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September Highlights

Things to watch for during September

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

Fungal disorders

Cooler morning and evening temperatures mean that many plants, including vegetables, are now very susceptible to white mold, caused by a fungus (Sclerotinia sclerotiorum) which can survive for several years in the soil. Beans, peas, lettuce, peppers, cabbages, cucumbers and others can all be infected by wind blown spores arriving in moist conditions on senescing blossoms and leaves as well as plant debris. Water soaked or dry white lesions rapidly progress to wilting and death of healthy plant tissue.

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garden pepper white mold
pepper

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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cucumber powdery mildew
cucumber

Cytospora canker in apple is caused by fungi that are vigorous wound invaders including wounds from sun scald and winter injury. They grow throughout the bark and cambium, and to a lesser extent the tree's structural wood. Overwintering is in infected stems and stem debris on the ground. These fungi have a wide host range and can infect virtually all stone fruit and pome fruit trees. Over-the-canopy and under-tree sprinkler irrigation can promote sporulation and disperse conidia. Cankers have been reported on apple in all Pacific Northwest regions, especially after severe winters. Cytospora fungi do not destroy the tree's structural strength, but other fungi commonly invade the Cytospora infection sites and cause rapid wood rotting, which adds to the severity of the disease's effects.

apple cytospora
apple

Verticillium wilt, which is caused by the fungus Verticillium dahliae, shows up during hot weather when the water conducting tissues of the vascular system are under stress. In trees and other woody ornamentals symptoms first appear in the leaves as yellowing, curling, and scorch followed by sudden wilting affecting whole branches, and then branch dieback.

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maple verticillium wilt
maple

In vegetables, in particular the solanaceous plants including eggplant, potato, pepper, and tomato, verticillium wilt symptoms often appear near harvest time with leaves becoming pale, rolling inward then dying and dropping off.

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eggplant verticillium wilt
eggplant

We have been seeing more Dutch Elm Disease, and symptoms are exacerbated by hot weather which further stresses affected trees.

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dutch elm disease
elm

Continue to be vigilant for late blight in tomato and potato (see August highlights), especially in the home garden.

tomato late blight
tomato

Abiotic disorders

We have had many hot spells in Western Oregon this year and sunscald is possible. Symptoms include burned or brown lesions on foliage, and necrotic areas on fruit and flowers;

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apple sunscald
apple

pumpkin sunscald
pumpkin


blackberry sunscald
blackberry


onion sunscald
onion

trees may show decline symptoms from heat and drought stress.

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maple drought stress
rocky mountain maple

maple heat stress
red maple

Tomato blossom end rot is a physiological problem associated with calcium deficiency and soil moisture fluctuation often seen in home gardens. The blossom end of the fruit develops a spot which enlarges and becomes dark brown to black.

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tomato blossom end rot
tomato

Bacterial disorders

Angular leaf spot on cucumber and other cucurbits is caused by a bacterium Pseudomonas syringae pv. lachrymans that overwinters in diseased plant material and on seed so DO NOT SAVE SEED FROM AFFECTED PLANTS. It is spread by rain, sprinkler irrigation, and on workers' hands and clothing.

Leaves, stems, and fruit may be affected and is seen as irregularly shaped spots with a water-soaked appearance. Bacteria may dry to a white residue on the spots. The water-soaked area later turns gray and dies, leaving an irregular shaped hole in the leaf tissue.

cucumber angular leaf spot
cucumber

Viral Disorders

Grapevine red blotch disease has been reported in the last few years and can be confused with grapevine leafroll virus. Red blotch is a distinct virus that the OSU Plant Clinic can now detect with its new PCR test. Small irregular red areas start to form as the days shorten in Fall. Vines from virus infected plants produce lower yields.

grapevine red blotch virus
grapevine

Arthropods

This is the time of year when the giant house spider (Eratigena atrica) tends to be more visible in your house as the males go in search of mates. They generally prefer secluded locations behind furniture but being more active at night you may see them running across the floor during the evening. Like the hobo spider (Eratigena agrestis), the house spider was introduced into North America from Europe. The fear associated with the hobo spider related to its so called venomous bite, may be unfounded. Nevertheless many people are not fond of sharing their home with these long-legged sprinters that are hard to tell apart, and so the Oregon Department of Agriculture has some recommendations.

Eratigene atrica
Eratigena atrica
Photo copyright: LadyIslay from BugGuide.net

 

Recently we have received some long-horned beetles (Prionus spp). Between June and September they may be seen at night attracted by lights. They cause no harm to humans.

Prionus spp
Prionus spp.

see also "The Plant Clinic Year"