by Fred Roth, Ph.D. Cal Poly University, Pomona and Consulting Arborist
Many American sweetgum or liquidambar trees are declining in inland Southern California due to infection by a bacterial pathogen that causes a disease known as “bacterial scorch.” This pathogen is not new in California, but it is relatively new in liquidambar trees because of the introduction, about 15 years ago, of a new insect pest imported from the southern states called the glassywing sharpshooter, or GWSS as we affectionately call it. This insect, with strong sucking mouth parts, is able to extract sap from twigs of many plants much like an aphid feeds on leaves. In the process of feeding, it injects bacteria from an earlier feed into the vascular tissue of the new host. The bacteria multiply to such high numbers that they literally clog up the plant and water cannot reach the leaves. You may also have noticed a fine “rain” falling from your trees, particularly in winter. This is a kind of sap which passes through the insect during feeding. Studies of the effect of this sap leakage on humans has shown that any more than 4 droplets a minute is quite annoying even if you don’t know it is bug poop. The bacterial disease had a much lower profile before the GWSS arrived because our native sharpshooter doesn’t reach the huge populations that the GWSS does, probably because GWSS is not kept under control by a host of parasitic and predatory organisms found in its native ecosystems.
There is no control for the bacterial disease currently. The course of the disease in any individual tree is unpredictable. Some trees seem to die in as little as two years, while others in thesame stand do not show symptoms for many years. Generally trees decline by death of individual branches at first (Figure 1). You may see many healthy leaves in a tree with major dieback.
This is not a specific symptom and may be caused by several problems, but none of them are as widespread as bacterial scorch. Usually there are few symptomatic leaves even on a tree in advanced decline, but marginal scorch, or “burning” of the leaf tips and edges caused by vascular disfunction is the most common (Figure 2). You may also see some yellowing, but this is also common on liquidambars growing in alkaline soil.Many important ornamental plants including oleander, purple plum, and olive are susceptible to bacterial scorch, but the greatest concern is for California’s grape and wine industries which are also likely to be affected. Efforts have been underway for some time to introduce biological controls for GWSS, but these have not been adequate to manage the populations of the vector yet. There is still hope as long as research efforts are properly funded. This is one of the things your tax money goes to.