Scientists estimate that 1 in 6 US tree species are at risk of extinction, largely due to pests, disease, and climate change, according to a recent study published in the journal “Plants People Planet.”
At the Arboretum, we’ve met the latest of these challenges to maintaining our conifers against needle cast and other conifer diseases. The Colorado blue spruce (Piceapugens) that were part of our Dwarf Conifer Collection have suffered the most, but our white spruces (Picea glauca) have also shown evidence of infection. We have been pruning out branches of our other spruces and mitigating their inevitable decline. We have also suffered a number of losses of our pine trees, in particular the Austrian pines (Pinus nigra), primarily to Diplodia tip blight. Nearly 50 of our heritage specimens have been lost to these fungal diseases.
By far, the most pervasive of these threats to conifers is needle cast. Needle cast is a fungal disease of evergreen trees, primarily pines and spruces, which causes infected needles to prematurely turn yellow and brown and then drop, especially on lower branches. Symptoms appear months after infection, initially as yellow spots that progress to brown or purplish discolorations, often with rows of black fungal structures visible on the needle surface. While not typically fatal in the early infection stage, repeated annual infections weaken the tree, making it susceptible to other stressors and leading to sparse foliage and eventual decline and death.
• Spruce (Picea pugens): Colorado blue spruce is highly vulnerable to Rhizosphaera needle cast. Other species such as white, black, Engelmann, and Sitka spruce can also be affected.
• Pines (Pinus): Austrian pines, Scotch pines, mugopines, and Japanese red and black pines are susceptible to various needle cast fungi, especially Lophodermium and Cyclaneusma.
• Douglas fir, balsam fir, and Siberian fir (Abies) can also be affected by needle cast diseases.
These fungal diseases are spread by wind and water, and control involves improving air circulation, removing dead needles, and applying fungicides. For the home gardener, probably the best solution is to plant disease resistant varieties of spruce, such as Norway spruce (Picea abies) and pine such as white pine (Pinus alba) and pitch pine (Pinus rigida.) Both white spruce (Picea glauca) and black spruce (Picea mariana) have shown some resistance to these fungal diseases. Hemlock is a possibility but there are long term concerns about the hemlock wooly adelgid, a rapidly spreading aphid-like insect that attacks North American hemlocks.
As we continue to experience the effects of rapidly changing and less predictable shifts in weather each season at Landis, we will continue to do our best to monitor our collections for signs of insects and diseases, especially in our most susceptible species. Each time we can identify symptoms early, we have a better chance of success.