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Things to watch for during November From our experience of samples we have received during November in previous years in the OSU Plant Clinic, this page will highlight some problems that may occur this month. Fungal disorders Downy mildew on cabbage is caused by the fungus Peronospora parasitica. Small yellowish spots appear on the upper surface of the leaves. As these enlarge they also become apparent on the underside of the leaves and are accompanied by a white moldy growth especially in conditions of high humidity. click on image for more information
If you are a nursery grower you can now learn about Phytophthora, including how to reduce the risk of disease caused by this organism, in a new online training course "OSU Phytophthora Online Course: Training for Nursery Growers". Yellowing of needles on spruce in late summer, which then turn brown or purplish-brown, are a sign of Rhizosphaera Needle Cast. Spores from the fungus causing this spread to current -season needles during wet conditions. click on image for more information
Abiotic disorders Abiotic disorders may predispose plants to secondary infection. This sample of delicata squash had secondary Pseudomonas syringae infection (bacterial) of the fruit which we suspect may initially have endured mechanical injury. click on image for more information
Bacterial disorders Leaf spots on nursery herbaceous ornamentals may be attributed to bacterial infections, including Pseudomonas syringae. click on image for more information
Bacteria and fungi capable of inducing plant disease may occur together and it is sometimes difficult to determine if both are contributing to the symptoms. In this coral bells sample, fungal species of Phoma were also present. Bacterial blight is an important disease of geraniums and pelargoniums. Symptoms can be seen in cuttings, on stems and on leaves. Disturbingly, some hardy perennial geraniums can carry the bacteria but remain symptomless. The disease is easily spread by physical contact, and in soil and irrigation water, especially during propagation. click on image for more information
Also important for nursery growers at this time of year during stock propagation are the two further bacterial diseases in herbaceous ornamentals; crown gall click on images for more information
and shoot proliferation and leafy gall.
The OSU Plant Clinic resource page for these two diseases has information and images of symptoms on many different plants. Viral disorders Barley Yellow Dwarf Virus can be cause of poor fall regrowth in perennial ryegrass and other grasses for seed in the Willamette Valley especially in cool moist conditions.
Leaf blackening, reddening and mottle can be sign of Grapevine Leafroll Virus which is transmitted by insects. Since there has not been any work on virus vectors of these viruses in grapes in Oregon, it is critical that growers continue to use only certified wood when establishing vineyards. click on image for more information
see also "The Plant Clinic Year"
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October 29, 2009
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Oregon State University Plant Clinic 
