Today’s disease photos come courtesy of Stuart Gauthier in Vermillion Parish. Several Chinese elms in a landscape bed had died suddenly, and when the bark was removed from the base of the tree, white fungal mycelium that is typically associated with Armillaria root rot was observed. This mycelial mat was even found under the bark several feet above the ground. Armillaria root rot (or oak root rot) is caused by Armillaria mellea, but a second species of Armillaria, A. tabescens, also occurs in the eastern U.S. Both of these fungi cause root rots on a wide range of hosts, and the disease is often found in areas where oaks or other hardwoods existed previously. Once the pathogen reaches the crown of the plant, it can girdle resulting in rapid death of the plant. Management of this disease is difficult because it has such a wide host range and the fact that it can survive in infected roots deep in the soil for many years. The only practical means of control is to replant with species of plants that are resistant or immune to the disease.
Donald M. Ferrin, Ph.D., Louisiana State University Agricultural Center, Baton Rouge, LA