A preformed chemical inhibitor of tobacco powdery mildew on leaves of Nicotiana glutinosa

A diterpene identified as 13(s)-hydroxylabda-8(20),14-dien-2-one (2-ketoepimanool) (C 20 H 32 O 2 ) was isolated from epicuticular leaf “waxes” of Nicotiana glutinosa , a species immune to powdery mildew of tobacco. When externally applied to leaf surfaces of susceptible tobacco plants this diterpene strongly suppressed the emergence of conidial germ tubes 2 and 3 (but not of germ tube 1) and hence inhibited mildew development (ED 90 = 1 μg cm −2 ). The compound was not detected in N. debneyii or N. tomentosiformis , two other species immune to mildew, nor in three immune cultivars of N. tabacum .