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August Highlights

Things to watch for during August

From our experience of samples we have received during August in previous years in the OSU Plant Clinic, this page will highlight some problems that may occur this month. As with July, August will see some hot days interspersed with cooler temperatures and even a little rain.

Abiotic disorders

Melodie Putnam says "Plants with any root disease will show wilting and likely scorch in hot temperatures. Some plants will do that anyway if the irrigation is not sufficient, just because the water demand is so high. Grapes with trunk diseases will show firing of leaves and poor vigor due to the stress of the heat and water demand that can't be supplied. Street trees are showing leaf scorch due to the sun, and may start to show early fall coloration." 

butternut squash scorch
scorch in butternut squash

blueberry heat stress
heat stress in blueberry

High temperatures and direct exposure to the sun may cause scorching due to sunburn, or sunscald. Symptoms show subsequently as stunted new growth, burned or brown lesions on foliage, necrotic areas on fruit and flowers, or stem cankers.

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rhododendron sunscald



Chemical injury
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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himalyan birch


spinach with chemical injury
snap bean chemical injury
snap bean

wine grape with chemical injury

Further information can be found at these links:

Diagnosing Herbicide Injury on Garden and Landscape Plants - Purdue Extension

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

Viral disorders

Iris yellow spot virus is a relatively recent disease on onion in the Pacific Northwest, transmitted by onion and tobacco thrips, but also transmitted in seed. Symptoms include elongate, pale colored ring spots and lesions on leaves and flower stalks.

onion iris yellow spot virus
onion with Iris yellow spot virus

Hop stunt viroid may be showing its spots on hop plants.  Look for speckling on leaves, especially the main bine leaves along the bottom 6 feet of strings.  Depending on the hop variety, speckling may range from a few yellow freckles clustered on the bottom lobe of a leaf to bright yellow leaf area atop leaf veins.  Plants are stunted with shorter internodes and sidearms. 

hop stunt viroid
hops with Hop stunt viroid

Grapevine red blotch disease is a relatively recently known virus that looks very similar to the symptoms caused by the grapevine leafroll associated viruses. However red blotch is caused by a different virus that is graft transmissable.

grapevine red blotch virus
grapevine red blotch virus

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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Fungal disorders

Samples of peppermint with verticillium wilt caused by the fungus Verticillium dahliae have frequently been received during August from Union, Marion, Linn, Lane, Benton and Baker counties. This fungus lives in soil where it infects roots and enters the plant's vascular system causing the characteristic wilt symptoms of leaf yellowing, browning and bronzing, and stunted, dying plants.

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peppermint, Verticillium wilt

Look out for tomato late blight caused by the fungus-like organism Phytophthora infestans. This organism is brought into the garden on transplants, and on seed potatoes on which it is also a problem. Although favored by cool, moist conditions, the disease becomes a problem in hot, dry conditions when foliage is irrigated later in the day and remains moist. Diseased leaves, petioles, and stems have green watersoaked spots which turn purplish-black, and a white mold may appear at the edges of the lesions. Fruits may also be affected.

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tomato, late blight

Watch for powdery mildew on cucurbits (cucumber, squash, pumpkin, melon) which starts with small white spots on leaves and stems. This fungus then spreads to cover many parts of the plants.

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Watch for Fusarium crown and stalk node rot  in commercial sweet corn and silage corn fields.  Hotter weather conditions seem to enhance this disease.  Watch for leaf firing, starting at the base of affected plants, or sometimes in the side tillers when produced.  If crown and stalk node rot is present,  longitudinal cuts down the base of the stalk and through the crown will expose the darker brown to blackish rot in the crown and lower stalk nodes. 

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corn fusarium stalk end rot

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. We are seeing many samples in the OSU Plant Clinic now because of the high temperatures that accentuate the symptoms of trees suffering this vascular disease.

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elm_Dutch elm disease

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 angluar leaf spot

cucumber angular leaf spot

Outbreaks of Walnut blight caused by the bacterium Xanthomonas arboricola pv juglandis can occur when there is rain just before, during, and 2 weeks after bloom. The developing nuts are damaged when the bacteria penetrate from spots on the hull, through the shell and into the meat.

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walnut blight


see also "The Plant Clinic Year"