Journal of Taiwan Agricultural Research

Author:Jin-Hsing Huang* and Jiunn-Feng Su

Abstract:

    A study revealed that conidial germination of Podosphaera xanthii was observed under light microscope when they were inoculated on water agar, acetyl cellulose or leaf discs of melon and retrieved by a plastic membrane method. However, mycelial growth was observed only by the plastic membrane method. When conidia of P. xanthii (race 1) were inoculated on melon leaf-discs (Iran H variety), incubated for 48 h at series temperatures, and prepared specimens with the plastic membrane method for observation under compound microscope, the range of optimum temperature for conidial germination was 20–28℃ with the germination rate of 40.7–48.3%. The range of optimum temperature for hyphal development from germinated conidia and mycelial growth was 20–26℃, with the hyphal development rate of 30.7–33.1% and the hyphal length of 179.0–228.0 μm. When conidia of P. xanthii (race 1) were inoculated on leaf-discs of the susceptible melon varieties, ‘Iran H’ and ‘Védrantais’, and incubated at 24℃ for 48 h, the range of conidial germination rate, hyphal development rate and hyphal length were 51.3–58.8%, 41.3–53.5% and 196.0–212.4 μm, respectively. They were significantly ( P < 0.05) higher than those (12.5–15.4%, 0–0.8%, and 0–82.1 μm) of the pathogen inoculated on the resistant melon varieties, ‘PMR-45’, ‘PI-124112’ and ‘MR-1’. When the conidia of P. xanthii (race 1) were inoculated on leaves of the tested melon varieties grown in pots, the conidial germination, hyphal development rate and hyphal length were similar to the results of conidia inoculation on the leaf-discs. The conidia of race 1 strains (Px-001 and Px-274) and race 5 strains (Px-359 and Px-360) of P. xanthii were respectively inoculated on leaf discs of the susceptible variety Iran H and the resistant varieties, ‘PMR-45’ and ‘MR-1’, the hyphal development rate (29.0–54.0%) on the susceptible melon varieties was significantly (P < 0.05) higher than that (0–6.8%) on the resistant varieties. These results suggest that the plastic membrane method for collecting samples from leaf discs or leaves of plants inoculated with conidia of P. xanthii may be a useful and practical method for effective rating of melon varieties for resistance to powdery mildew.

Key words:Plastic membrane method, Melon, Podosphaera xanthii, Conidial germination, Mycelial growth

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