By D. H. Gent and C. M. Ocamb
Cause Leveillula taurica (syns. = Oidiopsis sicula, O. taurica), a fungus that overwinters as chasmothecia on diseased tissue or on alternate hosts. The sexual stage has not been reported in the Pacific Northwest. Conditions favoring the disease are not well known, but infection appears to be favored by high relative humidity during warm weather. The spores do not require rain or persistent dew to cause infection. The disease apparently causes little damage to most onion varieties, but symptoms were observed on the following varieties in a trial in Franklin County, Washington in 2004: Gunnison', 'BGS 194', and 'BGS 196' (Bejo Seeds); 'SWO 6011' (Global Genetics); 'W20' (Nippon Norin); 'Golden Spike', 'EX 15122', and 'SVR 5819' (Seminis Vegetable Seeds); and 'Flamenco', 'Granero', 'Salsa', 'Tesoro', 'SX 7002', and 'SX 7004' (Nunhems). The disease appears to be most common and troublesome on varieties with very glossy leaves, which is associated with thin cuticular waxes.
Symptoms Circular to oblong (0.2 to 0.75 inches), yellow (chlorotic) to white (necrotic) areas on the leaves. Whitish, powdery patches, typical when the fungus is producing asexual spores (conidia), are apparent on infected tissues. Occasionally, lesions coalesce, and cover larger areas of the leaf surface. Symptoms can appear on both young and mature leaves.
- Plant varieties resistant to powdery mildew.
Chemical control The disease is not common in the Pacific Northwest and generally causes too little damage to warrant fungicide sprays.
References du Toit, L.J., Glawe, D.A., and Pelter, G.Q. 2004. First report of powdery mildew of onion (Allium cepa) caused by Leveillula taurica in the Pacific Northwest. Online. Plant Health Progress doi:10.1094/PHP-2004-1129-01-HN.
Mohan, S.K., and Molenaar, N.D. 2005. Powdery mildew caused by Leveillula taurica on glossy leaf genotypes of onion in Idaho. Plant Disease 89:431.