In issue No. 5 of this newsletter, we discussed scorch of trees. That discussion involved noninfectious leaf scorch, but there is also an infectious bacterial leaf scorch that is less well known. The infectious leaf scorch is caused by the bacterium Xyllela fastidiosa. Although this disease is found predominantly in eastern and southern states, it is also frequently found in western Kentucky and Indiana. It has been reported in several areas of Illinois, and we are aware of two laboratory confirmations of its presence. Its occurrence in the state is probably greater, but most growers cannot identify its symptoms.
The most frequent hosts of this disease include elm, oak, sycamore, mulberry, sweet gum, sugar maple, and red maple. A recent confirmation by AGDIA, Inc., in Indiana involved an Illinois river birch with bacterial leaf scorch. Look for scorch symptoms that occur in early summer (now) 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. Symptoms will show on oldest leaves first, and they progress toward the stem tips. Noninfectious scorch starts on the newest leaves and moves down the stem. Bacterial scorch often allows infected leaves to remain on the tree until the fall.
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.
If you have a sample with a history of progressive scorching and leaf retention, you may want to seek laboratory help. The Plant Clinic cannot perform the required serological test, but we can determine whether further assistance is needed. Fresh, scorched leaves, still attached to small stems, are needed for testing. Call before sending this type of sample so that the lab can be prepared to get it to the appropriate people before it dries.