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Light- and Scanning Electron Microscopy of Cucumber and Barley Powdery Mildew on Host and Nonhost Plants. T. Staub, Agrochemical Division, Ciba-Geigy Ltd., Basle, Switzerland; Heide Dahmen(2), and F. J. Schwinn(3). (2)(3)Agrochemicals Division, Ciba-Geigy Ltd., Basle, Switzerland. Phytopathology 64:364-372. Accepted for publication 18 September 1973. DOI: 10.1094/Phyto-64-364.

The development of Erysiphe graminis and E. cichorccearum on their respective nonhost plants, cucumber and barley, was studied and compared to normal development on their host plants. On cucumber leaves, total germination and rate of germination of E. graminis were only half as high as on barley. Nevertheless, the germinating spores formed mature appressoria and penetrated the epidermal cells of the cucumber leaves. The invaded cells responded with a hypersensitive reaction before branching of the haustorial initials. On barley, E. cichoracearum conidia germinated normally, but the germ tubes were unable to dissolve the epicuticular wax crystals and penetration of the epidermal cells was prevented. After the fungal structures were removed, scanning electron microscopy revealed the imprints of germ tubes, appressoria, and hyphae on the cuticular surfaces of both the host and nonhost plants. On barley leaves, the epicuticular wax crystals in contact with E. graminis had been dissolved. On cucumber, both fungi left traces, consisting probably, at least in part, of fungal secretions, on the cuticular surface. The absence of tearing around the penetration holes suggested the involvement of enzymes in cuticular penetration. The penetration holes of E. graminis on both barley and cucumber were usually in the center of appressorial imprints, whereas those of E. cichoracearum on cucumber also occurred along the center of hyphal traces. These observations indicate that germ tubes and hyphae of powdery mildew fungi may share in some functions previously attributed only to appressoria.