Cytological aspects of interactions between formae speciales of Erysiphe graminis and genera of gramineous plants, and their evolutionary implications

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Abstract
The progress of infection in leaves of wheat (Triticum aestivum), rye (Secale cereale), wheatgrass (Agropyron tsukushiense), barley (Hordeum vulgare), and oat (Avena sativa) inoculated with Erysiphe graminis f.sp. tritici, secalis, agropyri, and hordei was observed using a fluorescence microscope. Relative compatibility of each host–parasite combination was estimated by the incidence of cell wall penetration. Formae speciales tritici, secalis, and agropyri were highly compatible with any accession of wheat, rye, and wheatgrass, but less compatible with barley. On the other hand, f.sp. hordei was highly compatible with any of these four hosts. The four formae speciales had very low compatibility with oat, although f.sp. hordei was relatively more compatible with this host. These results suggested that (i) f.sp. tritici, secalis, and agropyri are phylogenetically very close; (ii) f.sp. hordei is less close to f.sp. tritici, secalis, and agropyri, and closer than these to the ancestral form of E. graminis; (iii) f.sp. avenae is remote from f.sp. tritici, secalis, agropyri, and hordei, but relatively less remote from f.sp. hordei than from the other three formae speciales.