Powdery mildew of Beans and Turnip and Rutabaga

Erysiphe polygoni

Powdery mildew, caused by the fungal organism Erysiphe polygoni, is one of the most commonly occurring diseases on many types of beans. Green bean, pole bean, long bean, Italian bean, and snow pea crops are all susceptible to powdery mildew in tropical and subtropical climates. Although the causal organism rarely causes extensive damage, significant yield losses (up to 69%) were reported in Columbia County when infection occurred in dry beans prior to flowering. Accurately identifying this disease and immediately taking action for control are critical to effectively prevent spread of powdery mildew in order to reduce significant losses of yield and quality.
Symptoms. Powdery mildew can affect all above-ground parts of bean plants. Initial symptoms appear as small and white talcum-like spots, which most commonly are seen on the upper surface of leaves. These spots increase in size and run together to form a whitish, powdery growth, gradually spread over a large area of the leaves, and can spread even farther to the stems. As the symptoms developed, infected leaves may gradually curl downward, pale yellow or brown, die, and fall off. Under severe conditions, the entire leaves and plants could be covered by white cottony mycelial growth of the fungus. Symptoms on infected leaves may vary with bean varieties, but powdery mildew may cause the leaves to be twisted, buckled, or distorted. The powdery mildew fungus usually does not grow on bean pods except pea pods. However, powdery mildew spots can develop on snap bean pods.

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