Developments in Research
In 1942 a report from the Wisconsin Agricultural Experiment Station revealed that the oak wilt disease was caused by a fungus, and research programs were started early in Wisconsin and Iowa. Neighboring states were quick to follow as surveys showed a wider distribution of the disease. Now almost every state in which oak wilt occurs is taking part in efforts to learn more about the disease and its causal agent so that practical control measures may be applied before the spread of the disease gets out of hand. The National Oak Wilt Research Committee at Memphis, Tennessee, supports in part an intensive oak wilt research program in coordination with several midwestern universities and with the U.S.D.A., Bureau of Forest Pathology.
Until recently the causal fungus of oak wilt was known only in its asexual or imperfect form living in the sap stream of infected trees. The most important question to be answered now is how the fungus spreads over long distances from diseased to healthy trees. Before this could be accomplished, however, we had to know how the fungus escapes from the inside to the outside of diseased trees where it can be exposed to agents of dissemination.
In the late summer of 1951 clearly visible mycelial mats of the oak wilt fungus were found in Illinois under the loose bark of wilt-killed trees. These mats were usually located beneath cracks in the bark; thus, they were exposed to the outside air and to visiting insects. Most wilt-killed trees contain beneath the bark numerous insect larvae of wood and bark boring beetles. Larvae were frequently found in direct contact with mycelial mats of the fungus. Larvae of the two-lined chestnut borer, Agrilus bilineatus, were most abundant, but larvae of species of the families Scolytidae and Cerambycidae were also present in large numbers.
In addition to the mycelial mat under the bark there was often present a thick dark pad usually in the center of the mat. It is not known yet what part this pad plays in the life history of the fungus but we do know that it is produced by the same fungus which causes oak wilt.
We also found in Illinois that the oak wilt fungus often develops into visible mats from chips of bark and wood that have been chopped from wilt-killed trees and allowed to lie on the moist forest floor. This should be remembered when considering sanitation as a partial means of controlling the disease.
In 1951 the sexual or perfect form of the oak wilt fungus was produced on laboratory media in Missouri by crossing different strains of the fungus. The sexual form is recognized by the appearance of microscopic, black, short-beaked fruiting structures or perithecia that are filled with sticky ascospores. This sexual form is a species of Endoconidiophora.
The sexual form of the fungus was first found in nature in Illinois in the autumn of 1951. The perithecia are produced on the mycelial mats beneath the loose and sometimes cracked bark of diseased oaks. Both the ascospores of the sexual form and the endospores or conidia of the asexual form will cause wilt if the spores are injected into oak trees.
From the foregoing information it is apparent that several methods by which the disease might be spread over long distances are possible. First, and what seems to be most probable, is transmission by insects. Adult beetles, such as the two-lined chestnut borer, which emerge from dead trees in the spring and feed on the leaves of healthy trees might transmit the spores of the fungus. Other insects might feed on the fungus mats that are exposed through cracks in the bark and carry both the sticky ascospores and conidia to other trees. Additional agents that must be considered are woodpeckers, squirrels and air currents.
Besides searching for the vector or vectors that spread the disease other important studies are in progress. Among these is the consideration of chemotherapy as a possible means of controlling oak wilt. For our purpose, plant chemotherapy may be defined as the control of disease by chemicals which are introduced into the plant. According to Dr. Paul Hoffman of the Illinois Natural History Survey, a number of chemicals have shown promise in curing small diseased oak trees when treated in a very early stage of the disease. In one instance, trees that were inoculated with the oak wilt fungus then treated with chemicals 2 years ago are still alive. The most promising results were obtained by injecting the chemicals into the soil where they are taken up by the roots and by applying chemicals directly to the foliage in a spray. Trunk injection showed least promise because of the limited distribution of the chemicals through the tree.