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Powdery mildew

powdery-mildew

Powdery mildew on 'Bali' pie cherry

Thaddeus McCamant, Central Lakes College

Powdery mildew, caused by the fungus Podospheara clandestine, is a disease found wherever stone fruit trees are grown. Cherry, plum, and apricot are all susceptible to powdery mildew. The disease primarily infects new leaves and young, green twigs. Leaves on branches on the inside of the tree canopy are most susceptible. Leaves with powdery mildew usually stay on the tree. Powdery mildew will reduce growth on infected branches and will occasionally kill small twigs, but rarely causes significant damage.

Identification

Powdery mildew infections start as white fuzzy patches on the lower surface of new leaves. As the disease progresses, the entire lower portion of the leaves become covered with the white mildew and small white patches can be seen on the upper surface of the leaves. Mildew can spread to young twigs with green bark. Later in the summer, the fungi form dark spore producing structures that look a little like pepper scattered on the lower portion of the leaves. Infected leaves tend to be small and distorted, with younger leaves being especially malformed. Infected leaves occasionally become brittle and fall off. In years with warm wet springs, powdery mildew can infect the fruit resulting in round sunken areas with white fluffy fungal growth on the surface.

Important biology

Powdery mildew primarily overwinters in infected buds. At bud break, the fungi produce spores that primarily spread through the air and will infect new leaves that are wet from dew or rain. Powdery mildew can spread when there is no rain, as long as the leaves are wet from dew each morning. Powdery mildew is most common during warm, dry summers.

Management

Powdery mildew in stone fruit trees in Minnesota rarely hurt the trees enough to require a fungicide spray. If severe disease has occurred several years in a row, fungicides with an active ingredient of sulfur, captan, myclobutanil or propicanazole can be applied to protect young leaves.

The name of the plant being treated MUST BE LISTED on the fungicide label or the product cannot be used! Some products are registered for use on ornamental Prunus species but are not safe to use on stone fruit intended for human consumption. Always completely read and follow all instructions on the fungicide label.


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