Earth & Sky

Our thanks to the following individual who assisted in the preparation of this script:

Dr. Jennifer Juzwik
Research Plant Pathologist
Project Leader
North Central Research Station
USDA Forest Service
Saint Paul, MN
also serving as
Adjunct Professor
Plant Pathology
University of Minnesota

The following articles and websites were used in preparing this script:

South, Carol. ““Oak wilt concerns area citizens.”“:http://www.record-eagle.com/herald/2000/30oak.htm Grand Traverse Herald, Grand Traverse, Michigan, August 30, 2000.

Bragg, Roy. “War on oak wilt takes root in Medina.” Houston Chronicle, Sept. 22, 1991, State section, page 1.

Rebuffoni, Dean. “A threat to the urban forest.” Minneapolis Star Tribune, August 19, 1996,. page 1 B.

Juzwik, Jennifer, Kory R. Cease, and Jason M. Meyer. “Acquisition of Ophiostoma quercus and Ceratocystis fagacearum by nitidulids from O. quercus-colonized oak wilt mats. Plant Disease, February 1998, 82 (2): 239-243.

Juzwik, Jennifer, and Jason M. Meyer. “Colonization of oak wilt fungal mats by Ophiostoma piceae during spring in Minnesota.” Plant Disease, 1997, 81 (4):410-414.

Author’s notes:

In 1930, an Asian fungus brought to the United States was the cause of Dutch Elm disease, which wiped out almost all of the nation’s majestic, vase-shaped American elm trees. Oak wilt, which is killing trees in the eastern half of the United States (from Minnesota south to Texas, eastward to South Carolina, and north to Pennsylvania), has been dubbed by some foresters “the new Dutch Elm disease.”

The fungus that causes oak wilt, Ceratocystic fagacearum, is native to the United States. It affects both the group of trees that biologists call “red oaks” (some species include northern red oak, pin oak, black oak, and Spanish oak) and the “white oak” group (species include burr oak and white oak). Red oaks are the most susceptible to the disease and usually die within the season that they are infected. White oaks are more tolerant of the infection; burr oaks may live for up to three years after being infected, and white oaks may linger for up to five years or even recover.

The fungus spreads from tree to tree in two different ways. One is via “root grafts”; when two oak trees of the same species grow close together, their roots tend to grow together and become a shared system, and the fungus is transmitted from one tree to another via the roots. (Root grafting happens most commonly between red oaks; in Texas, stands of live oaks are usually clones that share a single root system).

The oak wilt fungus can also be carried from tree to tree by insects. This happens in spring, when a “fungal mat” forms under the bark of an infected tree, causing the bark to break open. Beetlesin particular, the ones most people know as “picnic beetles” or “beer bugs” for their tendency to show up at outdoor meals-are attracted to the fungal mat and feed on it. While they are there, they accidentally pick up fungal spores, much the way a bee or hummingbird may accidentally pick up pollen when it sips nectar from a flower. If a beetle dusted with spores is then attracted to another tree that has a fresh wound oozings sweet sap, the spores enter the tree through the wound and the fungus starts to grow.

The fact that beetles are a vector for the disease make timing an important issue when applying a treatment like the biocontrol spray, says Dr. Juzwik. She says the spray should be applied early in the spring, so that the oak wilt spores are eliminated from the fungal mats BEFORE the beetles come to feed and accidentally pick up the spores.

Until the biocontrol spray is available, tree-owners can still take steps to help prevent the spread of oak wilt disease. One of the most basic steps is to avoid pruning or cutting down oak trees during the spring. An injured tree is more likely to attract the beetles that spread the disease.

Because infected trees can continue to spread fungal spores through their roots for years after they die, severing the root grafts between connected trees is another useful measure; researchers have developed a special “vibratory plow” that can make precise cuts and is maneuverable enough to use in a woodlot.

Dr. Juzwik says another recommended step is cutting down dead trees BEFORE spring arrives, cutting the wood into firewood lengths, and sealing the woodpile under plastic. After a few months of summer heat, any fungus in the tree will be killed.

Since oak wilt is native to North America, one question that arises is, why has this disease become so common in recent years?. Dr. Juzwik says, the disease seems to be intensifying mostly in urbanizing areas-in places where people are buying building lots in wooded settings. As the lots are prepared for homesites (most of this work occurs in the spring, when the disease is spread by the beetles,) trees are inevitably nicked and damaged by bulldozers or other machinery. Dr. Juzwik says homeowners can urge contractors to protect trees with fencing and otherwise work to minimize the damage to live trees.

Oak wilt fungus has been a serious problem for homeowners, because the value of your property goes down when your mature shade trees die. But ecologists say the fungus may actually be a GOOD thing for forest ecosystems. In recent years, the national policy to prevent forest fires may have allowed some stands of oak trees to become TOO dense. When trees die of oak wilt, however, GAPS open in the forest canopy-and these sunny clearings are attractive to wildlife.