Johann Bruhn

Johann Bruhn

Research Associate Professor Emeritus

Division of Plant Sciences

B.S., Forest Science, Utah State University
M.S., Forest Health, University of Michigan
Ph.D., Plant Pathology, University of California, Berkeley

Forest fungi are among the most influential components of forest communities. Nearly all plant species require mutualistic root associations with mycorrhizal ("fungus-root") fungi to survive (e.g., truffles). Another immense variety of fungi perform the indispensable function of nutrient recycling by decomposing the leaves and wood of plants (e.g., the edible wood decay fungi Lentinula edodes, Pleurotus spp., and Stropharia rugoso-annulata); some of these fungi (e.g., the wood decay genera Armillaria and Heterobasidion) have the ability to begin recycling plants even before they die. Still other fungi have become so specialized that they can only cause disease of living trees (e.g., the oak wilt fungus Ceratocystis fagacearum); they lack the ability to compete with the recyclers once the tree dies. Johann Bruhn is interested in improving our understanding of the ecology of these groups of fungi and developing tools for their management. His current research highlights primarily the ecology and epidemiology of Armillaria species, the cultivation of specialty fungi in agroforestry settings, and the epidemiology and management of oak wilt.

Armillaria is a wood decay genus comprising species that vary greatly in their behavior toward different kinds of trees (e.g., conifers vs. hardwoods, etc.), their abilities to colonize and even kill living trees, their competitive capabilities, and their environmental tolerances (e.g., drought, heat, and soil texture differences). Armillaria species also vary tremendously in the abilities of fungal individuals (genets) to grow through the forest floor, traversing among woody food bases by means of root-like rhizomorphs. As a result, the maximum potential territories occupied by individual genets also vary greatly by species (perhaps up to hundreds of hectares), and Armillaria genets can be mapped in the forest floor as mosaics of spatially continuous territorial individuals. In many cases, Armillaria genets are clearly the largest organisms in the landscapes they occupy. Because Armillaria species and genets differ in pathogenicity and virulence, the mosaic(s) of Armillaria genets occupying a landscape interact with existing vegetation and site factors to determine forest community structure. The dynamics of these relationships need to be explained if we are to understand forest community processes and predict the future effects of human activities on forested ecosystems. Bruhn is studying the behavior of various Armillaria species with respect to forest management activities and environmental variation.

Ceratocystis fagacearum causes the disease known as oak wilt. Asexual spores of the fungus are carried in the sapstream across root grafts from diseased trees into surrounding healthy trees of the same species. In most cases, tree-to-tree spread through root grafts continues indefinitely, resulting in expanding pockets of mortality. Barriers to expansion of mortality pockets can be effectively established by breaking the root graft connections between infected trees and the surrounding susceptible oak populations. Bruhn is currently developing improved techniques of root graft barrier establishment. To effectively apply these techniques, though, Bruhn and his colleauges need to know more about the spatial geometry of susceptible oak species' root systems and the relationships between stem and root system sizes in various soil types. They have developed techniques for modeling these relationships, and plan to expand their modeling efforts.