Needle Cast Diseases of Spruce: Diagnosis and Treatment (6/2/11)
Two needle cast diseases occur in North Dakota: Rhizosphaera needle cast and Stigmina needle cast. Similarities and differences between the two diseases exist. Symptoms of both needle cast diseases look similar to each other. The classic symptoms of needle cast include brownish purple discoloration and eventual death of older needles, while current-year needles show no symptoms (Figure 1). Another key characteristic of needle cast is the microscopic rows of small black dots (fungal fruiting bodies) that displace the normally white stomata along the length of the underside of needles. The fruiting bodies of Rhizosphaera kalkhoffii are spherical (Figure 2) while those of Stigmina lautii have what look like microscopic hairy projections emerging from a the fruiting body (Figure 3) . The fruiting bodies of both fungi are noticeable with a 10X hand lens and are located in the needles’ stomata which are normally white. Rhizosphaera needle cast (caused by the fungus Rhizosphaera kalkhoffii) primarily infects Colorado blue spruce, while Stigmina needle cast (caused by the fungus Stigmina lautii) affects both blue spruce and Black Hills spruce.
Needle cast diseases thrive in humid, wet environments and they are more common in the north and east parts of our state. Although we have seen needle cast diseases as far west as Minot, we expect it to occur rarely, if at all, west of U.S. Highway 83. It is most common east of U.S. Highway 52 and is especially prominent in the Devils Lake basin and the Red River Valley.
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. These other causes can be easily confused with a needle cast disease. Identifying the fungal fruiting bodies in the stomata is crucial before a needle cast disease can be diagnosed and before fungicide treatments are applied.
Needle cast diseases of spruce are treatable. Within a few years after treatment, an infested spruce tree can look beautiful again. Left untreated, a severe case of needle cast can lead to continual thinning and eventual decline of the affected tree if spring weather is conducive to infection year after year.
Needle cast diseases can be effectively controlled with fungicides containing chlorothalonil. For Rhizosphaera needle cast, two properly-timed applications per year for at least two consecutive years, and sometimes three years, is required for control. The first application should occur when the new needles are half elongated (50% elongation relative to previous years’ needle length). We usually say “around Memorial Day”, but actual timing depends on the year, the location, and the individual tree. The second application should occur three to four weeks after the first application. The timing of the two applications is the same for the second and third year. Timing of treatment for Stigmina needle cast is similar, except preliminary data suggest that the trees should be treated indefinitely, with at least two properly timed fungicide applications per year.
Proper timing of the fungicide application is critical for effective control. Spraying too early or too late will miss the stages when the tree can be protected from infection by the fungus. A lot of time and money has been wasted by applying fungicides at the wrong time. For further information on Rhizosphaera needle cast, see the NDSU Extension service publication PP-1276, “Spruce Diseases in North Dakota.” If a fungicide is used, be sure to read, understand, and follow the labeled instructions to avoid injury to the tree, the environment, or yourself.
If unsure whether a spruce has a needle cast disease, consider consulting with your local county agent.
Kasia Kinzer - Plant Diagnostician
Aaron Bergdahl - Forest Health Specialist