For those of you with oaks, it is time to start watching them for development of bacterial scorch. In fact, you may have already seen symptoms. Other tree species can also become infected, but oaks seem to be the preferred host species in Illinois. Scorch refers to leaves that are brown along the margins and sometimes between veins. Scorch is a symptom and does not identify a specific cause. It may be caused by environmental stress such flooding, drought, hot temperatures, drying winds, clay soil, compaction, construction injury, or any factor that inhibits water movement to the leaves. Scorch from these sorts of stress factors is usually not fatal. Plants generally recover with adequate moisture and time. Trees infected with bacterial scorch decline and die in about 3 to 6 years.
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 predominantly a disease found in eastern and southern states, it is now also frequently found in western Kentucky and Indiana. In the past few years, Illinois has had confirmed cases of BLS on pin oaks, shingle oak, and bur oak. The bacterial pathogen cannot be isolated 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 doing ELISA tests. One nearby is AGDIA, Inc., in Elkhardt, Indiana. The pin oak and shingle oak cases reported here were confirmed by AGDIA.
The most frequent hosts of this disease 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 species in Illinois. It may be that we have attributed scorching to other causes on the other hosts. 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. The 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 be better next year but 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. They drop leaves early. If you have seen a slow but progressive 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.
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. 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 prepare 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. Consult AGDIA at http://www.agdia.com or call (219)264-2014 or (800)62-AGDIA. If you prefer to go through the Plant Clinic, we can test for other problems but would 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 would do well in the site. Investigate drainage pattern, soil type, amount of sunlight, and any oddities about the location. There are not any fungicides, insecticides, or bactericides that can be sprayed on a tree to 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 provides 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. We will keep you posted as new information on this disease and its management is available.