Oregon State University Plant Clinic

Things to watch for during July

From our experience of samples we have received during July in previous years in the OSU Plant Clinic, this page will highlight some problems that may occur this month.

Viral disorders

Hosta Virus X is a virus relatively recently found in Hosta that can be spread by mechanical means from infected to healthy plants. Symptoms include mottling, mosaic and deformation of leaves. Some cultivars do not show symptoms even when infected.

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hosta

Fungal disorders

Dutch Elm Disease has continued to spread since it first appeared in Oregon in the early 1970s. Caused by the fungus Ophiostoma spp., the disease is actually spread by bark beetles. The beetles carry the fungal spores on their bodies to healthy trees where they feed on the bark. The deposited spores grow in the tree's vascular system and cause wilting, leaf yellowing, defoliation, brown discoloration in the sapwood, and ultimately death.

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elm

Abiotic disorders

Herbicides and other chemicals used in the landscape may inadvertantly cause injury to non-target plants. Uptake through roots, or through aerial contact will cause a variety of different symptoms that include leaf yellowing and browning, characteristic leaf distortion, proliferation of growth at some sites, fruit russeting, stunting, and swelling. The pattern of injury on the plant and the demarcation between affected and unaffected parts and other plants will be important. A diagnostician can distinguish chemical injury from disease.

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maple	arborvitae
himalyan birch	blueberry
potato

Further information can be found at these links:

How Herbicides Work - Oregon State University Extension Service

Herbicide Injury Symptoms - University of California Cooperative Extension

Diagnosing Herbicide Injury on Garden and Landscape Plants - Purdue Extension

Preventing Herbicide Drift and Injury to Grapes - Oregon State University Extension Service

Bacterial disorders

Cucurbits, including squash and cucumbers, often show symptoms of angular leaf spot caused by the bacterium Pseudomonas syringae pv. lachrymans. Irregularly shaped spots bounded by the veins occur on leaves. Stems and fruit may also be affected. This bacterium overwinters in diseased plant material and on seed and is then spread by rain, sprinker irrigation, as well as on hands and clothing.

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squash

cucumber