Phytoalexin activity in barley powdery mildew.

Phytoalexin activity was detected in an obligate parasitic host-parasite combination, barley and Erysiphe graminis. There are two phases of phytoalexin production in the pathogenesis. In the first phase, phytoalexin activity was found 12hr after inoculation and was more prominent in incompatible cultivar-race interactions than in compatible ones. Both resistance and the ability to accumulate phytoalexin activity in leaves of the incompatible cultivar during the first phase were lost by heat treatment at 50C for 5min. In the second phase, phytoalexin activity was detected around the fungal colonies formed on leaves of the compatible hosts. The antifungal activity of the second phase phytoalexin was almost the same against several races. The role of phytoalexin in the pathogenesis in powdery mildew of barley are discussed.