Issue 6, August 15, 2023
Bacterial Leaf Scorch of Shade Trees
Bacterial leaf scorch (BLS) symptoms are evident on many oaks in central Illinois. This disease causes a slow, multiyear decline and eventually death of the host tree. BLS is caused by the bacterium Xylella fastidiosa, which is also responsible for Pierce’s Disease in grapes and is currently causing widespread damage to the Italian olive industry.
Bacterial leaf scorch affects a wide variety of Illinois tree species. The most common hosts in our state are oak (red oak group), elm, sycamore, London plane, sweetgum, hackberry, ginkgo, and maple (sugar and red). Many other woody and herbaceous plants can be susceptible to the pathogen. The bacteria is found only in the xylem (water-conducting) tissue of the plants and is spread from host to host xylem-feeding leafhoppers, treehoppers, and spittlebugs. X. fastidiosa is known to pass between certain host species through root grafts. However, root grafts do not appear to be an important transmission route between shade trees.
Red oak leaves from a tree infected with BLS. Note the yellow band between green and scorched tissues. Travis Cleveland, University of Illinois
Scorch symptoms appear on leaves in mid to late summer and gradually intensify as the season progresses. Affected leaves may turn a yellow/green color before turning brown, usually from the margin of the leaf inwards. Older leaves are often affected first, and an individual branch or section of branches usually becomes discolored at the same time. Symptoms are generally not scattered throughout the crown. Branches will leaf out the following spring, but symptoms will re-appear and slowly spread through the canopy over subsequent seasons. Except in oaks, leaves generally do not drop until autumn.
Shingle oak tree infected with BLS year-to-year comparison. August 2021, August 2022, and July 2023. Travis Cleveland, University of Illinois.
It’s important to note that BLS symptoms can easily be confused with drought stress, cultural problems, cankers, and, in oak trees, oak wilt. They can also be confused with Verticillium wilt in some trees. Submitting a sample to a plant diagnostic laboratory is the only way to diagnose the disease definitively.
The University of Illinois Plant Clinic uses an antibody test to determine the presence of X. fastidiosa in symptomatic tissue. The test requires a high population of bacteria to be effective. Thus, the plant clinic offers BLS testing in late August or early September when populations are high enough for detection. BLS tests conducted in spring or early summer may result in a false negative due to the population of bacteria being too low.
If you suspect that a tree or shrub is affected by BLS, you may submit a sample to the University of Illinois Plant Clinic. Samples should consist of symptomatic leaves complete with the petiole (the structure that attaches the leaf to the branch). Ideally, at least a few of the leaves would be transitioning from green to brown. There is a $25 fee for this test. To download a sample submission form, visit the Plant Clinic’s website at https://extension.illinois.edu/plant-clinic and click on the “Download Forms” tab. Please indicate that you wish the sample to be tested for BLS.
Management for trees affected with BLS consists of increasing tree vitality by mulching the base of the tree to retain moisture, watering during periods of dryness lasting more than two weeks, pruning out dead branches, and fertilizing when appropriate. While trunk injections with antibiotics have been shown to be effective at delaying symptom development, 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.
Authors:
Travis Cleveland
Diane Plewa