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ABSTRACT
Damage from Sirococcus and Diplodia shoot blights of red pine is widespread and periodically severe in the Lake States. An outbreak of shoot blight occurred in red pine sapling plantations across northern Wisconsin, northern Minnesota, and the Upper Peninsula of Michigan in 1993. We established monitoring plots in red pine sapling plantations severely affected by shoot blight in Michigan and Wisconsin to assess the influence of residual overstory red pine and the presence of Sirococcus and Diplodia shoot blights on disease severity and their long-term impact on tree survival and growth. Fourteen years after the initial outbreak, many red pine saplings had recovered from serious damage (^ 1/3 of new shoots affected) that had occurred in a single outbreak year followed by lower levels of disease in subsequent years. However, the most severely affected trees sustained higher mortality and reduced growth. Forking or development of crooks of the main stem was common if terminal leaders were killed by shoot blight. Results from this study suggest that the impact of a shoot blight outbreak on red pine saplings largely depends on the presence or absence of an inoculum source in residual overstory trees, the species of pathogen present in the stand, and the initial disease severity.
Keywords: Pinus resinosa, plantations, Diplodia pinea, Sirococcus conigenus, multicohort stands
The fungal pathogens Sirococcus conigenus and Diplodia pinea can be highly damaging to red pine (Pinus resinosa), often killing seedlings and young trees during episodic outbreaks (Nicholls and Ostry 1990, Ostry et al. 1990, Bronson and Stanosz 2006, Oblinger et al. 2013). First reported in Wisconsin in 1959, S. conigenus infects young needles then grows into and kills current- year shoots, reducing the live crowns of affected trees (O'Brien 1973). Sirococcus shoot blight is favored by cool, wet weather and low light conditions (Nicholls and Robbins 1984). Diplodia pinea was first reported killing seedlings in a Wisconsin nursery in 1975 (Palmer et al. 1988) and since has become widespread in the Lake States on red pine, causing a collar rot of seedlings, shoot blight, and branch and stem cankers. Trees under drought stress and wounded by hail, heavy snow, and various insects are highly susceptible to infection by D. pinea (Peterson 1981, Nicholls and Ostry 1990, Feci...