Symptoms and Signs
Yellow foliage, shriveled fruit, and little or no new growth are symptoms of Rosellinia root rot. Cottony, white mycelia cover small feeder roots, and roots decay. Mycelia grow into soil and upward in the tree, forming small, pale patches under or in bark of major roots, the root crown, and lower trunk, which eventually decay. Older mycelium become gray or black. The fungus can also cause a purple canker in wood at the root crown of young trees. Diseased trees will defoliate and always die prematurely, usually within 1 to 3 years of initial infection.
Comments on the Disease
Rosellinia root rot is not common in avocado in the United States. Although uncommon, when present, it is a very serious disease and requires prompt action to prevent its spread to other trees.
Rosellinia root rot is also called white root rot in reference to its pale mycelium. The fungus persists for years in buried wood and organic matter in soil. It spreads to nearby trees through root grafts and can also be moved longer distances in infected soil or wood. Spores apparently are not important in causing disease.
The whitish mycelial patches of Rosellinia resemble those of Armillaria, but Rosellinia mycelia lack the characteristic mushroomlike odor produced by Armillaria. One method to diagnose Roselliniais to seal infected wood, roots, or soil in a moist container. Extensive white mycelium will grow within a few days. However, because of its severity and persistence, seek expert assistance if Rosellinia root rot is suspected.
The biology and management are much the same as described for Armillaria. Uproot and dispose of infected trees and roots. Remove immediately adjacent trees that may also be infected. Remove as many root pieces from soil as possible and trench around the infected site to break root grafts. Establish a dry zone and prevent soil movement or water runoff from that site. Fumigate or solarize the ground well before replanting.