The Influence of Machine Oil on Morphology of Cucumber Powdery Mildew Fungus(II). Electron Microscopic Observation on Hyphae and Haustoria of Sphaerotheca fuliginea.

Sphaerotheca fuliginea is the causal agent of cucumber powdery mildew, which is one of the most serious diseases in cucumber cultivation. Various fungicides have been used for controlling this disease in Japan, but with unsatisfactory results in the field because of fungicideresistance problems4,5). Machine oil, a well recognized dormant/pre-blossoming insecticide, is also used as a fungicide against powdery mildew. Machine oil is reported to be able to control the strains of cucumber powdery mildew resistant against several other fungicides6). In our previous study7) by light and scanning electron microscopies, machine oil was found to cause structural deformation of conidia and mycelia of S. fuliginea. Ultrastructural studies have been reported on cucumber powdery mildew fungus1-3) but not on the fungus treated with fungicides. In this paper, we examined the effect of machine oil on the ultrastructure of hyphae and haustoria of S. fuliginea. Cucumber plants (Cucumis sativus L. cv. Tokiwahikari No.3, Type-P) were used. Each of test plants was grown in a plastic pot under the powdery mildew-free condition in a plastic box, and cotyledons of 10to 14-day-old seedlings were used in the experiments. Inoculation was carried out by tapping freshly formed conidia onto the adaxial surface of healthy cotyledons. After inoculation, the seedlings were kept in a closed plastic box to avoid contamination by other powdery mildew fungi from outside. Machine oil suspension (10,000ppm) or distilled water was sprayed onto the seedlings with a glass sprayer 4 days after inoculation. Tissue samples (2mm2) were cut from infected portion of cucumber cotyledons 10hr and 24hr after spraying, placed in 2.5% glutaraldehyde in 0.1M phosphate buffer at pH 7.2, and infiltrated under suction for 2hr at 25•Ž. The specimens were then postfixed with 1% OSO4 in the same