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

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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 sicula Scalia). The teleomorph of this fungus was not observed. Greenhouse inoculations were made onto leaves of three 8-week-old plants of a glossy leaf onion genotype (94P240) and three 6-week-old plants of tomato (cv. Payette) grown in pots. Conidia from sporulating lesions on onion leaves were brushed onto the leaves, and the inoculated plants were covered with a plastic bag for 72 h to maintain high humidity. Inoculated leaves on both hosts developed chlorotic lesions with sporulation of L. taurica after 14 days at 25 to 28°C. Noninoculated onion and tomato plants (one of each) did not develop powdery mildew symptoms. This selective susceptibility of onion genotypes with glossy leaves to L. taurica warrants careful consideration by onion breeding programs in utilizing such germ plasm as a potential source for other desirable characters such as nonpreference to thrips. To our knowledge, this is the first report of powdery mildew of onion caused by L. taurica in Idaho. This disease has been reported on onions in California (3) and Washington (2). However, it has not been observed in commercial onion crops in Idaho, even though the cultivars involved apparently vary as to the amount of cuticular wax on their foliage.