ARMILLARIA ROOT ROT ( Armillaria ostoyae , formerly A. mellea )
Armillaria root rot is one of many micro-organisms that naturally decay stumps and roots of trees. But Armillaria can also attack living trees. While growing on a dead stump, Armillaria produces root-like structures called rhizomorphs which can grow out into the soil away from the infected stump for distances of up to 60 feet. Larger stumps support more extensive rhizomorph growth, and hardwoods may produce more rhizomorph growth than conifers. Where rhizomorphs contact the root system of a suitable host, they may be able to infect those roots, causing a reduction in growth and/or girdling of the tree.
Fig. 1. Trees killed by Armillaria root rot
Large trees may be able to confine the fungus in infected roots and thereby survive for years, but they grow progressively weaker as roots are killed and eventually become subject to windthrow. Smaller trees may be killed much more quickly. An infected tree in a Christmas tree plantation first shows symptoms of trouble with off-color foliage, and perhaps, stunted growth. Then, usually within two months, browning of the needles and death of the infected tree occurs.
Many other agents may cause the above symptoms. To check for Armillaria, gently remove the bark from the root collar area of a suspicious-looking tree. Look for a dense, white "mat" of fungal tissue growing between the bark and the wood. Armillaria is not likely to be the culprit if the mat is not present.
No chemical treatments are available for use in controlling Armillaria root rot. Often, trees killed by Armillaria have first been weakened by some other agents such as insects or other pathogens, so anything you can do to prevent stress on the trees should help prevent Armillaria from gaining a foothold.
Fig. 2. This white mycelial fan can often be found under the bark of trees infected with Armillaria root rot.
Fig. 3. Honey-colored Armillaria mushrooms may grow from the rhizomorphs on a decaying tree.
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