Bacterial leaf scorch affecting red oak. (photo by Pam Zipse)

by Pam Zipse (Rutgers Urban Forestry Program) and Rosa Yoo (DEP State Forestry Service)
This article originally appeared in the NJ Woodland Stewards Program online newsletter, and was reprinted with permission.

There are several serious diseases that can attack our oak trees in New Jersey. Two that are of increasing concern are bacterial leaf scorch and oak wilt. This article is intended to help increase your awareness of these two diseases of oaks, explain the differences and similarities between the two, and provide some direction if you suspect the oaks on your property may be affected.

Oak wilt. (photo by Rosa Yoo)

Bacterial leaf scorch (BLS) of oak has been present in NJ for many years. It mainly affects the red oak group (red oak, pin oak, black oak, scarlet oak, etc.), however trees in the white oak group (white oak, swamp white oak, chestnut oak, post oak, etc.) are occasionally impacted. BLS is caused by a bacterium, Xylella fastidiosa, which clogs the xylem vessels preventing water transport in the tree. BLS is spread from alternate host vegetation and from tree to tree by several species of leafhoppers and treehoppers; insects that feed in the xylem. A tremendous amount of research has been conducted regarding BLS, however no permanent cure has been identified. There are antibiotics that can be injected into the tree to kill back the bacteria, but if these injections are not repeated annually the populations of bacteria build up again quickly. Drought heightens the negative effects of BLS, so keeping your oaks watered through dry patches is a good cultural practice. BLS acts slowly, but will eventually kill a tree, although it may take a decade or more.

Oak wilt. (photo by Rosa Yoo)

Oak wilt is a new problem for oaks in NJ, in fact it has not yet been confirmed in NJ, but trees as close as Brooklyn and Long Island have been found infected with oak wilt. Oak wilt can impact trees in both the red oak and white oak groups, however the effects are much more drastic, and progress much more quickly, in red oaks. Oak wilt is caused by a fungus, Ceratocystis fagacearum, which clogs the xylem and phloem vessels cutting off the transport of water and nutrients throughout the tree. Oak wilt is spread by a variety of sap and bark feeding insects (mainly nitidulid beetles) as well as through root grafts (when the roots of nearby trees fuse together). This makes the spread of the oak wilt fungus very difficult to control. There is no chemical treatment for oak wilt, and states that are dealing with this disease are removing infected trees and chipping, burning, or covering the wood to prevent the spread of the fungus. Root grafts are severed by cutting deep trenches at a distance of 100 feet or more from the infected tree. It is believed that the fungus can survive for 5 to 7 years in the soil, so oaks should not be supported in infected areas (replanted or allowed to seed in or sprout) until sufficient time has passed, in order to prevent the fungus from continuing to spread.

The visual symptoms of bacterial leaf scorch and oak wilt can be difficult to tell apart since both manifest as scorched leaves. The most prominent difference should be in the timing of scorch development. Oak wilt will affect the majority of the crown, causing leaves to scorch and both green and scorched leaves to fall, in July. Scorching caused by BLS affects individual branches or sections of the crown and tends to appear in mid to late August and into September, and trees will hold on to these scorched leaves for longer. However, timing of leaf discoloration can vary, creating overlap between oak wilt and BLS symptoms. A red oak tree with oak wilt can scorch, drop all leaves, and die in a matter of weeks or months. Trees in the white oak group can tolerate the oak wilt fungus for several years. Trees can be infected by BLS and oak wilt at the same time, along with other secondary problems, which can make it very difficult to diagnose a specific disease.

Bacterial leaf scorch. (photo by Pam Zipse)

Oaks are important in NJ – the Northern red oak is our state tree, oaks are great mast producers for wildlife, and are important timber species. We should be aware of the problems they face. It may be too late to notice this year, but next year, keep an eye on your oaks (especially the red oaks)!

If you notice leaf scorch or if your oaks are dropping their leaves early, read up on these two diseases of oaks (http://www.state.nj.us/dep/parksandforests/forest/njfs_forest_health.html). There are cultural things you can do to slow the progression of BLS, like watering through droughty times, and pruning out infested branches. Although there is no cure for oak wilt, eradication is most feasible when detected and managed early.

For more information on oak wilt, please contact the NJ Forest Service Forest Health Program at foresthealth@dep.nj.gov. If you suspect that you may have oak wilt on your property, please contact the NJ Department of Agriculture at 609-406-6939.