Probably the No. 1 problem that frustrates homeowners and gardeners is spots on the leaves of trees and shrubs.
It bothers them so much that at least 291 people sent samples of their spotty leaves or ailing branches to Purdue’s Plant and Pest Diagnostic Laboratory in 2018, said Director Tom Creswell. At the recent Marion County Master Gardener’s Spring Garden Clinic, Creswell reviewed the frustratingly frequent leaf disease problems submitted to the lab last year.
Most of the time, the spots on leaves are more of a cosmetic issue caused by a fungus disease rather than a killer of the plant, he said. Here’s a rundown of two common leaf spot problems.
This fungus disease can affect all oak trees, but red, black and pin oaks are most susceptible. Sometimes a secondary fungus, spot anthracnose, shows up as tiny black specks on leaves affected by tubakia. Another sign of tubakia is the yellowing of the leaves with spots along the veins.
What to do
Don’t worry about it. The spots appear late in the season, so the overall health of the tree is not affected. An early loss of leaves is not a concern. Although tubakia may return every year, fungicide treatment is not recommended.
Yellow leaves on pin oak could be a soil problem. Indiana soil tends to be alkaline, which impedes pin oaks and many other plants from taking up nutrients, such as manganese and iron. The lack of these nutrients causes chlolosis. For this, a soil test is recommended. The test results will provide recommendations on how to correct the problems.
This is the most common leaf spot problem among shade trees, such as maples, elm, birch, sycamore, dogwood and many others. It appears differently on various species, but it always causes leaves to die and fall off.
On dogwoods, anthracnose can cause cankers and branch dieback. I know this first hand. Two of my native flowering dogwoods (Cornus florida) had anthracnose last year and were treated by a certified arborist. I’m having them treated again this year. If I can’t get it under control, hard decisions will have to be made.
It’s important to know that if you have anthracnose on a maple, it will not spread to another species, such as birch, because the fungus disease is species specific. It does not normally cause tree death.
What to do
Make sure to clean up any fallen leaves from infected trees or shrubs. Since environmental stresses contribute to the problem, water the trees during hot dry spells and develop a fertilization plan and stick to it. The latter helps the tree grow new leaves in midsummer after the infested ones have fallen. If a fungicide treatment is undertaken, ensure the proper timing of application, which is critical for success.
Consider downloading Purdue’s “Leaf Diseases” pamphlet for more help with diagnosis and treatments. Or submit a generous sample of your disease (or insect infested) plant to the lab.