How to Manage Pests
UC Pest Management Guidelines
Armillaria Root Rot (Oak Root Fungus)
Pathogen: Armillaria mellea
(Reviewed 9/16, updated 9/16)
In this Guideline:
SYMPTOMS AND SIGNS (View photos to identify disease signs and symptoms)
Armillaria is a soil-borne fungus that causes a root and trunk rot of avocado. The fungus can become well established in roots and the root crown before any symptoms become visible above ground. Infected trees usually die prematurely, and young trees often die quickly after infection. Mature trees may die quickly or slowly, or may recover at least temporarily if conditions become good for tree growth and poor for disease development.
Wilted, downward-hanging foliage is often the first obvious symptom of Armillaria root rot. Other symptoms include foliage yellowing, leaf drop, and dieback of upper limbs. During the rainy fall and winter, groups of short-lived mushrooms often grow around the base of Armillaria-infected trees.
The most reliable sign of Armillaria root rot is fungal growth in cambial tissue. If trees exhibit aboveground symptoms of infection, cut off bark at the base of the tree and crown to diagnose the presence of Armillaria mycelium. Fungal mycelia are whitish and have a strong mushroom odor. Growth typically occurs in patches in the cambium and inner bark. Large roots can be infected throughout their diameter.
COMMENTS ON THE DISEASE
Armillaria root rot infects many crops and native and ornamental plants. Common hosts include avocado, cherimoya, citrus, and oaks. The fungus persists in infested roots, stumps, and wood in soil, infecting new plantings and spreading to infect nearby plants.
Armillaria mycelia persist for years under the bark of diseased roots or the root crown. Armillaria spreads by any activity that moves soil containing infested wood fragments, such as during cultivation. Most infections are thought to occur when a healthy root grows into or near an infested dead root piece and the fungus moves over. In some situations, the fungus can also spread from tree to tree by cordlike rhizomorphs, which resemble small dark roots. In contrast, healthy avocado roots are lighter-colored, usually light brown to whitish. When pulled apart, rhizomorphs have a cottony interior, while the center of a healthy root is solid and woody. Rhizomorphs grow on buried wood, the surface of diseased roots and root crowns, and short distances on or through soil. Infection occurs when rhizomorphs contact and directly penetrate the healthy roots of adjacent trees which is rare in CA.
Long after the aerial parts of a tree are gone, Armillaria can remain alive in roots and stumps. When avocado trees are planted, new roots grow into contact with Armillaria-infected roots or infested wood pieces, and the new tree becomes infected. Armillaria can also be introduced on infected nursery stock.
Look for diseases and disease-promoting conditions regularly throughout the grove, see MONITORING DISEASES AND DISEASE-PROMOTING CONDITIONS. Provide a good growing environment and proper cultural practices and use good sanitation to manage Armillaria root rot. Providing good drainage and avoiding excess irrigation are important. Armillaria fungus is very susceptible to drying. Excavating soil around the trunk to temporarily air-dry the root crown can prolong the life of citrus trees and may also be effective on avocado, but apparently has not been tested on avocado. Shade any exposed root crowns from sunburn. Once trees die, remove them and any immediately adjacent trees that may also be infected. Remove the stumps and as many root pieces from the soil as possible. Thoroughly clean all soil from equipment and leave soil on-site before removing equipment. Consider replanting only with crops not susceptible to Armillaria.
UC IPM Pest Management Guidelines:
A. Eskalen, Plant Pathology, UC Riverside
Acknowledgment for contributions to Diseases:G. S. Bender, UC Cooperative Extension, San Diego County
H. D. Ohr (emeritus), Plant Pathology, UC Riverside
J. A. Menge, Plant Pathology, UC Riverside
L. J. Marais, Plant Pathology, UC Davis
R. Hofshi, Hofshi Foundation, Fallbrook, CA
J. S. Semancik, Plant Pathology, UC Riverside
J. A. Downer, UC Cooperative Extension, Ventura County
U. C. Kodira, Plant Pathology, UC Davis