Spruces (Picea spp.) are eye-catching at this time of the year, especially now that deciduous trees have lost their leaves. Spruces are popular in the Pennsylvania landscape, but unfortunately, the state’s mild, humid climate supports a disease that significantly threatens their health: needle cast.

Needle cast disease has been a long-standing problem for spruce trees in Pennsylvania. The two most common needle cast pathogens affecting spruces are caused by the fungi Rhizosphaera spp. and Stigmina spp. These pathogens are consistently identified as the most prevalent diseases on spruce trees each year at the Penn State Plant Disease Clinic, and they often occur together on the same tree. The disease symptoms and controls are similar for these two pathogens, and both thrive in the mild, humid climate of Pennsylvania. The name “needle cast” describes how the disease causes the infected needles to prematurely drop from the tree. Needle cast pathogens can infect all ages of needles through small, natural openings the plant uses for gas exchange called stomata. These pathogens thrive in environments with high humidity, so they frequently infect needles on the interior branches of the lower canopy first. As the disease progresses within the tree, the lower branches will become bare of needles, and the infection will continue spreading upward in the canopy (Figure 1).

Once a needle is infected, it may take up to a year before symptoms appear. Infected needles show yellow, brown, or purple discoloration. If you look closely, or with a 10X hand lens, you may be able to see signs of the pathogen on symptomatic needles. Look for small, black, circular fruiting bodies emerging from the needle stomata (Figure 2, top of ). These fruiting bodies contain the infectious spores of the pathogen that are spread by rain and wind.

Recently, I conducted a fun exercise —fun for a plant pathologist at least— where I closely examined Rhizosphaera needle cast under the microscope. In one fruiting body, I counted 60 spores. On a single spruce needle, I counted 150 fruiting bodies. If each fruiting body contains a similar number of spores, that amounts to 9,000 spores on just one needle, and each spore has the potential to be infectious! Now imagine that multiplied across an entire tree… wow, that’s a lot of needle cast! While my spore counts are based on a small subsample and do not represent every situation, I share this exercise to highlight the astonishing capacity of spores in spreading the pathogen.

Managing Needle Cast

Spruces range in their susceptibility to needle cast disease. Colorado blue spruce (P. pungens) is considered highly susceptible to needle cast, while Norway spruce (P. abies) is considered “tolerant” of the disease. Tolerant trees can still get the disease, but the damage is usually less severe, and it tends to only be destructive on trees already stressed by something else. If you are looking for an alternative conifer that is resistant or tolerant of the disease, check with your local Extension office for species recommendations for your site. Planting the right tree in the right place can go a long way in overall plant health care.

Needle cast pathogens overwinter in infected needles, including diseased needles that fall to the ground. By raking up and removing fallen needles, you can reduce the source of the pathogen. Newly emerging needles can be protected with preventative fungicides, but they need to be applied at regular intervals and will not cure already-infected needles. Consecutive years of treatment may be necessary for heavily infected trees. Visit the Penn State Extension website or your local Extension office for more information on product selection.

Increasing light and air flow into the canopy lowers humidity, reduces needle wetness, and helps control needle cast. Select planting sites with full sun exposure. Improve air circulation and needle drying by spacing trees appropriately and controlling weeds. Prune dead and dying branches to increase airflow within the canopy.

Needle cast disease management is most effective if done proactively. If the disease has progressed to the point where less than 30% of the tree’s canopy has live foliage, no amount of disease control will restore these trees to full health. Invest some care into your spruces now to keep them healthy and thriving.

By Jennie Mazzone, Horticulture Educator, Penn State Extension