Diplodia sapinea (Diplodia) is a fungus that causes shoot blight (dead branch tips), branch death, decreased growth, top-kill, and sometimes death, especially to pine trees with needles clustered in groups of two and three such as red, Austrian, jack, ponderosa, and Scots pines. Diplodia is seldom a concern on white pine, a five-needled pine. It is common in the Lake States and was first identified as a problem in the region in the 1970s. It is not known if Diplodia is native to Minnesota.
Diplodia can be a severe problem for young red pines growing near older diseased pines. Research on the long-term impact of shoot blight disease on red pine saplings found that 30 percent of red pine saplings died from Diplodia and Sirococcus (a nearly identical disease) over 13 years when grown within roughly 130 feet of mature red pines. The saplings were about 15 years old at the start of the study, and presumably more died before the research started. Only one percent of saplings that were far away from mature pines died from these diseases.
Diplodia can take an even bigger toll on young red pine seedlings growing in shade or after severe stress. A late spring frost in 2016 was the likely cause for elevated levels of Diplodia shoot blight in Minnesota and Wisconsin DNR bareroot nurseries. The affected fields in Minnesota were destroyed. To understand the fate of red pine from diseased fields, the DNR forest health team planted an experimental subset. More than 40 percent of the experimental planting died from Diplodia collar rot after only six months. The seedlings appeared healthy at the time of planting.
Diplodia is widespread across Minnesota. In 2005, DNR surveys found Diplodia in 97 percent of 92 red pine stands and in all 28 jack pine stands surveyed from Morrison County north to Lake of the Woods County and east to St. Louis County.