Effects of foliar washing on subsequent infection of leaves of swede ( Brassica napus) by Erysiphe cruciferarum

Abstract

Washing the surfaces of fully expanded leaves of swede ( Brassica napus) with distilled water for 1 h prior to inoculation with conidia of Erysiphe cruciferarum had no effect on germination and appressorium formation, but caused a reduction in the number of conidia which produced primary and secondary hyphae. Washing also caused a reduction in the lengths of primary hyphae. When inoculation of washed leaves was delayed, the number of conidia which produced primary and secondary hyphae gradually increased up to a maximum on leaves inoculated 5 days after washing. Washed leaves inoculated after this time showed a reduction in the number of conidia producing hyphae. All leaf washings collected during the first 5 days after the initial washing stimulated spore germination and germ tube growth of Botrytis cinerea used in vitro as a test fungus, but those collected later were inhibitory. Scanning electron microscope studies showed that wax was removed from the leaf during the washing process and that little of this wax was regenerated on fully expanded leaves. Analyses of leaf washings using gas chromatography showed that carbohydrates were also lost from the leaf surface during washing, but within 7 days these were replaced and accumulated at the surface in excess of the amounts found on unwashed leaves. It seems unlikely, however, that the inhibition of either fungus was due to the increased carbohydrate concentration but, rather, to the accumulation of some unknown inhibitory substance.