S U M M A R YInocula of spores of Penicillium digitatum in water applied to apparently uninjured skin of oranges do not cause lesions to develop. Addition of citric acid, orange juice, or various extracts of rind had little effect on susceptibility to infection.When spores in water are applied to wounds made between oil vesicles, lesions develop only from wounds that penetrate deeply into the albedo. The flavedo of most oranges seems to be resistant to infection even when damaged, but in a few consignments it showed much less resistance. Increasing the number of conidia in the inoculum caused more lesions to develop but some fruits developed lesions from inocula containing very few spores. T h e method and timing of spore application to wounds had a considerable effect on the incidence of lesions; emanations from infected fruit had no effect.Lesions developed more rapidly and readily when suspensions of spores in water were applied to wounds in the skin that damaged oil vesicles; wounds as shallow as 0.25 mm allowed lesions to develop.
Cystosori of Spongospora subterranea (Wallr.) Lagerh., with cysts at different stages in development, were found on the surface of a potato powdery scab lesion in situ at harvest time and are described using light and transmission electron microscopy. The resting cyst cytoplasm has a five-layered envelope separating it from periplasmic space and is contained within a wall consisting of three layers (W1, W2, and W3). Protuberances on the outer surface of the cysts are outgrowths of the outermost (W1) wall layer. The differentiation of a primary zoospore within each cyst is marked by a reorganisation of the cytoplasmic envelope, the appearance of multivesicular bodies in the cytoplasm, and a germ pore with a thickened W3 wall rim. The ultrastructure of the cysts is compared with the resting spore and encysted primary zoospore of Plasmodiophora brassicae.
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