture of powdery mildew on grapevine callus.