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Amy K. Stone, Ohio State University Extension, Lucas County; Joseph F. Boggs, James A. Chatfield, Mary Maloney, Erik A. Draper, Hannah Mathers, Pamela J. Bennett, Jane C. Martin, Marianne Riofrio, |
A Scorching Devastation Hits Oak Trees
John Hartman conducted an information session on bacterial leaf scorch of oaks. The problem was of great interest to the group since it has been devastating to a number of oak species in central Kentucky, but it has yet to be found in Ohio. The impact was dramatic at Gainesway Farm as a large number of mature pin oaks had already been lost and more were infected.
The causal agent is a xylem-inhabiting bacterium (Xylella fastidiosa) that is thought to be transmitted from tree to tree by xylem-feeding insects such as leafhoppers (e.g., sharpshooter leafhoppers) and treehoppers (e.g., oak treehopper). However, since the disease spreads slowly from tree to tree, it is speculated that the insects are not very efficient vectors of the disease.
Bacterial leaf scorch has long been noted in the Atlantic and Gulf states, and it has spread inland to the Ohio River valley. The disease has been detected in Kentucky for a number of years and has recently been found in southern Indiana.
Ultimately, infected trees die from the disease, and John indicated that some Lexington neighborhoods have lost more than 70% of their mature red and pin oaks. While oaks remain the most dominant species affected in Kentucky, the disease has also been found on maples including red, silver, and sugar maple, as well as sycamore.
Symptoms vary somewhat between oak species. On pin oaks, scorching appears along the leaf margins and progresses inward toward the midvein. There is often a yellowish margin between the scorched leaf tissue and green tissue. On red oaks, the scorch typically appears at the leaf tip and progresses up the leaf towards the petiole. The overall pattern makes the leaves look like they had been dipped in chocolate.
The timing of leaf scorch symptoms can present a diagnostic challenge. Infected trees often show no leaf scorch in the spring and early to mid summer. Symptoms usually first appear as a few scorched leaves sometime in mid-to-late August, but the scorching expands rapidly to involve other leaves in September and October.
The challenge is that the symptoms can be easily mistaken for physiological leaf scorch or early fall color. However, since a number of other diseases, as well as cultural problems, can mimic bacterial leaf scorch symptoms, John strongly recommended that suspected infections be confirmed by sending samples to a diagnostic clinic before concluding the tree is infected with the bacterium.
There is no cure for bacterial leaf scorch, so one should expect diseased trees to be gradually lost over the years. The best remedy for bacterial leaf scorch is tree replacement. However, in the meantime, infected trees can be made to look somewhat presentable for a few more years if the dead wood is pruned out. Since little is known regarding which species of insects serve as vectors for the disease, or exactly how disease transmission occurs, there are no insecticide recommendations available for disease prevention.