You have probably seen environmental leaf scorch before. The leaf edges turn brown, and sometimes that necrosis moves between veins as well. Another kind of scorch you need to be aware of is bacterial leaf scorch (BLS). Although environmental leaf scorch is not pretty, it does not kill trees as does BLS.
“Leaf Scorch of Woody Plants,” Report on Plant Disease, no. 620, is available on the Internet at http://www.ag.uiuc.edu/%7Evista/horticul.htm or in local Extension offices. The report discusses symptoms, causes, and management of scorch. We usually see environmental scorch on trees and shrubs after prolonged periods of dry, windy weather in the spring and summer. The same could occur after extremely wet, windy conditions. Scorch is also common on trees in new subdivisions or on newly transplanted trees and shrubs with inadequate root establishment. Any factor that inhibits water absorption by roots may result in leaf scorch. Environmental leaf scorch is not infectious and does not move from plant to plant. Of course, plants in a similar environment with similar root stress would be expected to show these symptoms, so sometimes the problem seems to spread. Look for healthy, live buds and live, new stem material as an indication of the plant’s ability to recover. Scrape the new growth with your fingernail. It should be green under the bark. Live buds are green inside.
Bacterial leaf scorch is an infectious plant disease caused by a bacterium called Xylella fastidiosa. It may cause initial symptoms similar to environmental scorch, but symptoms appear in midsummer to late summer. Each spring, the tree leafs out normally; but by midsummer, the symptoms spread further in the tree. By the fourth season, the tree may be dead.
The most frequent hosts of this disease in the United States include elm, oak, sycamore, mulberry, sweetgum, sugar maple, and red maple. Oak seems to be our most common host species in Illinois. We have confirmed it on pin, red, shingle, bur, and white oaks. Kentucky reports BLS on pin, red, scarlet, bur, white, willow, and shingle oaks; silver, sugar, and red maples; sweetgum, sycamore, planetree, hackberry, American elm, and red mulberry. 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, but not in all cases. The symptoms occur first on one branch or section of branches and slowly spread in the tree from year to year. It is a situation that you hope will be better next year but only gets worse. If you have seen a slow but progessive 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.
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. The transmission methods must not be very effective, though, because we do not see rapid spread of the disease from tree to tree.
The bacterial pathogen cannot be isolated in the lab as most other bacteria. It can be confirmed using serological techniques. We cannot test for this bacterium at the Plant Clinic, so we send our samples to a private lab such as AGDIA, Inc. That lab has a serological (polyclonal antibody) test for the bacterium that can be done on young twigs and leaves. There is a fee. As of this writing, the fee was $48.25 for one sample and $6.25 for each additional sample using the same test. It is suggested that you call ahead to be certain you have properly prepared the sample. Leaf petiole tissue is preferred for this test, so leaves with green petioles are the usual request. Consult AGDIA at http://www.agdia.com or call them at (219)264-2014 or (800)62-AGDIA. If you prefer to go through the Plant Clinic with your sample, we can test for other problems but would have to bill for AGDIA testing as well. Call if you have questions.
What can you do if bacterial scorch is present? There is probably nothing you can do to keep the tree from dying. You can help by pruning out dead wood as it appears. Start thinking of tree-replacement options and plant something that is not known to host this disease. Be sure to pick a species that does well in the site. Investigate drainage pattern, soil type, amount of sunlight, and any oddities of the location. No fungicides, insecticides, or bactericides can be sprayed on a tree to positively, effectively prevent or cure this disease. There is an antibiotic called oxytetracycline present in some commercially available injectable products intended to combat Xylella. There is not a great deal of research in this area; but work shows that in some cases, oxytetracycline suppresses Xylella and may provide temporary symptom suppression when injected into trees. Researchers in Kentucky have tried such injections and do not see any benefit. National Park Service researchers have seen only short-term benefits. Injections may need to be repeated as frequently as every year, can be costly, and afford no guarantees.