Ultrastructure of Pseudoperonospora cubensis in muskmelon genotypes susceptible and resistant to downy mildew

The causal agent of muskmelon (Cucumis melo L.) downy mildew, Pseudoperonospora cubensis (Berk. et Curt.) Rost., produced 10-15-mm lesions in the susceptible cv. Ananas-Yokneam (AY) but 1-3-mm, water-soaked lesions in the resistant cv. PI 124111F (PI). Sporulation on cv. PI was extremely limited compared to the conspicuous sporulation on cv. AY. In both cvs the fungus produced intercellular hyphae, rich in β-1,3-glucans, from which digitate (cauliflower-like) haustoria grew into the host cells. The haustoria had induced only minor chemical and ultrastructural changes in the cytoplasm and/or walls of cv AY 144 h after inoculation, whereas in contrast major changes were observed in cells of cv. PI as early as 20 h after inoculation. Changes induced in cv. PI included a heavy deposition of paramural, layered, callose-like materials along the inner surfaces of the host cell walls; enrichment of host cell walls with lignin-like materials (staining with phloroglucinol and chlorine sulphite); and encasement of the haustoria with heavy deposits of callose-like materials staining with resorcinol blue and lacmoid, fluorescing with aniline blue, and from TEM observations). In addition, the intercellular hyphae in cv. PI, but not in AY, sometimes produced haustoria (and necrosis) in epidermal cells, which gave rise to the early appearance (72 h) of water-soaked lesions. It is proposed that the containment of host cells and haustoria by callose- and lignin-like materials may interrupt the flow of nutrients from and into the invaded host cells.