Diseases and Pests

by Heather Prince

Tubakia leaf spot (Tubakia dryina) is often confused with anthracnose on oaks. However, anthracnose is a disease of late spring and early summer. Tubakia appears in July to September. It begins as small watersoaked areas that become more evident as they enlarge and transition to a reddish-brown color. Severe infections may cause premature leaf drop and it tends to be most prevalent on lower branches where moisture collects. Tubakia leaf spot is most common with abundant rainy weather and moderate temperatures.

While the symptoms may cause clients alarm, the disease develops late enough in the season that there are no long-term adverse effects on tree health. Since it’s primarily cosmetic, treatment with fungicides is not usually recommended. Rake and remove leaves in the fall to reduce inoculum in the surrounding area, thus limiting disease occurrence the following season. Promoting tree vigor and alleviating any potential stresses is also recommended.

Two-lined chestnut borer (Agrilus bilineatus) is a small, native, jewel beetle that primarily impacts species of oaks in our area and occasionally beech trees. It is usually a secondary pest of already stressed trees, laying eggs under the cambium layer of bark. The adult beetles are greenish-black with two white to gold stripes running down their back. Adults are active beginning in June and can be seen in flight and on sunny tree trunks. The larvae feed in the cambium, creating random galleries and disrupting nutrient flow to the canopy. They create D-shaped exit holes of about 1/8 inch diameter upon emerging as adults. Signs of two-lined chestnut borer are wilted foliage in late summer. Leaves may dry and turn brown and remain. Twig and branch dieback starts at the top of the tree and moves downward.

Because two-lined chestnut borer attacks stressed trees, the best prevention is maintaining tree vigor. Avoid stressors for oaks such as soil compaction, construction damage, and mechanical injury. Water during dry periods to prevent drought stress. Treatment is available for two-lined chestnut borer. When adults are active, they can be controlled by spraying the lower 4 ½ feet of the trunk with imidacloprid. Larvae can also be treated with a soil-drench of imidacloprid. Because it is a broad-spectrum insecticide, use care in deciding to use it as imidacloprid will negatively impact the myriad insect species that call oaks home.

University of Illinois Extension Service https://web.extension.illinois.edu/state/ horticulture/index.php 217-333-0519 The Morton Arboretum http://www.mortonarb.org/Plant Clinic: http://www.mortonarb.org/trees-plants/ tree-and-plant-advice/ 630-719-2424 Chicago Botanic Garden Plant Information Service: https://www.chicagobotanic.org/ plantinfoservice 847-835-0972