‘Summer slump’ no cause for alarm
Q: In the past week, many leaves on trees in our yard have suddenly turned yellow and are falling in great numbers. The problem seems to be most severe on serviceberry, hydrangea trees and even some maples. What could be causing this? Would the addition of fertilizer help?
A: We are at that time of the growing season when we experience a “summer slump” in the foliage of many trees, shrubs and perennial plants. Some cultivars of species, such as serviceberry, typically begin their normal seasonal leaf drop around July 4, so I wouldn’t be concerned about those.
Right now, we are seeing a lot of physiological leaf scorch on many different plants, particularly young or newly transplanted trees. Leaf scorch can be caused by environmental (physiological) conditions or a bacteria.
The leaf scorch that we are seeing now seems to be a direct result of the weather that we experienced in late June and July. Unrelenting rains for weeks caused some soils to remain waterlogged for days, and then extreme heat, lack of precipitation and hot, drying winds caused tissue around the leaf margins and between the main veins to yellow, wither and fall.
The good news is that most trees recover from this condition and an application of fertilizer in midsummer is not recommended.
Q: We have a beautiful collection of dogwood trees that had spectacular blooms this spring. Recently, we noticed that something is chewing on a lot of the leaves. We do not find Japanese beetles or other insects present, but the damage is getting worse. Some of the leaves have white growths on the underside of the leaf, which looks like bird droppings. Any clue what could be causing this?
A: I believe that the white “growths” are the larval form of the dogwood sawfly. The larvae of this midseason pest is covered in a white powder-like material. Often these insects are found on the underside of the leaves, and, while resting, can curl up in a ball giving the impression of bird droppings.
These insects can be voracious feeders. Often the first symptom you will see are almost completely skeletonized leaves with not much more than the mid-vein of the leaf present. At this time of the growing season, trees can withstand significant defoliation without permanent damage.
The simplest course of action would be to shake the leaves containing the insects over a bucket to remove them from and then squash them. Insecticides, such as spinosad and pyrethroids, are also effective.
Mike Hogan, an Ohio State University Extension educator and associate professor, writes this monthly column. Send your questions to hogan.1@ osu.edu. Questions not answered in print will be answered by OSU Extension master gardeners.