Oregon State University Plant Clinic

Things to watch for during May

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

^UPDATE The heat wave in the middle of the month will be a test for the vascular system of most woody plants. If there has been winter injury or root rot, many of these plants will appear to collapse suddenly. Although the heat will bring on these symptoms, the damage occurred many months ago.
Use a pocket knife to look for discolored vascular systems. Where does the damage tissue transition into healthy tissue? Root rots will show this transition from the roots to the stems. Winter injury or above ground problems will show the transition from the stems to the trunks or crowns.
To figure out the problem the clinic will need the tissue from around this transition zone. This is where most of the organisms would be active, if they are responsible for the problem.

Abiotic disorders

Sudden low temperatures during spring continue to be a problem this month and can injure leaf and flower buds on flowering trees, small fruits and fruit trees that bloom early such as such as strawberry, cherry, pear and apple.

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Pluot

Strawberry

An excellent discussion of Winter Injury in Landscape Plants can be found in the PNW Plant Disease Management Handbook.

Bacterial disorders

Damage caused by species of Pseudomonas including Pseudomonas syringae, on woody ornamentals are typically seen at this time of year particularly in variable weather conditions and are especially favored by frosty mornings followed by moist or wet conditions. These bacteria overwinter in diseased twigs or as epiphytes on healthy wood. Tissues that have suffered injury from pruning, frost, unsuitable soil pH, poor nutrition, or infection by other pathogens, are predisposed to Pseudomonas diseases and exhibit a variety of symptoms.

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Cherry

Raspberry

Fungal disorders

Again, the majority of the fungal diseases we receive on samples of woody ornamentals are due to species of Phytophthora. Symptoms may occur both above and below ground including root rot, branch dieback, stem canker and are associated with poor drainage. Jay Pscheidt discusses symptoms, detection and management of Phytophthora diseases in his PNW Plant Disease Management Handbook article.

Dying or dead raspberry plants with decayed roots, received during May, have had Raspberry root rot due to Phytophthora fragariae var. rubi.

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Raspberry

The primary symptoms of Blueberry Mummyberry caused by Monilinia vaccinii-corymbosi could be expected to show up as the plants begin to flower.

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Blueberry

Viral disorders

Lately we have received samples of wheat from eastern Oregon with wheat soilborne mosaic virus, showing as pale green to bright yellow mosaic on leaves and stunted plants.

Blueberry Shock Ilarvirus (BSIV) may become apparent if flowers and young shoots suddenly die when the flowers are about to open. Entire bushes or parts of branches may show symptoms as a shock reaction to this viral infection transmitted in pollen by bees and other pollinators.

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Blueberry

We continue to see symptoms of Fall infection of Barley Yellow Dwarf Virus (BYDV) on fescue, rye, wheat and many other cultivated and wild grasses that may first appear in the spring as yellowish leaf tips (reddish in oats). BYDV is transmitted by several species of aphid.

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Wheat