October 11, 1996
Many silver maple leaves showing signs of tar spot have been arriving in the Plant Disease Clinic. Tar spot is caused by the fungus Rhytisma acerinum and related species. As the common name suggests, the fungus causes slightly raised, tar-like spots on leaves. These black spots are often one-half inch in diameter. By fall, spots appear ridged or wrinkled. The "tar" is actually stromata (a fungal structure which contains fruiting bodies).
Continuing instructional courses (CIC's - 1996) for commercial pesticide applicators are listed below. Continuing education courses are approved for specific certification categories. Mailings with registration information for these programs are sent to companies or applicators who are certified in each of these categories. If you are a certified commercial pesticide applicator but do not receive registration information 3 weeks before the program, call the Entomology Department at 515-294-1101.
While most gardeners are familiar with spring-flowering bulbs, such as tulips and daffodils, many don't realize that some bulbs actually bloom in the fall. These fall-blooming bulbs make unexpected, colorful additions to the fall garden.
Nothing sends gardeners running faster than a weather forecast of FROST. Cool air, clear skies and light or calm winds are necessary for frost to occur. Cool air permits temperatures to drop low enough to freeze moisture in the air which would otherwise form dew. When skies are clear, heat from the soil is able to rise, allowing the cool air to settle close to the ground and chilling the plants as they lose heat. Calm winds allow the cool air to settle without mixing it with warm air.
Strawberries should be mulched in the fall to prevent winter injury. Temperatures below +20 F may kill flower buds and damage the roots and crowns of unmulched plants. Repeated freezing and thawing of the soil through the winter months can heave unmulched plants out of the soil and also damage plants. Excellent mulching materials include clean, weed-free straw and chopped cornstalks. (Leaves are not a good mulch for strawberries. Leaves tend to mat together and do not provide adequate protection.) Apply 3 to 5 inches of the material.
A serious needle disease of pines, arborvitae, and spruce is not "running rampant" across the state. This is the time of year when the oldest needles of evergreens turn yellow or brown and fall from the tree. This normal senescence is often mistaken for a disease problem. Environmental stresses can influence the timing of needle drop. Seasonal needle drop is usually most noticeable on white pine and arborvitae.
This article originally appeared in the October 11, 1996 issue, p. 162.
The first symptoms of powdery mildew are light spots on leaves. When the mildew growth becomes more dense, infected areas look as though they have been lightly sprayed with white paint or "powdered". The white substance observed on leaves is composed of millions of spores of the powdery mildew fungus. Heavily infected leaves turn yellow and may eventually brown and die. Infected plants are weakened, causing them to be more susceptible to other stresses, such as drought or low temperature injury.