"Drought, environmental stress, root injury and many other factors are causing leaf margin necrosis, or scorch, to show up in trees throughout the state," said Stephanie Porter, U of I visiting plant diagnostic outreach specialist in the Department of Crop Sciences. "Bacterial leaf scorch (BLS) is infectious and spreads systemically and causes a slow decline and death of the tree."
Although BLS is not new, it's beginning to appear more frequently in the Midwest, Porter said. The infectious leaf scorch is caused by the bacterium Xylella fastidiosa. The bacterial pathogen is found only in xylem tissue. Xylem-feeding leafhoppers and spittlebugs are thought to spread the bacterium in landscape trees.
"It can also be transmitted between trees through root grafts," she added. "The transmission methods must not be very effective, though, because we do not see rapid spread of the disease from tree to tree."
The most frequent hosts of this disease include elm, oak, sycamore, mulberry, sweetgum, sugar maple, and red maple. Look for scorch symptoms that occur in early summer to midsummer and then intensify in late summer. The scorched leaf edges or tissue between veins may be bordered by a yellow or reddish brown color.
"The symptoms occur first on one branch or section of branches and slowly spread in the tree from year to year," Porter said. "It is one of those situations that you hope will be better next year, but only gets worse. Symptoms will often show on oldest leaves first, distinguishing this disease from environmental scorch that first appears on newest leaves."
However, diagnosis is never that simple, she said. For example, most references say that oaks show symptoms on an entire branch at once. BLS often allows infected leaves to remain on the tree until the fall.
"Oaks are an exception," she said. "If you have seen a slow decline in your oak, leaf scorch symptoms showing each July to August, and fall leaf drop about a month ahead of healthy oaks, BLS may be present."
To determine if you have BLS, submit a sample to the U of I Plant Clinic. A fee will apply for submitted samples. Porter suggested calling ahead to be certain you have prepared the correct sample and to avoid resampling at your expense.
Leaf petiole tissue is preferred for this test, so leaves with green petioles are the usual request, she said. Please send samples in the next several weeks. The U of I Plant Clinic will collect samples, store them, and then run ONE test on all the submitted samples at the end of August.
If BLS is present, Porter said there is probably nothing you can do to keep the tree from dying. However, she recommends pruning out dead wood as it appears. She also encourages tree owners to think about planting a tree replacement not known to host this disease.
"Be sure to pick a species that does well in the site you have in mind," Porter said. "Investigate drainage pattern, soil type, amount of sunlight, and any oddities about the location. There are no fungicides, insecticides, or bactericides that can be sprayed on a tree to effectively prevent or cure this disease. There may be antibiotics that can be injected, but they may need to be repeated as frequently as every year, can be costly, and afford no guarantees."
For more information, contact web.extension.illinois.edu/plantclinic.
Digital photos available for three months at www.aces.uiuc.edu/news/News_Photos/BLS