Most gardeners, nurserymen, and arborists are aware of noninfectious leaf scorch. Few are aware of an infectious leaf scorch caused by a xylem-inhabiting bacterium (xib).
The noninfectious leaf scorch has been common of late. It occurs whenever water cannot be translocated to the foliage rapidly enough to replace lost moisture. The causes vary and might include root injury, root rot, poor soil conditions, high winds, flooding, drought, transplant shock, and many other scenarios. A thorough discussion of this problem is given in Report on Plant Diseases No. 520. Symptoms generally include a necrosis of leaf edges and foliar tissue between the veins. Symptoms are most intense on the newest leaves. These areas are the last to receive water from the roots and the first to show a lack of uptake.
The infectious leaf scorch is called bacterial leaf scorch and is caused by a bacterium called Xylella fastidiosa. It takes its name from the fact that it is limited to the xylem; it cannot be cultured in traditional laboratories. Positive diagnosis relies on serological methods or specialized staining of the xylem tissues.
Bacterial scorch occurs mostly in the eastern and southern states but has been reported in Illinois and nearby states. The scorch occurs most commonly on elm, oak, sycamore, mulberry, and red maple. Look for symptoms that occur in early summer to midsummer and intensify in late summer. The symptoms spread within a tree from year to year, branch by branch. Unlike noninfectious scorch, bacterial scorch develops first on the oldest leaves and progresses toward the branch tip. Infected leaves often remain attached until fall.
If you have a tree showing a history of progressive scorching as described above, you may wish to seek laboratory help to test for the possibility of bacterial scorch. Call the Plant Clinic at (217) 333-0519 if you have a suspect tree. Staff can help with sampling and mailing advice.