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July Highlights

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. Drought, accompanied by high temperatures, will impact many plants, even with irrigation. Any diseases or injuries that obstruct water flow in woody plants will show up in July with the intense heat: that includes cankers, vascular disease, wounds. The forecast for July is damp and cool, but the notes below should be borne in mind as the summer heats up later on.

Abiotic disorders

Scorch and heat stress will affect all sorts of woody plants that are in harsh areas, even if they receive intermittent irrigation. The burning bush pictured below is in a parking lot and gets a lot of reflected heat from the pavement. The bright red fall color shows early in such conditions. In unirrigated landscapes, this can also occur with normally tolerant plants if the soil is too dry.

burning bush
burning bush

noble fir drought stress
noble fir

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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birchhimalyan birch




Further information can be found at these links:

Weed control and herbicides - NPIC

Plant herbicide symptoms - NPIC

Diagnosing Herbicide Injury on Garden and Landscape Plants - Purdue Extension

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

Fungal disorders

Last year we saw a lot of anthracnose in sycamore. This is caused by a fungus that overwinters on leaves and twigs, then becomes noticeable as the leaves expand in spring. New leaves turn brown and in severe cases the entire tree can be defoliated.


Trunk cankers can be caused by the fungus Phytophthora, especially in landscape trees where watering systems are wetting the trunks. Bark that continually gets wet is a good environment for development of this disease.

oak_Phytophthora cinnamomi
small oak with Phytophthora cinnamomi

mature oak with Phytophthora showing as dark staining on bark 

Look for Cytospora in apples. This disease can be mistaken for fire blight, and usually follows stress including wounding from winter injury. The fungus spreads through the bark and cambium eventually forming calluses.  The wounds are frequently invaded by wood rotting fungi.

apple with Cytospora

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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Dutch elm disease

The blackleg story continues now in commercial fields of broccoli transplants. Cindy Ocamb notes crucifer vegetable growers should be on the look-out for plants infected with black leg, including from infected seeds.  Seeds infected with the fungus that incites black leg may exhibit symptoms similar to damping-off or wirestem, but closer examination will reveal the presence of pycnidia characteristic of the fungal pathogen, although sometimes they are only produced below the soil line on hypocotyl tissues.  

Fusarium crown and stalk node rot will probably make an earlier appearance in commercial sweet corn and silage corn fields this season.  Hotter weather conditions seem to enhance this disease, as well as drier soil conditions.  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.

PNW Plant Disease Image

Powdery mildew has been severe this season, with the warm dry weather that we have been having, so now squash plants and other plants from fruit to ornamentals will face stronger pressure from powdery mildew during July.  Look also for hop powdery mildew, as disease pressure continues to build.

Fusarium canker is showing up in hop plantings as stunted, unthrifty plants that wilt suddenly in hot weather and this disease freqently occurs together with Hop stunt viroid (see below).

Viral disorders

Barley Yellow Dwarf Virus is very common in the Willamette Valley in all grass crops, including oats and wheat.

Hop stunt viroid has also appeared in hop plantings as stunted, unthrifty plants. Thought to be spread by mechanical means, this does not apparently affect yield in PNW varieties.

Bacterial disorders

Look for fire blight in the Willamette Valley on apple, pear, serviceberry and other fruiting, flowering trees in the Roseaceae.

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apple fireblight

pear fireblight


see also "The Plant Clinic Year"