FOREST PATHOLOGY Root and Butt Rot Diseases

Diseases caused by root rots figure prominently amongst the most-studied pathologies of forest trees. Indeed, root and butt rots cause more economic damage to commercial forestry in the temperate world than any other known type of disease. While this notoriety underlines the negative impact of this type of disease on timber production, a much more dynamic and positive role can be assigned to root rots in natural ecosystems. Root rots are one of the driving forces ensuring spatial and temporal diversification of forests. While root rots as a whole encompass both generalistic and host-specific pathogens, aggressive primary microbes, and secondary opportunistic ones, their overall effect at the stand level is to accelerate, and sometimes cause, a patchiness in which some tree species are preferentially affected and weaker individuals culled. Because of their selectivity at the species and at the individual tree level, root diseases play a significant role in determining the structure and composition of a forest. Individual trees and/or clusters of individuals are taken out, and the gaps created allow for tree regeneration. Often, more resistant seral species will substitute the more susceptible pioneering species leading to forest succession. Ecotones between gaps and closed canopy offer rich and diverse habitats, home to a substantial amount of the local biodiversity. Finally, a further outcome of root rots is nutrient recycling: this is achieved by breaking down the chemically complex woody substrate in a synergistic activity with other wood decay fungi, bacteria, and wood-boring insects. As stated above, concerns about root rots become serious when timber production is involved. Unfortunately, most habitat modifications, including but not limited to those related to logging, appear to increase the damage caused by root rots. In many cases, human activities allow for the establishment of or increase in root rots. Once disease is established, an irreversible process starts in which root rots will play a significant role in shaping the future of that forest. When root diseases affect ecosystems characterized by poor soils, limited host variability, or limiting climatic conditions, their impact may be significant even in the absence of further human activities.