A research team from the Faculty of Natural Sciences at Mendel University has identified a new species of harmful fungus beneath the bark of a royal walnut tree near Brno. The new species has been described and named Ophiostoma juglandis. Scientists have established that this fungus acts as a disease, spreading gradually on trees and causing necrosis. The results of the study will be used by institutions dealing with plant protection and the monitoring of harmful organisms.
The fungus is being spread by a non-native bark beetle that has been found on walnut trees in Europe for the past 15 years. The insect carries the spores of the fungus on its body, housed in small cavities on its shell.
The spread of a non-native species within the country has raised significant interest among scientists. Jiří Foit, a professor from the Department of Forest Protection and Wildlife Management at MENDELU, explains that “five years ago, the walnut bark beetle (Dryocoetes himalayensis) originating from Asia was discovered in the Czech Republic. This species is spreading mainly in the lowlands in the Czech Republic and is involved in the withering of black walnut and king walnut. Years ago, scientists also noticed dark necrosis forming around the furrows created by the bark beetle in the bark of trees, and a dark liquid oozing from the surface of the bark.”
Scientists from MENDELU and the Institute of Microbiology of the Czech Academy of Sciences have confirmed that the walnut ophiostoma is actively spread by the bark beetle. Inoculation tests showed that the fungus causes damage to the living trunk of the tree even without the presence of a bark beetle, and that both the king walnut and the black walnut are susceptible to fungal infection.
The scientific team later isolated a fungus from the affected wood, which could not be classified among previously known species.
The genus Ophiostoma causes so-called tracheomycosis disease in trees, which leads the crown to dry out, associated with drooping and yellowing of the leaves. For example, Dutch elm disease (Ophiostoma novo-ulmi), another disease well-known by scientists, has been significantly reducing the elm population in Europe for over a century.
“Unlike stem-spore fungi, such as those of the genus Ophiostoma, these fungi are not visible to the eye,” explained Michal Tomšovský, one of the co-authors of the study. “The infected tree is manifested by drying out and dark necrosis forms on the trunk, around the holes created by the bark beetle, which is caused by this fungus. Given the common occurrence of the beetle and the fungus, it is difficult to clearly determine which of the two organisms is the immediate cause of the deterioration of the health of the tree. It is probably their simultaneous action.”
Neither type of walnut tree is native to the Czech Republic, yet both are considered important tree species. “The king walnut tree, which grows mostly in the open countryside, gardens and orchards, stands out for its fruits,” said Josef Kašák from MENDELU University. “Its nutritionally rich walnuts are popular and widely used in the food industry. In contrast, the black walnut tree, grown in lowland forests, is valued primarily for its high-quality and decorative wood. Therefore, it is necessary to pay due attention to the health of these tree species.”
This new discovery provides valuable findings for pathogen detection, and will assist researchers and institutions involved in plant protection and the monitoring of harmful organisms.
