Plant Disease Management Handbook

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Note: Phytophthora spp. have been identified on rose samples with root rot from the PNW several times by the OSU Plant Clinic.

photo of Rose (Rosa spp.) and hybrids-Phytophthora Root Rot

Phytophthora was found on necrotic roots.

OSU Plant Clinic Collection, 2013.

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Rose (Rosa spp.) and hybrids - Black Spot

Rose (Rosa spp.) and hybrids - Rust

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Rose (Rosa spp.) and hybrids - Brand and Common Canker

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Rhododendron - Gray Blight

Azalea cutting rot along with leaf spots due to Pestalotiopsis sp. complicate matters.

OSU Plant Clinic Collection, 2013.

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Seed Crop, Crucifers (Brassica, Eruca, and Raphanus spp.) - Powdery Mildew

Cause Lilac ring mottle virus can be mechanically transmitted from one plant to another.

Symptoms Leaf deformation, reduction in leaf size, ring spots, and line patterns were observed on cultivars President Grevy and Krasavitsa Moskvy. Symptom expression was influenced by environmental conditions and may be erratic, thus infections may remain undetected for years.

Photo showing Lilac (Syringa spp.)-Lilac Ring Mottle

Leaf deformation observed on 'President Grevy'.

OSU Plant Clinic Collection, 2015.

Ring spot and line pattern symptoms observed on 'President Grevy'.

OSU Plant Clinic Collection, 2015.

Line pattern symptoms observed on 'Krasavitsa Moskvy'.

OSU Plant Clinic Collection, 2015.

Cause The fungus, Erysiphe cruciferarum (syn. E. polygoni), can infect most crucifer crops and cruciferous weeds, but strains exist that have specificity and will infected only a subset of the crucifers. Stressed plants will be more susceptible to disease. The fungus survives on infected hosts or as chasmothecia (formerly cleistothecia), sexually-produced survival structures on infected crop debris.

By N. K. Osterbauer and C. M. Ocamb

A young hemp plant dying due to Pythium root rot.

Photo by Lloyd Nackley, Oregon State University.

Young hemp plant dying due to Pythium root rot; note the brown root coloration.

Photo by Lloyd Nackley, Oregon State University.

By C. M. Ocamb and N. K. Osterbauer

Hemp plant with gray mold symptoms on flower buds. Note the dead tissues that have turned brown.

Photo by Cynthia M. Ocamb, Oregon State University, 2020.

Hemp plant with gray mold symptoms on the terminal flower (cola). The affected tissues have turned brown.

Photo by Cynthia M. Ocamb, Oregon State University, 2020.

Hemp plant with gray mold symptoms on a flower. The affected tissues have turned brown and the fungus is sporulating on the tissues as denoted by the fuzzy brownish-gray mass.

Photo by Cynthia M. Ocamb, Oregon State University, 2020.

By N. K. Osterbauer and C. M. Ocamb

Cause The fungi, Cercospora cannabis and Pseudocercospora cannabina, which overwinter on infected crop residues on the soil surface. More commonly observed in field-grown hemp. In greenhouses, most commonly observed in plants near open greenhouse doors or near vents.