Now that nearly every shade tree and ornamental are in full leaf, gardeners have been spotting some "spots" out there. Those spots can range in color from light green on a very green leaf (oak leaf blister) to black dots coming together to give a much larger blotch of black (tar leaf spot on maples).
In general, leaf spot diseases are rarely fatal to a tree so that is good news. What is often disturbing is those spots appear on the lower branches, right at eye level for all to see, especially while we sit on the patio chairs gazing out into the home landscape. These spots can also deform the shape of the leaf, causing a curling or appear raised or sunken on the leaf surface. About the only time a fungal leaf disease can be impactful is if every leaf is covered on a young seedling.
Homeowners also are spotting galls. Galls show up as the leaf develops. Typically, a gall-forming insect will lay its egg(s) right into swelling bud scales in early spring, as the leaves are ready to emerge. Many oak galls do this. Instead of a leaf, the egg transforms the leaf tissue into a thick walled gall or a portion of the leaf is transformed. The egg hatches and the young larvae consume the gall from the inside out through its various stages, and the adult emerges via a small hole in the now hollowed out gall.
Earlier this spring, silver maples were seen with the Maple Bladder Gall Mite creating small bright red galls that later turn black when the mite emerges. They occur in large numbers usually. The Spindle Gall is more solitary with only several on any one leaf and sticks up off the leave nearly half an inch. Just like the leaf spots, a plant would need to be covered before a treatment may be warranted.
If leaf spots and galls have been heavy this year, then planning now for next year is in order if they are to be prevented. Since leaf spots and galls get started early in the spring, the treatments are needed then to protect the plants. Foliage leaf spots invade new developing tissue as the leaves just begin to emerge from the overwintering bud scales. Gall forming insects can lay eggs in those same buds and on young emerged tissue.
Leaf spots are a fungal disease and a fungicide would need to be applied preventing fungal spores from invading that tender tissue. Similarly, an insecticide or miticide, depending on the insect, would need to be used for galls. Since some of these insects overwinter right on the tree in bark crevices, thorough coverage is needed.
The bottom line here is most of these problems are not life threatening to the trees, and if you are going to prevent them, timing is everything!
Richard Hentschel is a Horticulture Extension Educator with University of Illinois Extension, serving DuPage, Kane and Kendall counties. Stay tuned to more garden and yard updates with "This Week in the Garden" on Facebook at facebook.com/extensiondkk/videos. The 2017 Kendall County Master Gardener Help Desk currently is open Monday, Wednesday and Friday from 9 a.m. to 12:30 p.m. at 630-553-5823 or at uiemg-kendall@illinois.edu.
About the author: Richard Hentschel’s expertise extends across several subject areas with specialties in lawn care, fruit tree production, woody ornamentals, and home and community gardening. During his 45-year career in horticulture and agriculture, Hentschel became a well-known and respected expert for commercial and homeowner audiences, industry organizations, and media. He retired from University of Illinois Extension in April 2022 with nearly 30 years of service as a Horticulture Specialist and Educator in northern Illinois.