Special Feature
Pest of the Month: Box Tree Moth Disease of the Month: Powdery Mildew
Pest of the Month: Zimmerman Pine Moth
Disease of the Month: Bacterial Leaf Scorch
Bacterial leaf scorch (Xylella fastidiosa bacteria) is a serious disease of a wide range of tree species, most commonly red oaks, elm, sycamore, sweetgum, hackberry, ginkgo, and maple. The bacteria is found only in the xylem tissue of the plants, and is spread by root grafts. Xylem-feeding leafhoppers, treehoppers, and spittlebugs are also possible vectors. Fortunately, the disease does not spread quickly. Scorch symptoms appear on leaves in early to midsummer, becoming more intense as the season progresses. Affected leaves may turn a yellow/green color from the margin of the leaf inwards, eventually turning all brown. Older leaves are usually affected first, with individual branches or sections of branches become discolored. Branches will leaf out the following spring, but symptoms will re-appear and slowly spread through the crown of the tree over time. Symptoms are often confused with drought stress, oak wilt, cankers, etc., so this is a disease that should be submitted to the U of I Plant Clinic for clinical diagnosis.
Treatment:
Bacterial leaf scorch can be managed by increasing tree vitality by mulching to retain moisture, watering during periods of dryness lasting more than two weeks, pruning out dead branches, and fertilizing when appropriate. Antibiotic trunk injections may be effective at delaying symptom development, but they do not cure the tree, and the injection sites open new paths of entry for organisms that decay wood. Over time, repeated treatments can severely weaken the tree. Choosing non-susceptible hosts to plant near affected trees is also recommended to prevent the spread of disease.
Zimmerman pine moth (Dioryctria zimmermani) is a common pest of Austrian, red, and Scotch pines, injuring tree trunks and often killing branches. Injury on trunks are crusted areas of white pitch one to two inches in diameter or white cones of pitch that are about one inch in diameter. Heavily infested terminal leaders may bend downward like a hook. Adult moths emerge from the bark around mid-August. They mate and deposit eggs in crevices or wounds in the surface of the bark and at terminal buds. Hatching about 10 days later, the young larvae are tan or green with small dark spots. They will begin to move across and feed on the surface of the bark for several weeks. Mature caterpillars spin cocoons where they overwinter. In spring, the larvae become active again, moving across the bark for several days before boring into the tree to pupate.
Treatment:
Caterpillars are best controlled when young and actively feeding, so August is the time to scout and treat. Pyrethroid insecticides such as bifenthrin, cyfluthrin, permethrin, or other labeled insecticide to control Zimmerman pine moth are effective. These pests usually only attack the trunk in trees under 8 inches in diameter in the lower 10 to 12 feet. In larger trees, look for caterpillars in the branches.
Additional resources: 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 The Landscape Contractor August 2021
Chicago Botanic Garden Plant Information Service: https://www.chicagobotanic.org/ plantinfoservice 847-835-0972 53
