Powdery Mildew Caused by Leveillula taurica on Glossy Leaf Genotypes of Onion in Idaho

Plant Disease
S K Mohan, N D Molenaar

Abstract

Onion (Allium cepa L.) plants growing in a breeding nursery in Canyon County, Idaho were observed with leaf symptoms of powdery mildew in August and September, 1994. The affected leaves showed roughly circular, 10- to 15-mm-diameter, powdery, white-to-grayish white patches with irregular margins. The leaf area under the fungal growth turned chlorotic and later necrotic. Occasionally, lesions coalesced, covering larger areas of the leaf surface. Symptoms were seen on mature and young leaves. Of 620 lines examined, only the 10 lines which had glossy leaves (no or low levels of cuticular wax) displayed symptoms of powdery mildew. Microscopic examination revealed that the white, powdery growth on the lesions consisted entirely of conidiophores and conidia. Conidiophores emerged through stomata, singly or in groups of two or three, were branched, and bore either terminal, pyriform (59 to 68 × 16 to 19 μm), apically pointed (lanceolate) primary conidia or cylindrical (57 to 62 × 18 to 20 μm) secondary conidia. On the basis of the presence of endophytic mycelium, morphological characteristics of the conidiophores, and the dimorphic conidia the fungus was identified as Leveillula taurica (Lev.) G. Arnaud (1) (anamorph = Oidiopsis sicul...Continue Reading

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