Infection of near-isogenic susceptible and resistant (Algerian M1a) barley withErysiphe graminisf. sp.hordeiculture CR3 was studied with the aid of the light and electron microscope. Barley coleoptiles were used extensively because mildew conidia, appressoria, haustoria, and mycelium were more readily found and observed on the non-pigmented two- to four- layered coleoptiles than on leaves. About 80 of the germinated conidia produced haustoria in the susceptible line. Electron micrographs revealed that, in the infections attempted in the resistant line (73), the penetration peg passed through the barley epidermal wall but grew little further into an aggregation of the host cytoplasm and (or) reaction products which occurred also in the cell of the susceptible coleoptile. Interaction between the host and fungus must have been very rapid because the infection peg had developed to the same extent as one in a susceptible host about 11ensp;h after inoculation. In the 14 of the germinated conidia that penetrated the resistant host successfully, the haustorium developed fully and secondary mycelium was formed but no conidiophores were produced. By the fourth day, fungus growth was greatly retarded. From the fifth day onwards, growth ceased and the colony showed signs of shrivelling and death. Two types of resistance occur, one which is effective during or immediately after host wall penetration, and another which is effective 2ndash;4 days later.
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