Inoculum Reduction Measures to Manage Armillaria Root Disease in a Severely Infected Stand of Ponderosa Pine in South-Central Washington: 35-Year Results

A stand of ponderosa pine (Pinus ponderosa) severely affected by Armillaria root disease was treated with five different levels of sanitation by root removal to reduce root disease losses in the regenerating stand. Treatments included the following: (1) all trees pushed over by machine, maximum removal of roots by machine ripping, and visible remaining roots removed by hand; (2) all trees pushed over by machine and maximum removal of roots by machine ripping; (3) all trees pushed over by machine with no further removal of roots; (4) smaller trees pushed over by machine but large stumps left, otherwise maximum removal of roots by machine ripping; and (5) all trees felled and removed by skidding, area cleared of slash, sod scalped, and no removal of roots. After 35 years, we found that the more intense and thorough root-removal treatments were generally more effective in reducing the occurrence of Armillaria root disease. However, even the most intensive treatment (treatment 1), which experienced significantly less disease than most other treatments, had 23% of the area expressing mortality. The only operationally feasible treatment (treatment 3) also reduced levels of mortality, but not significantly (40% mortality versus 52% in the control, treatment 5). Although these results support the concept that inoculum removal can reduce root disease levels, the treatment necessary to provide a meaningful reduction in disease loss does not seem to warrant its cost.