Plant Disease Management Handbook

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Greenhouse Plants, Ornamental - Gray Mold

Gray sporulation of Botrytis indicates colonization of senescent flower parts. Also note dieback of flower stem.

Jay W. Pscheidt, 2019.

Cause The fungus Peronospora lamii. The disease has been found on Lamium in Oregon. The cultivars Beacon Silver and White Nancy are susceptible. The disease is favored by cool, wet conditions. Spores are produced under high humidity and are released in large numbers when relative humidity drops (particularly in the morning hours). Spores are carried through the air, spreading the disease under favorable environmental conditions.

Downy mildew sporulating on lesion of lower leaf surface of ‘Beacon Silver’.

OSU Plant Clinic image, 2011.

Downy mildew lesion on upper leaf surface of ‘Beacon Silver’.

OSU Plant Clinic image, 2011.

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Greenhouse Plants, Ornamental - Gray Mold

Older flowers of this Lamium maculatum 'Orchid Frost' have been colonized by Botrytis.

Jay W. Pscheidt, 2019.

Spent flowers of Lamium maculatum 'Orchid Frost' are easily colonized by Botrytis.

Jay W. Pscheidt, 2019.

Cause Phytophthora spp., fungus-like microorganisms that survive in soil and can be carried in irrigation water from surface sources. Prolonged soil saturation is optimal for the pathogen to infect roots. The OSU Plant Clinic has found the disease. Studies in China have found that lower soil pH (from a range of 4.2 to 8.8) was associated with more pathogen. Warmer soil temperatures (from a range of 12.7 to 24.3°C) were also associated with more pathogen.

Image related to Kiwifruit (Actinidia spp.)-Root and Crown Rot

Dark roots are rotting due to Phytophthora.

OSU Plant Clinic collection, 2012.

Black root lesions due to Phytophthora sp.

OSU Plant Clinic Collection, 2011.

Crown region of the plant showing discoloration of the cambium.

Photo by Melodie Putnam, 1999.

Cause Botrytis cinerea, a fungus. This is an important postharvest problem in cold storage. Overwintering sclerotia and mycelium produce abundant conidia in winter and spring. Conidia infect petals and anthers under warm, moist conditions during bloom. Infection periods occur when the temperature is between 57°F and 77°F during a wetness event of at least 6 hours. Sepals and receptacles can become infected from 30 days after flowering through harvest.

These kiwi fruits had many different fungi isolated including Botrytis, Penicillium, Cladoporium and Epicoccum.

OSU Plant Clinic image, 2019.

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Spruce (Picea spp.) - Rusts

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Rhododendron - Phytophthora Blight and Dieback

Note the dark rotted roots at the base of this plant.

OSU Plant Clinic Image, 2011.

Cause Several fungi have been associated with leaf spots of Arctostaphylos uva-ursi (kinnikinnick or bearberry). Gloeosporium arctostaphyli has been reported from Idaho and Washington. Phyllosticta sp. has been commonly isolated from leaf spots by plant clinics in Oregon and Washington. Phyllosticta pyrolae and P.