Saving the trees that kill each other - oak wilt

American Forests, Jan-Feb, 1995 by Norah Deakin Davis

Trenching and neighborhood teamwork are two of the tactics used to fight oak wilt, a wicked disease that is epidemic in Austin.

BILL PETERS went to a meeting of his neighborhood association and learned to his horror that his yard is one of the centers of a virulent disease attacking oak trees. "I had seen dead trees, but I didn't know there was a real problem until people focused on my yard," recalls Peters, who has been a resident of Austin, Texas, for 25 years. "They had made drawings of the neighborhood from aerial photographs, and I recognized my corner."

It was too late to save his trees, but Peters found himself called on to help save his neighbors'.

The epidemic attacking trees in Austin is called "oak wilt." The disease is caused by a fungus that clogs the water-conducting vessels of infected trees. The trees become dehydrated as water is prevented from moving from the root system to the crown. The leaves wilt, and, almost invariably, a quick death follows. During the past 20 years, oak wilt has killed more than 10,000 trees in Austin. It is considered one of the most virulent tree diseases in the United States.

Disease or natural disaster can strike any community and can cause a devastating loss of tree canopy. Should that happen, the effect is significant: Your neighborhood could be stripped of its trees, your house could lose its shade, and, as your air conditioner works harder to cool your home, you could find your utility bills rising. The best way to protect your trees? Plant a diversity of species. That way, you will not lose your entire tree canopy (see "Whose Trees Are at Risk?" on page 38).

Bill Peters, 61, whose business is wholesale industrial supply, has lost six trees so far; in the end, all his oaks will probably die. A few elms, ashes, and magnolias will remain, leaving less than half the original tree canopy to protect his home from the harsh Texas sun. The challenge for Peters and many who have faced such a loss anywhere in the U.S. is to find ways to reestablish a healthy tree canopy.

Oak wilt is caused by a fungus called Ceratocystis fagacearum. Many oaks are resistant, but live oaks (Quercus fusiformis and Q. virginiana) and the red-oak family, particularly Spanish (Q. texana) and blackjack oaks (Q marilandica), are quite susceptible. Mortality is 100 percent in the red-oak family, only slightly less in live oaks.

The fungus is transmitted in two ways--by airborne spread and underground through roots.

In late fall and early spring, diseased red oaks produce "fungal mats"--masses of oak-wilt spores--beneath their bark. These sweet-smelling mats attract free-flying, sap-feeding insects called nitidulid beetles. The fungal spores adhere to the beetles' bodies as they feed. If air currents blow the insects to uninfected oak trees, the beetles deposit the spores as they feed on sap oozing from any fresh wounds caused by pruning cuts or broken limbs. Scientists believe the beetles can spread the spores from fungal mats to uninfected trees a mile away. Only red oaks--not live oaks--produce the fungal mats that initiate this process of establishing new centers of disease. For this epidemic, the red oak is the Typhoid Mary.

Live oaks generally grow in large stands with an interconnected root network, so they are particularly susceptible to the second, localized form of transmittal. A characteristic trait of live oaks is prolific root sprouting, and when the sprouts become mature trees, the root connections remain. The disease travels from an infected tree along this communal root structure to adjacent healthy oaks. Oak wilt can expand in this manner by as much as 100 feet a year.

In live oaks, the symptoms are a distinctive yellowing and browning of the mid-vein and lateral veins of the leaf while the remainder stays green. The incubation period--from the time the fungus is introduced to the appearance of symptoms--is brief. Once live oaks are infected, most die within three months to a year. Symptoms in red oaks are less distinctive. The sign to watch for is leaves that turn pale green to brown very quickly. Red oaks die within two weeks to several months after symptoms appear.

At this time there is no cure for oak wilt, so the only treatment is preventative measures and suppression. Precautions include removing diseased red oaks before they can form fungal mats. Bill Peters cut a couple of his dead trees even before he knew what had killed them.

Another preventative measure is to avoid pruning during fall and spring when the fungal mats are forming and the nitidulid beetle is most active. In addition, an extremely important precaution is to seal wounds immediately by covering them with an asphalt-based paint. Normally foresters no longer recommend this treatment for pruning cuts or other wounds, but in the case of oak wilt, research has shown that sealing prevents sap from attracting the beetles. The paint must be applied within 48 hours. After that, the wound is no longer attractive to the insects, and thus it is unnecessary to paint old wounds.