The release of diffusible substances from plant leaves, fungal spores or from pollen into water droplets on the surface of leaves is discussed in relation to (1) direct and indirect effects on spores of pathogens and (2) the effect on the growth of saprophytic micro‐organisms. Evidence suggests that competition for nutrients in droplets on leaves may limit germination of spores of some pathogens. Modification of the chemical environment of leaf surfaces by saprophytic micro‐organisms is discussed in relation to lytic enzyme and antibiotic production, cuticle degradation, production of growth substances and fixation of nitrogen.Examples are given of the effect on pathogenic fungi of diffusible toxins from leaves and inhibitors associated with surface wax. Treatments of leaves that remove or reduce surface wax can result in a stimulation of spore germination of some pathogenic fungi. The possible occurrence of phytoalexins on leaf surfaces prior to entry of pathogens into leaves is discussed in relation to differences in host‐pathogen interaction.
SUMMARYSakuranetin, a flavanone aglycone, was present in glands on the adaxial surfaces of leaves of most varieties of Ribes nigrum L. These glands senesce early in the season, releasing their contents on to the surface of the leaf. Glands on the abaxial surfaces of leaves,, which are different in structure frotn those of adaxial surfaces, do not contain sakuranetin. Conidia of Hotrytis cinerea Pers. ex Fr. were inhibited from germinating on adaxial but not on abaxiai surfaces of leaves at the time of adaxial gland senescence. Later in the season there was no inhibition of germination of conidia on either leaf surface. Because of the low activity of sakuranetin in vitro it was concluded that other factors were also likely to affect germination of conidia on adaxial leaf surfaces.
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