Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
8/9/99
Contacts:
Paula Flynn, Extension Plant Pathology, (515) 294-0581,
pflynn@iastate.edu
Elaine Edwards, Extension Communication Systems, (515)
294-5168, eedwards@iastate.edu
Yard and Garden Column for the Week of Aug. 13
What can I spray?
By Paula Flynn
Extension Plant Pathologist
Iowa State University Extension
When the leaves of valuable landscape plants show spots of brown, peculiar growths or the presence of insects, the question often arises, "What can I spray?" In many cases the answer is to spray nothing. Spraying nothing is free, easy on the environment and requires no effort.
Consider leaf diseases of large trees. Many different species of fungi can infect and cause brown spots on the leaves of shade trees such as oak, maple and ash. Such problems often are referred to as leaf blight or anthracnose diseases. Their occurrence can be especially common after wet-weather periods in the spring. Homeowners get worried when some of the affected leaves fall to the ground prematurely.
Fortunately, established trees have plenty of leaves. Scattered brown spots on the leaves and the early loss of a portion of the leaves is usually just a minor annoyance to the tree. The warmer and drier conditions of summer are not as favorable for these fungi and disease development slows down. Later flushes of growth often mask early loss of leaves.
Fungicides can protect trees from infection by these leaf spot fungi. The key word is protect. Once the disease is established and symptoms are observed, it's usually too late to apply a product. One also has to consider the feasibility of achieving good coverage of a large tree that may span the entire yard. With these considerations in mind, spraying a fungicide is generally not warranted.
How about applying fertilizer to sickly plants? Some ads want you to believe that brown plants will turn green with just the addition of a little fertilizer. Fertilizing isn't going to help, however, if lack of nutrients is not the problem. In fact, excess nutrients can be toxic. Each situation should be evaluated individually to find the cause of the problem. Stunted leaves, abnormal leaf color or poor growth may be related to compacted soils, poor drainage or a restricted rooting area. Yellowing or browning of leaves may be caused by drought conditions. If you suspect a nutrient deficiency, a soil test can help you determine whether there is a need for fertilization. The Iowa State University Soil Testing Laboratory and other private testing laboratories can provide this service for a fee.
Finally, should bug-infested trees be sprayed? In general, insect infestations do not seriously affect the vigor of established trees and shrubs. A variety of organisms, including mites, midges, wasps, aphids and psyllids, can cause the formation of galls on leaves or twigs. These galls surround the insect or mite and provide protection from the weather and predators and also provide a source of food. Maple bladder gall, caused by a mite, is common on silver maple trees. The small galls are red or black and irregularly spherical. Nipple gall is so common on hackberry that it is often used to identify the tree. Chemical applications are an option, but must be timed to coincide with the initial insect/mite activity before the galls are formed. Once the galls form they protect the causal organisms and control is impractical. Even though their form and color may be alarming, galls seldom threaten the health of established trees.
In some cases, the insect or mite is not protected by a gall and can be easily spotted on the leaf tissue. Aphids, for instance, may cause leaf curling, discoloration and defoliation. Damage is usually more ugly than harmful. Sprays would have to be applied prior to observable symptoms to be effective.
The most important first step in deciding what to do about a plant problem is to accurately diagnose the problem. Take time to learn the common problems of your favorite plants. In many cases, sanitation practices such as raking and removing diseased foliage, or pruning out branch terminals that are deformed by aphids will give you adequate control.
ml: isugarden
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