Even though the growing season is in full swing now, some tree species are losing their leaves. The extended cool, wet weather this spring has created prime conditions for foliar diseases, especially anthracnose and apple scab.
Anthracnose is a common foliage disease of shade trees in Iowa. Symptoms occur on sycamore, ash, maple, oak, walnut, linden, hickory, and other deciduous trees. Anthracnose is caused by a number of different but closely related fungi. Each fungus is specific to the host tree it infects. Anthracnose on walnut will not cause anthracnose on ash, as so on.
In most cases symptoms may appear serious, but damage caused by anthracnose is minimal and does not seriously harm established shade trees. Symptoms vary from small, circular to irregular spots that are tan, dark brown, or black, to larger blotches that are usually associated with midribs and veins. When immature leaves are infected, these leaves may become distorted from abnormal leaf expansion. Young leaves may die and fall soon after a heavy infection. If a severe infection occurs early in the growing season and the trees defoliate, a new set of leaves may emerge. Sycamores may also show bud, shoot, and twig blight in addition to blighted leaves.
The following suggestions will aid in decreasing the severity of anthracnose and minimize its impact on tree health.
- Clean up and destroy as many fallen leaves as possible. This will help reduce the overwintering population of anthracnose fungi.
- Prune the tree to remove diseased twigs and branches (primarily for sycamore anthracnose) and to open up the canopy for better air circulation and light penetration.
- Maintain tree vigor with proper watering, fertilization, and other cultural practices such as mulching.
- Select species that are resistant or less susceptible to anthracnose.
- Apply a labeled fungicide when warranted. Research and experience show that fungicide control is rarely warranted because anthracnose usually does not seriously damage tree health and adequate control is seldom achieved.
If your crabapples are losing their leaves, the culprit is apple scab. Apple scab is caused by the fungus Venturia inaequalis. Apple scab usually appears as spots on leaves. In early stages the spots appear as small black or olive-green, velvety lesions, but later they become more distinct and have well-defined margins. A leaf with several spots will later turn yellow and drop. Highly susceptible varieties may lose most of their foliage by midsummer. The heavy defoliation will weaken the trees slightly, but will usually not kill them. Scab may also infect the fruit. Infected fruit have distinct brown or black spots with irregular margins. When severe, the skin splits open and fruit becomes irregularly shaped.
Cultural methods of disease management produce better results in relatively dry years and, in some situations, may even eliminate the need for fungicides. A properly pruned open tree canopy has better air circulation which allows for faster drying of leaves and reduction of the number of infections. Because the apple scab fungus overwinters in apples and leaves on the ground, raking and removing fallen leaves and apples is recommended.
Selection of high quality, disease resistant varieties is desirable since there are differences in susceptibility. A list of scab resistant crabapple varieties was printed in HHPN 6/23/95; this list is also available from your county extension office.
Fungicides can be used to protect scab susceptible crabapples. Sprays need to be applied when growth first appears and repeated at 7 to 10 day intervals. Thorough coverage is essential. Fungicides for scab control include Daconil 2787 (chlorothalonil), Cleary's 3336, Funginex or products that contain captan, maneb or mancozeb. Follow label directions.
This article originally appeared in the June 14, 1996 issue, p. 98.
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