Symptoms and Signs
Trees with Armillaria root rot have thin canopies with yellow leaves and twig, shoot, or limb dieback. Infected roots have white rot wood decay with white to yellowish fan-shaped mycelial mats (plaques) between the bark and the wood. Dark brown to black rhizomorphs sometimes can be seen on the root surface.
Comments on the Disease
Armillaria survives on dead roots. Paradox, Northern California Black, and English rootstocks are susceptible, but disease response is variable. It is common to see circular patterns (i.e., infection centers) of dead or missing trees in orchards with an A. mellea problem. These centers expand outward as roots of bordering trees come in contact with infected roots.
Management of Armillaria root and crown rot relies primarily on preventing infection of new trees. You can reduce the chances of infection by carefully preparing planting sites for new orchards and by practicing good sanitation and early detection. Do not rip or disc in infected orchards to avoid spreading the inoculum. Overly wet soil conditions favor development of this disease, so take measures to correct this condition throughout the orchard.
Choose a less-susceptible rootstock if Armillaria root rot is present. Avoid using English rootstock, which is highly susceptible, in sites known to be infested with A. mellea. Paradox rootstock is generally more tolerant of Armillaria root rot than Northern California Black rootstock. However, neither of these rootstocks is tolerant if an extremely aggressive strain of Armillaria is present.
No effective fumigants are currently available for managing Armillaria in walnut.
Be careful not to introduce Armillaria into an established orchard via diseased root pieces that may be transported on equipment or in surface water.
Check trees for symptoms of Armillaria root rot in late summer when dead or declining trees are most obvious. Look for mycelial plaques under bark or mushrooms at the bases of trees after a rain, generally from October to April.
Saving Infected Trees
Once symptoms of Armillaria root rot appear, it may be possible to slow or stop spread of the pathogen within an infected tree by exposing the crown and upper roots and allowing them to dry out.
- Remove soil from the base down to a depth of 9 to 12 inches in spring.
- Keep the crown and upper roots exposed to the air and avoid wetting them for the duration of the growing season.
- Fill the soil back in before rains start in the fall.
This procedure may allow the diseased tree to regrow, but is not always successful.
If trees cannot be saved, remove infected and dead trees without delay. The fungus can transmit more quickly through roots of dead trees to nearby healthy trees. The disease spreads as healthy roots come in contact with infected roots of adjacent trees.