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Armillaria ostoyae causes a destructive root disease in northern conifers. Most infections result from root invasions by rhizomorphs growing through the soil from previously colonized stump and woody root debris. Armillaria longevity in woody debris is related to the colonized volume. Stump and root removal by root raking alone often does not satisfactorily reduce subsequent root disease mortality. This investigation characterized residual woody debris volumes in three conifer seed orchards in northern Ontario. Site preparation for stump and root removal in two of the orchards consisted of root raking alone, vs. Madge Rotoclear™ treatment ("rotoclearing") in the third orchard. Fallow periods between site preparation and planting were 1, 5, and 8 yr in the three orchards, respectively. Mortality related to colonized residual woody debris is continuing in the raked orchards, but appears to have ended in the rotocleared orchard. Root systems of 32 orchard trees recently killed by Armillaria root disease and 9 apparently healthy (otherwise comparable) trees were excavated. Regardless of site preparation method, Armillaria-killed trees were associated with greater total volumes of residual woody debris (comprising larger pieces on average) than were healthy trees. Also, healthy trees in the rotocleared orchard were associated with smaller debris pieces on average than were healthy trees in the root-raked orchard. Size of individual woody debris pieces which contained viable A. ostoyae was highly variable. Even small pieces of colonized debris close to a root crown were apparently capable of causing lethal infection. Nevertheless, compared to root raking, rotoclearing apparently breaks A. ostoyae-colonized woody debris into smaller pieces resulting in more rapid displacement of A. ostoyae. We conclude that rotoclearing followed by a fallow period of 8 yr before planting merits consideration as an inoculum reduction treatment for site preparation. A 10 yr fallow period might have prevented nearly all root disease mortality under the conditions of this study. North. J. Appl. For. 15(4):191-196.

Armillaria Root Disease Inoculum Remaining in Young Ontario Conifer Plantations Following Root Raking or Madge Rotoclearing