Plant Disease Management Handbook

Cause Peronospora sp., a fungus-like organism. Has not been reported in the Pacific Northwest but has been found in Washington. Favored by high moisture and crowded plant conditions.

Symptoms Yellow spots with either diffuse to angular borders and white to grayish-purple, fuzzy fungal growth under leaves.

Cultural control

Salvia farinacea 'Evolution Blue' with purple leaf spots due to downy mildew.

OSU Plant Clinic Image, 2016.

Sporulation of downy mildew on the underside of this leaf of Salvia farinacea 'Evolution Blue'.

OSU Plant Clinic Image, 2016.

The cultivar 'Victoria Blue' (Salvia farinacea) shows purple spots.

Photo by Melodie Putnam

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Greenhouse Plants, Ornamental - Gray Mold

The initial symptom is a dark, water-soaked, slimy lesion but under humid conditions gray, powdery spore masses form.

Jay W. Pscheidt, 2025.

Spent flowers are very susceptible and under humid conditions gray, powdery spore masses form.

Jay W. Pscheidt, 2025.

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Turnip and Rutabaga (Brassica spp.)

Cause Sclerotinia sclerotiorum, a fungus that produces hard, black sclerotia, which can survive many years in soil. Sclerotia germinate best when soil temperature is 55°F to 60°F. The fungus can colonize dead organic plant matter and survive many years. As a rule, these plants tend to be susceptible to overwatering. In the potato system, flowers are very susceptible to infection and are the main source of stem infection.

Cause Golovinomyces cichoracearum (formerly Erysiphe cichoracearum) has been reported from Idaho and it is a common garden problem in Oregon. The fungus is favored by conditions that produce high humidity but dry leaves. They are highly specialized pathogens that form a close association with the host. Conditions that favor the host also favor the pathogen.

Image related to Rudbeckia spp. (Black-eyed Susan)-Powdery Mildew

Powdery mildew can cover leaves late in the growing season.

Jay W. Pscheidt, 2014.

Powdery mildew may cover much of the plant late in the season.

Jay W. Pscheidt, 2014.

Cause Several fungi can cause leaf spots. Ramularia rudbeckiae has been reported from Idaho, and Septoria rudbeckiae has been reported from Idaho and Washington. Generally, weather or irrigation that keeps leaves wet for extended periods favors these diseases.

Cause Several root rotting organisms have been detected in rosemary root rot samples coming to the OSU Plant Clinic. Pythium, Berkeleyomyces sp. (formerly Thielaviopsis basicola), and Rhizoctonia are among the organisms found. Rosemary is native to seaside cliffs and thus over watering and too much fertility inhibit good growth. These same soil conditions favor many root-rot organisms.