SCHMIDT: Diagnosing and treating spruce diseases
Spruce trees often thrive in North Dakota under the right conditions. However, spruce trees, like all plants, are also susceptible to attack by pathogens and arthropods, and they can suffer from environmental stress and other ‘non-contagious’ factors.
This week I’d like to address the two most common ailments affecting spruce trees that I receive calls about in our area: needlecast and spider mites.
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. 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 while those of Stigmina lautii have what look like microscopic hairy projections emerging from a the fruiting body. The fruiting bodies of both fungi can be seen using 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 needle cast diseases have been seen as far west as Minot, it is expected 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.
As with the treatment of any ailment be it one affecting plants, animals or humans proper diagnosis is necessary. In the case of needle cast, proper diagnosis 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, however, be advised that new needles will not grow back to replace those that have fallen off. 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” and F1680, “The Old and the New: Two Needle Diseases of Spruce 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.
Spruce Spider Mites
Spruce spider mites are cool-season mites and are often active very early in the growing season. If temperatures consistently remain above 90 F, these mites will lay eggs and become dormant. Mites will cause tiny yellow or white speckles on needles that can be seen with the naked eye and are easily identified with a hand lens. We often notice damage during hot, dry summer days. The feeding injury often occurred in the previous spring or fall and became evident under the stress of hot, dry weather. Since miticides are usually more effective against adults than eggs, summer applications of miticides may not be effective.
Mites are only 0.5 mm long and are very difficult to see without magnification. As with most spider mites, a good test for spruce spider mites is to place a white piece of paper under needles which are believed to be infested and tap the branch. Mites appear as mobile specks on the white paper. Generally, if ten or more mites are found per sample, some type of control may be necessary.
Syringing, cultural control, biological control, and chemical controls are all options that can be used in controlling spruce spider mites. Spraying mites with a forceful jet of water (syringing) can be an effective method for controlling mite populations in home landscapes while maintaining natural predators. Lace wings and lady beetles are naturally occurring predators of spruce spider mites in North Dakota. Insecticidal soaps can be used to manage spruce spider mites in warm weather, while horticultural oils (1-2% rate) may be used during the summer and dormant oils (3-4% rate) can be used to kill mite eggs and adults during the spring and fall. Horticultural oils can injure conifers if applied when temperatures are not appropriate. Read the labels carefully.
Chemical control involves the use of insecticides that control mites. These include: malathion, bifenthrin a.i. (Talstar), spinosad (Conserve), and Acephate (Orthene and Isotox). Miticides should be sprayed when adults are active (April-May) with a follow-up spray 5-7 days after the first spray to control later hatching nymphs. However, use of a miticide is not recommended unless active mites are numerous (10 or more found using white paper test) and natural enemies (lace wings and lady beetles) are not present. Additionally, entomologists tell me that using chemicals to control mites often results in a resistance buildup that takes place quite quickly leading to population explosions following chemical applications. Always read and follow pesticide labels.
Schmidt is NDSU Extension’s agent for Pierce County.
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