Plant Disease Management Handbook

Cause Erysiphe calocladophora, Erysiphe penicillata and Phyllactinia alnicola, (formerly Microsphaera sp.), fungi that have been reported from Oregon and Washington. The conidia are readily wind disseminated. Chasmothecia develop within the external hyphae and form appendages late in their development. This fungus is favored by conditions that produce high humidity but dry leaves.

Note the two white colonies on the center leaf.

Photo by Jay W. Pscheidt, 1992.

Image related to Oak (Quercus spp.)-Powdery Mildew

Lesions can be difficult to see unless you look indirectly with the sun over your shoulder.

Jay W. Pscheidt, 2012.

White powdery growth on the leaf surface due to powdery mildew.

Melodie Putnam, 1990.

Cause Phoradendron villosum, a parasitic plant that affects white oak (Quercus garryana), black oak (Q. kelloggii), red oak (Q. rubra), scarlet oak (Q. coccinea), and pin oak (Q. palustris) and occasionally is on alder, aspen, birch, locust, manzanita, buckeye, sycamore, walnut and willow. Generally found in western Oregon.

Picture was taken in the dormant season with frost on the ground. All the foliage in this tree is due to mistletoe.

Jay W. Pscheidt, 2014.

Note the large green areas of mistletoe on this oak tree that has not even reached bud break. Pear trees in the background are in full bloom.

Photo by Jay W. Pscheidt, 1990.

Flowers are inconspicuous and fruit are shiny, whitish berries.

Jay W. Pscheidt, 2022.

See:

Maple (Acer spp.) - Leaf Scorch

Note the necrotic tissue along the margin of these leaves. This slide is labeled as salt burn."

OSU Extension Plant Pathology Slide Collection, 1984.

Although leaf scorch symptoms are associated with Xylella fastidiosa infection, this disease has not been found in the PNW.

Melodie Putnam slide collection.

See:

Peach (Prunus persica) - Leaf Curl

Top side of oak leaves with many blisters.

Jay W. Pscheidt, 2019.

Image related to Oak (Quercus spp.)-Leaf Blister

Quercus rubra with leaf blisters.

Neil Bell, 2010.

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Peach (Prunus persica) - Cytospora Canker

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Plants Resistant or Susceptible to Armillaria Root Rot

Mushroom of Armillaria indicate infection of this tree.

Alan Kanaskie, 2012.

Notice the fruiting bodies of this root-rotting fungus.

Photo by Jay W. Pscheidt, 1994.

Black shoestring-like strands called rhizomorphs that look like roots.

OSU Plant Clinic, 2022.

Cause Apiognomonia errabunda, a fungus widespread and common in western Washington, Oregon, and coastal British Columbia and found occasionally in eastern Washington. It is of little significance most years but can be serious in prolonged moist weather in spring and summer. Large trees can sustain severe disease without permanent damage. The disease can be a problem in production nurseries growing susceptible species.