Leveillula taurica (anamorph: Oidiopsis sicula)
Brazil, Israel, Italy, Turkey and USA (California, Idaho, Utah and Washington)
Circular to oblong chlorotic lesions 5-20 mm (0.2-0.8 in.) in diameter develop on older leaves and rarely on younger leaves prior to bulb initiation. Sporulation gives lesions a gray to white powdery appearance. Chlorosis and eventually necrosis may develop around areas of sporulation. Lesions may coalesce to cover large areas of the leaf surface. This disease appears to be most common on varieties with glossy leaves, which are associated with thin cuticular waxes.
Conditions for Disease Development
Leveillula taurica overwinters in crop residue and many alternate hosts. Conidia are spread primarily by wind. Environmental conditions that favor infection include relatively warm temperatures and low relative humidity.
Following harvest, removal of crop residue, deep tillage and rotation to a non-host crop for at least one year will help eliminate the pathogen. Fungicide sprays to control this disease are available. Avoid excessive nitrogen fertilization and moisture stress.