Amy Ramsey-Kroll, DNR Forest Pathologist
This spring DNR’s Forest Health section received numerous inquiries about the health of pine trees in eastern Washington. Concerned landowners have sent numerous pictures and samples from various locations across the state, from Cle Elum to Colville. Observations have included an abundance of red, dead, needles in the two year and older foliage, with concerns primarily focused on ponderosa pine. After looking over the pictures and samples, and visiting some of the areas in eastern Washington with the most dramatic symptoms, the cause appears to be Pine Needle Cast.
Pine Needle Cast is caused by fungi that infect the needles of the host trees. Multiple species of fungi can cause this infection, with Elytroderma, Lophodermella, Lophodermium, and Dothistroma species being the most common in our area. Susceptible foliage in the tree usually includes new and one-year old needles.
Population levels of needle cast fungi are cyclic and are strongly associated with local weather. Most needle cast fungi require cool, moist conditions to persist. Dry spring and fall months normally result in low levels of needle cast the following year, while wet spring and fall months normally result in higher levels of needle cast the following year. September 2013 was an unusually wet month across the state of Washington, providing ideal environmental conditions for new foliar fungal infections. If spring and fall this year are relatively dry, there will likely not be much needle cast observed next year.
Needle casts rarely cause mortality in trees, but can reduce the growth of the host if infection levels are severe enough. Below are some management options to reduce potential growth impacts or address the aesthetic issue of Pine Needle Cast.
- Leave the trees. Trees affected by needle casts will shed their foliage, usually during a large wind event or storm. The current red foliage will eventually fall off the host. As new foliage emerges, the green plant tissue will mask some of the red foliage remaining on the tree. While not all the red needle cast- infected foliage will be cast from the tree, the symptoms of the needle cast will not be as obvious and pronounced.
- Remove infected trees. Infected trees may be cut in order to remove the visual impact of the disease. This is a high intensity management strategy, but one that can be used to achieve reduced stocking objectives in affected stands.
- Low intensity prescribed fire. Needle cast fungi reside in the foliage, even after the needles have been shed from the tree. A low intensity ground based prescribed fire will reduce the amount of fungal inoculum present in the effected ponderosa pine stands, and therefore will reduce the amount of inoculum that could cause new infections.
- Reduce the stocking in some of the stands to change the microenvironment conditions of the stand. If the basal area is reduced and space is created in between the crowns of trees, it will promote more air flow and likely drier conditions that are not conducive for foliar fungi germination. This provides a proactive approach for managing the disease over time.
- Ensure seed zone of any planted ponderosa pine stock matches the area is it planted in. Genetic differences in host and pathogen can affect the amount of disease present. It’s always best to obtain tree and plant stock that is native and adapted to the specific area where it will be planted.