Long-Term Impact of Shoot Blight Disease on Red Pine Saplings

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.