Plant Disease Management Handbook

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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.

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Photinia - Physiological Leaf Spot

Hawthorn (Crataegus spp.) - Leaf Spot

Small, dark reddish leaf spots of various sizes scattered on the older leaves.

Jay W. Pscheidt, 2020.

Note the numerous, reddish leaf spots on these leaves.

Photo by Jay W. Pscheidt, 1996.

Distinctive spores of Entomosporium mespili that appear insect-like.

Phil Hamm, 1993.

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Fire Blight of Apple and Pear

Pear (Pyrus spp.) - Fire Blight

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.

Under a microscope these leaves would been teeming with foliar nematodes.

Photo by Kathy Merrifield

Notice the necrotic leaves and twisted stems.

Photo by Melodie Putnam, 1998.

The stem nematode Ditylenchus dipsaci.

Melodie Putnam collection, 1998.

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.

Sporulation of the fungus can be seen on the leaves.

Photo by Melodie Putnam, 2005.

Many leaves are infected here.

Melodie Putnam, 2010.

Upper leaf shows normal trichomes along the margin while the leaf near the center of the image has sporulation of the fungus in the center of the leaf.

OSU Plant Clinic Collection, 2011.

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Greenhouse Plants, Ornamental - Black Root Rot

Stunted growth due to black root rot.

OSU Plant Clinic Image, 2013.

Dark sections of root that are black.

OSU Plant Clinic Image, 2013.

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Petunia (Petunia spp.) - Impatiens Necrotic Spot

Image related to Petunia (Petunia spp.)-Viruses

Plants infected with TMV have mottled green and yellowish green deformed leaves.

OSU Plant Clinic Image, 2014.

Plants infected with TMV can also have necrotic lesions as well as deformed leaves.

OSU Plant Clinic Image, 2014.

Plants infected with TMV may have very subtle mottling of the leaves.

OSU Plant Clinic Image, 2014.

Cause Sclerotinia sclerotiorum, a fungus that produces hard, black sclerotia, which can survive many years in soil. Sclerotia germinate to form hyphae that may attack plant stems and roots or produce apothecia that contain sexual dispersal spores in the spring. Spores may come in from nearby vegetable fields with the same disease. The disease develops in cool, wet conditions and is more prevalent when plants form a complete, compact canopy that holds high relative humidity.

Note mycelial webbing on this poor petunia plant that had stem rot and dieback.

OSU Plant Clinic image, 2010.

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Greenhouse Plants, Ornamental - Shoot Proliferation and Leafy Gall

Image related to Petunia (Petunia spp.)-Shoot Proliferation and Leafy Gall

OSU Plant Clinic Image, 2013.