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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 Fungal diseases are dominant again this month. 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. click on image for more information
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. click on image for more information
Many plants, including vegetables, are 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. click on image for more information
We have been seeing more Dutch Elm Disease, and symptoms are exacerbated by hot weather which further stresses affected trees. click on image for more information
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. click on image for more information
Since it was first reported in Oregon in 2006, we have continued to see the powdery mildew fungus Oidium ericinum, on nursery Leucothoë plants. Symptoms include yellow to red leaf spots, with spores on the underside of the leaves. The leaf spots become progressively necrotic. click on image for more information
Abiotic disorders The excessive heat we had at the end of last month has increased the number of samples we have seen with sun scald, including burned or brown lesions on foliage, and necrotic areas on fruit and flowers; Click on images for more information
as well as trees showing decline symptoms from heat and drought stress. click on images for more information
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. click on image for more information
Bacterial disorders We generally receive few bacterial samples at this time of year. We have found Pseudomonas syringae causing leaf spots and shothole on cherry laurel, and leaf spots on rose-of-sharon in nursery stock. We have also seen this bacterium form lesions on the skin of pumpkin in the field situation after rain or hail damage. In the fruits of squash, bacteria can enter through wounds and result in internal fruit decay. click on images for more information
Viral disorders Tomato spotted wilt virus is spread by small insects (thrips) and is mainly a problem in greenhouse tomatoes but can also occur in the home garden. Symptoms include stunted plants, irregular shaped leaves, bronze to purplish markings on the leaves, ring spots on leaves and fruit, wilt, and poor fruit yield. click on image for more information
Very infrequently we see apples with circular spots, or bands of dappling or scarring on the fruit skin; a condition that is caused by a viroid, click on image for more information
see also "The Plant Clinic Year"
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August 28, 2009
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