Tree Q & A per the
Chouteau County Extension Office
• Why did two of my cottonwoods turn yellow in late July? According to Eva Grimme (MSU Plant Disease Diagnostician), symptoms are consistent with Marssonina leaf spot. Symptoms include dry, brownish lesions with yellowing borders. Margins are often irregular, indistinct, and not restricted by leaf veins. Infection is favored by wet weather, especially at the time when the leaves emerge from buds. Management includes pruning out, discarding, raking and destroying affected leaves. Water adequately during the growing season. When trees are defoliated two or more consecutive years, fungicides with active ingredients such as chlorothalonil or copper can be applied at bud break. Most products need to be applied two or three times during the growing season. Follow the instructions on the product label carefully. Please keep in mind that fungicide applications prevent only new infections but won’t cure leaves already infected.
• My Ash trees have distorted leaves, is it herbicide damage? According to Laurie Kerzicnik (MSU Insect Diagnostician) the problem comes from ash plant bugs (Family Miridae, Tropidosteptes sp.). Several of the leaves had stippling, which is characteristic of the ash plant bug. Plant bugs puncture the plant tissue and cause discrete, white speckling on the upper surface of the leaves. They also leave brown “fecal” spots, which are obvious when you look on the underside of the leaf. Emerging leaves often show spotting and distortions, which is evident with your sample. The damage from the ash plant bug is often mistaken for other issues. Sometimes, if infestations are heavy, these leaves can become severely distorted and drop to the ground. The leaves start to show stippling on the upper surface of the leaf. Some of these areas will coalesce to form larger chlorotic spots. This can often cause the leaf to dry up and prematurely drop. The plant bugs lay their eggs under loose bark, and the nymphs or immatures hatch around the beginning of August. Immatures will begin feeding on lower leaves. Usually there are two generations per year. Even though the leaves seem very mottled and look concerning, the damage from this plant bug is usually minor and the leaves generally recover from the plant bugs. You can shake smaller trees or hit them with a jet of water several times to dislodge the nymphs. The trees that suffer the most are small or newly transplanted trees. One of your best controls overall is to maintain the health of your tree through extensive watering and mulching. You probably won’t need to go the chemical route. If you see populations build up too much, you can also use horticultural oils (such as Volck oil) or insecticidal soaps and apply these to the branches and leaves. Soaps and oils should be applied when the leaves are expanding in the spring.
• What is up with the shepherd’s crook formation on the new growth of my Cottonwoods? According to Eva Grimme, the tree was probably infected by Venturia leaf and shoot blight. Initial infection takes place in the spring. Symptoms include dead leaves and succulent shoots not long after bud break, terminal leader mortality, large irregular lesions appearing on leaves, and a shepherd’s crook formation on infected twigs. The fungus overwinters in fallen leaves and infected branches. Management includes raking and destroying fallen leaves, pruning out infected branches, and making sure the sprinklers do not hit the tree canopy. Support tree health by providing adequate water and nutrients.
Montana State University, U.S. Department of Agriculture and Montana Counties Cooperating. MSU Extension is an equal opportunity/affirmative action provider of educational outreach.
