SummaryThe water·soluble fraction of the ether·soluble moiety of the glaucous coating on the juvenile leaves of seedlings of Eucalyptus bicostata Maiden et al. inhibited the in vitro germination of the conidia of Phaeoseptoria eucalypti (Hansf.) Walker. A negative correlation was obtained between susceptibility of leaves to P. eucalypti leaf spot and the weight per unit leaf area of ether-soluble substances also soluble in water. Glaucousness of seedling leaves and inhibition of spore germination both increased acropetally, glaucousness also increasing when seedlings were raised at lower temperatures. The glaucous coating of leaves was strongly hydrophobic and affected the deposition of conidia from water suspensions as did the leaf angle.Hydrophobicity of glaucous surfaces and leaf angle are probably more important than chemical inhibition of spore germination in affecting field infection of E. bicostata leaves by P. eucalypti.