Powdery mildew is an important disease of cucumber and melon caused by the species of the order Erysiphales. In 1991, Erysiphe (Golovinomyces) cichoracearum was reported as the only causal agent of powdery mildew in Cucurbitaceae and other plant families in the Culiacan Valley center of Sinaloa Mexico. Due to the prevalence of powdery mildew on cucumbers grown in greenhouses and problems related to its control, the objective of this research was to characterize the causal agent of the disease. This research consisted of the light microscopy analysis of both its anamorph and teleomorph; the molecular analysis of the nucleotide sequence obtained from the region comprising the ITS1, 5.8 S rDNA and the ITS2, amplified by PCR, and the assessment of physiological races by the use differential melon cultivars growing in greenhouse and growth chamber. The powdery mildew growing on cucumber showed euoidium type conidiophore, crenated marginal outline and fibrosin bodies. Chasmothecia had large peridial cells, filamentous appendages, one ascus with ocular opening diameter of 20 µm, containing six to eight ascospores. These characteristics and the sequenced PCR amplified fragments matched with those described for Podosphaera xanthii that was composed of the physiological races 1, 2F, 4 and 5.
Key words: Podosphaera xanthii, powdery mildew, chasmothecia, Golovinomyces cichoracearum, physiological race.
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