Plant Disease Management Handbook

Cause Alternaria alternata, Pestalotia sp., and Phyllosticta sp. have been found by the OSU Plant Clinic. Many leaf-spotting fungi overwinter in diseased leaves and are favored by wet weather. Alternaria gains entry to leaves through wounds.

Symptoms Circular to angular necrotic spots on leaves.

Cause A fungus-like organism, Phytophthora sp., has been found in rotted roots by the OSU Plant Clinic. Poorly drained soil or flooding encourages disease development. The organism survives unfavorable periods in soil and infected plant debris. Under favorable conditions, spores germinate and infect roots. After infection, the microorganism spreads mainly in the inner bark tissues of the root.

Cause The fungus Podosphaera pannosa (formerly Sphaerotheca pannosa) has been reported on photinia in Oregon. This fungus is a common problem on apple and pear. Conidia are readily disseminated by wind. Chasmothecia develop within the external hyphae and form appendages late in their development.

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Photinia - Leaf Spot

Cause Physiological leaf spot is a problem on photinia in western Washington and Oregon. The cause is unknown. About 25% of the leaf spots samples that have come into the OSU Plant Clinic are of an abiotic nature. Leaf spotting appears to be more severe on plants in low-lying or shady areas. Cold temperatures also appear to be involved.

See:

Photinia - Physiological Leaf Spot

Hawthorn (Crataegus spp.) - Leaf Spot

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Pear (Pyrus spp.) - Fire Blight

Cause Erwinia amylovora, a bacterium that survives in affected wood of this and many other ornamentals. This is the same fire blight that attacks apple, pear, and other closely related plants. It usually goes unnoticed on ornamentals such as cotoneaster, hawthorn, photinia and pyracantha.

Cause The most frequently reported genera causing powdery mildew on Phlox are Golovinomyces magnicellulatus and Podosphaera sp. The fungi Golovinomyces cichoracearum (formerly Erysiphe cichoracearum) and Podosphaera macularis have been reported from Oregon (found by the OSU Plant Clinic) and Washington.

Cause The nematode Ditylenchus dipsaci enters through natural openings, such as stomata, of young shoots and through wounds. It lives and feeds inside the stem, moves up as the shoot grows, and can exist under the seed coat. It can infect many hosts. Movement and reproduction rates are high between 69°F and 77°F. The nematode can be spread via contaminated soil or media, water, garden tools, shoes, and infested seed. Detached or dried leaves can also serve as a source of the nematode.

Cause Downy mildew on phlox is caused by the fungus-like organism Peronospora phlogina. Although not formally reported from the Pacific Northwest, it has been found by both the OSU and WSU Plant Clinics.

See:

Greenhouse Plants, Ornamental - Black Root Rot