It's time to watch oaks for bacterial scorch. Other tree species can also become infected, but oaks seem to be the preferred host in Illinois.
Bacterial leaf scorch (BLS) is an infectious disease that spreads systemically and causes a slow decline and death of the tree. The disease is caused by the bacterium Xylella fastidiosa. Although BLS was mainly found in eastern and southern states, it is now now frequent in western Kentucky and Indiana. Last summer, we confirmed six cases on pin oaks in cen-tral Illinois. A case on shingle oak was also confirmed in Springfield. The bacterial pathogen cannot be iso-lated in the lab like most bacteria. It can be confirmed using serological techniques. Because our lab is not equipped for this test, we refer people to private labs. One nearby lab is AGDIA, Inc., in Elkhardt, Indiana.
Frequent hosts in the United States include elm, oak, sycamore, mulberry, sweetgum, sugar maple, and red maple. As stated, however, oak seems to be our most common host in Illinois. Look for scorch symp-toms 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. Symptoms occur first on one branch or section of branches and slowly spread in the tree from year to year. It is one of those situations that you hope will improve next year but that only gets worse. Symptoms often show on oldest leaves first, distinguishing this disease from environmental scorch, which first appears on newest leaves. Of course, diagnosis is never that simple, and oaks are an exception. We did not observe this pattern on pin oaks in Illinois. In fact, most references say that oaks show symptoms on an entire branch at once. We saw symptoms on new leaves on some branches, on older leaves on others, and scattered throughout the tree. Bacterial scorch often allows infected leaves to remain on the tree until the fall. Oaks are again the exception, dropping leaves early. 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 tis-sue. Xylem-feeding leafhoppers and spittlebugs are thought to spread the bacterium. It can also be transmitted between trees via root grafts. The transmission methods must not be effective, though, because we do not see rapid spread from tree to tree.
We recently sent a suspect sample to AGDIA and obtained a report that the sample was elevated above negative but not high enough to call positive. The folks at AGDIA are not trying to be difficult: Samples of BLS do not have high bacterial populations at this time of year. Watch your suspect tree. If conditions worsen by midAugust, try sending a sample.
AGDIA, Inc., has a serological (polyclonal antibody) test for the bacterium that can be done on young twigs and leaves. As of this writing, the fee was $48.25 for one sample and $6.25 for each additional using the same test. It is suggested that you call ahead to be certain you have prepared the correct sample and avoid resampling at your expense. Leaf petiole tissue is preferred, so leaves with green peti-oles 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, we can test for other problems but would have to bill for AGDIA testing as well.
What can you do if bacterial scorch is present? First, probably nothing can keep the tree from dying. You can prune out dead wood as it appears. Start thinking of tree-replacement options, and plant some-thing that is not known to host this disease. Pick a species that does well in the site. Investigate drainage pattern, soil type, amount of sunlight, and oddities about the location.
No fungicides, insecticides, or bactericides can be sprayed on a tree to effectively prevent or cure this disease. There is an antibiotic (oxytetracycline) in some commercially available injectable products intended to combat Xylella. There is not much 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 it and seen no benefit. National Park Service researchers have seen only short-term benefits. Injections may need to be repeated every year, can be costly, and afford no guarantees.