The belowground incidence of trees infected with Armillaria root disease (DRA) ranged from 23-33% in Douglas-fir plantations, and 80% in one mid-rotation stand. The increase in incidence of newly infected trees peaked about age 16-20, consistent with root closure in the stand. The fungus continues to infect both within and between roots as the trees age, but symptoms are also mostly not apparent because of building host resistance as the trees age. Juvenile trees with all root lesions contained (callused) increased with age from 18% to 38%. In mid-rotation trees, 52% of the trees had all lesions contained. There are still many trees with spreading lesions that probably represent new infections as the fungus repeatedly infects new areas (primarily with rhizomorphs) in the root system. Mortality is expected to stabilize somewhat between the ages of 30 and 60, but then is expected to increase after this time with mortality between 20 to 60% by age 100 due to the repeated root infections occurring belowground. These figures do not include in-growth that will occur in larger openings in the more severely affected stands, but growth is expected to be poor in light of the high inoculum left by the dead fir. Trees that are able to contain the fungus completely and survive suffer growth losses of about 7-10% for the five juvenile and one mid-rotation stands. Stand losses are underestimated in older stands because the remaining healthy trees tend to be small and losses are calculated by the difference between healthy and infected tree growth. The healthy population is represented by smaller trees because their size reduces the probability of contact with the fungus. Mike Cruikshank, Duncan Morrison.
Cruikshank, Mike, Morrison, Duncan J.; Canadian Forest Service. 2003. Impacts of Armillaria root disease on stand productivity in the southern interior. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report. FIA2003MR354