Purple Blotch and Stemphylium Leaf Blight

Purple blotch and Stemphylium leaf blight occur as oval-shaped, tan-to-brown lesions on leaf blades. Yellow streaks, which later turn brown, extend along the blade in both directions from the lesion. As it progresses, purple blotch lesions become deep purple and may develop dark-brown concentric circles within the lesion. Purple blotch lesions may expand, girdling and killing the leaf. Likewise, in advanced stages, Stemphylium leaf blight may exhibit multiple small lesions, which coalesce and cause blighted leaves. Both diseases can also attack and girdle seed stems.

Stemphylium leaf blight is more common than purple blotch in California, but symptoms and management are similar for both diseases. These fungi are often found infecting downy mildew lesions on onion but occur less commonly on garlic.

Stemphlyium leaf blight is a common late-season problem in processing onions grown in the Tulelake area. It commonly occurs in onions that are maturing and starting to lodge. Infection of seed stalks can reduce seed yield and quality.

Stemphylium vesicarium is a weak pathogen, so it often infects onion plants secondarily, after other diseases develop or when the plant is otherwise stressed. Stemphylium leaf blight develops in iris yellow spot lesions, as well as on dead onion leaf tips following severe heat stress. Both fungi are also commonly found infecting downy mildew lesions on onion but occur less commonly on garlic.

Moderate temperatures and increasing canopy humidity favor both diseases. Heavy dew favors their development in desert areas, but foggy and rainy weather promotes their development in other regions. The optimal temperature for disease development is in the mid-70s (°F). In most areas, these diseases are usually not a problem after the end of the rainy season. However, in the Imperial Valley and in Tulelake, they can cause damage in the absence of rain, when there are conditions of high humidity, nighttime dew, and moderate temperatures. The spores are airborne.

Because these diseases often occur after damage from onion DOWNY MILDEW, controlling downy mildew is a critical strategy for preventing problems with purple blotch and Stemphylium leaf blight. However, in some regions, such as in the Tulelake area, these diseases can also occur in the absence of downy mildew. In such cases, management must be focused specifically on fungicide applications for purple blotch and Stemphylium leaf blight.

The amount of time leaves are consecutively wet each night (leaf wetness) can be used to inform when to apply fungicides for control of purple blotch and Stemphylium leaf blight.

• When leaves are wet for less than 12 consecutive hours per night, the interval between fungicide applications can be increased by several days.
• When leaves are wet for more than 12 consecutive hours per night, fungicide application intervals for controlling these diseases can be shortened in a manner consistent with the pesticide labels.