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Cause The fungus Erysiphe azaleae (formerly Oidium ericinum) has been found in Washington. Powdery mildew on this host is common in west-side locations in Oregon and Washington. Wind blows conidia to healthy foliage where they start new mildew colonies. The fungus does not grow in a leaf but rather across the surface. Small anchor cells of the fungus, haustoria, remain in the leaf and take nourishment from the plant, but the main filament and the spores it produces are outside the leaf. The disease can develop under relatively dry conditions in spring and summer so long as the air is humid. Plants in shade tend to have more problems because that environment favors disease development. Multiple disease cycles occur during the growing season.
Symptoms Leaves may show pale, yellow-green, irregularly shaped blotches. These areas enlarge to about 0.5 inch and turn reddish-purple. Close observation reveals sparse whitish or tan fungal mycelium on the leaf undersurface, growing in association with the spots. Heavy infection may lead to defoliation.
Cultural control
- Remove and destroy infected leaves.
- Rake up and destroy fallen leaves.
- Plant in an area with good air circulation and morning sun.
- Avoid humid planting sites.
- Prune to allow better air circulation.
Chemical control Carefully monitor plants for the very first symptoms and then begin treatment. Alternate or tank-mix products from different groups that have different modes of action. Limit the use of any one group during crop production.
- Armada 50 WDG at 3 to 9 oz/100 gal water. Do not use a silicone-based surfactant. Not for nursery or greenhouse use. Group 3 + 11 fungicide. 12-hr reentry.
- Eagle 20 EW at 6 to 12 fl oz/100 gal water. Group 3 fungicide. 24-hr reentry.
- Myclobutanil 20 EW T&O at 6 to 12 fl oz/100 gal water plus spreading agent. May observe a PGR effect. Group 3 fungicide. 24-hr reentry.
- Seido at 4 to 5 fl oz/100 gal water plus an adjuvant. Group 50 fungicide. 4-hr reentry.
- Spectracide Immunox Multi-Purpose Fungicide Spray Concentrate for Gardens at 1 fl oz/gal water. Group 3 fungicide. H
Reference Putnam, M.L. and Glawe, D.A. 2007. New North American records of ananmorphic powdery mildew fungi (Erysiphales) parasitizing species of Leucothoe and Limnanthes. Pacific Northwest Fungi 2:1-6.