Al: How can landowners identify Rhizosphaera needle cast?
Linda: Rhizosphaera needle cast is a fungal needle disease that causes needles to die and drop off the tree prematurely. Another needle disease with similar symptoms that we’re seeing quite a bit of this year is Stigmina needle cast. Symptoms of these needle cast diseases will be worse near the bottom of the tree, while the tops are typically not heavily impacted. Older needles will become infected and turn rusty brown color and drop off prematurely. Close examination of those needles will show rows of tiny black dots (you may need a hand lens to see these), which are the fungal fruiting structures that produce the spores of the fungus. Blue spruce trees are much more susceptible to the disease than white spruce, but we’re seeing heavily impacted white spruce this year too. Norway spruce is more resistant but we’re even seeing some of those trees being impacted.
Al: Why does this seem to be such a problem this year?
Linda: Rhizosphaera needlecast does very well during wet years. Last year (2017) was a very wet year all throughout the growing season, so the problems we’re seeing are directly tied to the weather last year.
Al: What does this disease do to the tree?
Linda: It causes the tree to drop needles prematurely. Spruce ideally hold its needles for several years, but older needles infected by rhizosphaera drop off years before they normally would. Repeated defoliations make the tree appear thin, with very little foliage (often only current year foliage) on the branches. Severe repeated infections can cause branch mortality.
Al: What can be done to prevent it?
Linda: There are fungicides that can be used to prevent needle infection. These fungicides will either contain copper (e.g., Bordeaux mixture) or chlorothalonil, and must be sprayed early in the spring when emerging needles are about half their expected size, with a repeat application 2 weeks later. Complete coverage of the needles is important to protect them. At least 2 years of treatment will be necessary to allow trees to begin to recover their full complement of needles. But, some blue spruce will be much more susceptible to this needle disease throughout their lives, regardless of the weather, and may need continued treatment to maintain a full complement of foliage.
At planting time, landowners should consider trees other than blue spruce, since it is prone to infection by Rhizosphaera.
Also, another prevention strategy for landowners to use is to prune the lower branches of their spruce. Pruning the lower branches off will allow better air flow under the tree and dry off the needles more quickly after rain or dew events. Since the fungus needs wet needles to infect them, pruning the lower branches can help limit the amount of infection on a tree.
Al: What can be done to treat it?
Linda: There are no “cures” for the needles once they’re infected. Prevention is important if control is desired.
Photo credit: M. Grabowski