I receive several calls asking about what is wrong with older spruce trees. One of the most common spruce tree diseases here is needle cast. Two needle cast diseases occur in North Dakota: Rhizosphaera needle cast and Stigmina needle cast. The classic symptoms of needle cast include brownish purple discoloration and eventual death of older needles, while current-year needles show no symptoms. A key characteristic of needle cast is rows of very small black dots (fungal fruiting bodies) that displace the normally white stomata along the length of the underside of needles. Rhizosphaera needle cast primarily infects Colorado blue spruce, while Stigmina needle cast affects both blue spruce and Black Hills spruce. A lab test is the only way to determine the difference between the two different needle cast diseases but treatment for both is similar.
Proper diagnosis of needle cast is recommended before treatment is initiated since other non-disease factors can cause similar symptoms. Other pests and environmental problems can cause browning and death of older needles, including normal needle death that occurs simply as a function of needle age or shading. Fungal fruiting bodies in the stomata should be identified before treating a tree with fungicide.
Needle cast disease is treatable. Within a few years after treatment, an infested spruce tree can start to produce healthy new needle growth again. Left untreated, a severe case of needle cast can lead to continual thinning and decline of the tree. Needle cast diseases can be effectively controlled with fungicides containing chlorothalonil (Bravo is an example of one tradename). The fungicide should be applied to the whole tree to prevent the disease from spreading to the new needle growth.
Treatment for needle cast is two properly-timed applications of chlorothalonil per year. The first application should occur when the new needles are half elongated (50% elongation relative to previous years’ needle length). A general rule of thumb is typically around Memorial Day for the first application. The second application should occur two to three weeks after the first application.
NDSU Extension has two publications with more information about needle cast: Diagnosing Spruce Disorders in North Dakota (F1818) and Two Needle Diseases of Spruce in North Dakota (F1680). They can be found online or by contacting the Extension office. For more information about needle cast or other questions, please contact the Stutsman County Extension office at 701-252-9030 or e-mail Alicia at firstname.lastname@example.org.