Using spatial and temporal patterns of Armillaria root disease to formulate management recommendations for Ontario's black spruce (Picea mariana) seed orchards

Between 1982 and 1989, 22 black spruce (Picea mariana (Mill.) BSP) seed orchards were established on cleared jack pine (Pinus banksiana Lamb.) forest land in northwest Ontario. These orchards were located on stressful sites for black spruce to hasten seed production. Mortality caused by Armillaria ostoyae (Romagn.) Herink was observed in most of these orchards within 3 years of establishment. This study was initiated to quantitatively describe the temporal progress and spatial patterns of Armillaria root disease mortality in five representative orchards, to determine future operational management implications. In the four orchards where epidemics developed, temporal disease progress was nonlinear and was better described by the monomolecular function than by the Gompertz or logistic functions. Monomolecular rates of disease increase were 0.0062-0.0346. Applying these rates, we estimated that cumulative Armillaria root disease mortality will be 9-41% and 25-79%, at 20 and 50 years after planting, respectively. Armillaria root disease mortality was spatially aggregated in all four orchards. Trees adjacent to Armillaria-killed trees had an increased probability of mortality from Armillaria root disease. Successive epidemics may develop in these orchards; their timing and severity will be affected by orchard management practices. Measures of spruce family performance in these orchards are compromised by the aggregated distributions of different A. ostoyae genets and the root disease they cause.