Key Features
- Canopy dieback from top down
- Black stringy web beneath bark
- Mushrooms emerging from roots
Symptoms
Armillaria root rots affects many species of deciduous and evergreen trees, shrubs, and vines. The pathogen grows on underground portions of the plant and under the bark making detection difficult unless characteristic mushrooms are produced around the base of the tree or symptoms become obvious in the crown. Above-ground symptoms on individual trees are variable and not specific to this disease or even to root diseases in general. Often, no symptoms are present even when the disease is in a very advanced stage. Commonly observed symptoms include thinning foliage, leaf discoloration, chlorosis proceeding to browning, branch die back, and reduced vigor. Heavy production of cones or fruit (stress crop) is an indication that the tree may be stressed. Sap bleeding from the crown may be seen in conifers. If Armillaria is present, removing the bark covering infections will expose the white mycelial mats, or black, shoe-string like rhizomorphs that grow between the wood and the bark. The white mycelial mats have irregular, fan-like ridges. The thick mats decompose, leaving impressions on the resin-impregnated inner bark.
Biology
The pathogen has three means of spreading to other trees. Infection of neighboring trees occurs when mycelia grow directly between the root systems of two trees. Rhizomorphs can stretch up to 10 feet through upper soil layers to infect stressed trees. Mushrooms produce spores, which are wind-dispersed and can infect trees in new locations. Armillaria can survive in tree stumps and roots. East of the Mississippi, armillaria rarely infects healthy trees, but stressed trees are highly susceptible.
Management Recommendations
The key to successful management of Armillaria is to keep plants healthy in the first place. Reduce damage to the crown and roots caused by lawn care, garden, and other equipment. Maintain plant vigor by irrigating and fertilizing according to the plants requirements. Remove stumps and root systems of previously affected trees. Planting more resistant species will avoid the disease.
Effective Pesticides
Pesticides are neither available nor recommended for managing this disease.
Resources
