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May Highlights

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.

High humidity caused by showers and warm temperatures forecast for this month will likely exacerbate plant diseases.

Fungal disorders

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 in the past during May, have had Raspberry root rot due to Phytophthora fragariae var. rubi.

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Raspberry

Watch for Blueberry Mummyberry caused by Monilinia vaccinii-corymbosi.

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Blueberry

Look for rusts on pear and serviceberry.

Viral disorders

Blueberry Shock Ilarvirus (BSIV) symptoms include tip dieback and bud death. 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, and different aphids can transmit different strains of the virus. The OSU Plant Clinic can differentiate these different strains.

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wheat - bydv
Wheat

In the past we have received samples of wheat from eastern Oregon at this time with wheat soilborne mosaic virus, showing as pale green to bright yellow mosaic on leaves and stunted plants.

Bacterial disorders

Damage caused by species of Pseudomonas including Pseudomonas syringae, on woody ornamentals are typically seen at this time of year although less has been observed so far this year. These bacteria overwinter in diseased twigs or as epiphytes on healthy wood.

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Cherry


Raspberry


Blueberry


Dappled willow

Abiotic disorders

Sudden low temperatures during spring can 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.

see also "The Plant Clinic Year"