Colorado blue spruce, one of the most popular and loved landscape trees, is ready to put forth its new seasonal growth. This signals the time to consider protecting them against a troublesome fungus disease. The Colorado blue spruce is beautiful and makes a bold statement in the landscape but, unfortunately, it is also one of the most vulnerable trees to decline and diseases related to stress. If you notice that the branches of your Colorado blue spruce trees are beginning to die from the ground upward, it may be infected with Rhizosphaera needle cast disease. Rhizosphaera needle cast is a stress-related disease that is infecting many established Colorado blue spruce trees. The needle cast fungus infects the current year’s needles causing them to turn purple to brown in color and fall from the tree prematurely. This loss of needles leaves the inner portion of the branches bare and as the disease progresses, severely infected branches die, leaving the tree with a hollow or thin appearance. The disease starts near the base of the tree where humidity levels are highest and then continues to spread upward. As the disease progresses, trees become unsightly and lose their value as lawn specimens, visual screen or privacy fences. The first sign of infection occurs in late fall or in the spring a year after infection. At that time, the fruiting bodies of the fungus emerge from the stomata or “breathing pores” of infected needles. The fruiting bodies resemble tiny black dots in neat even rows and can be observed by examining needles with a hand lens. Because of the long delay between infection in the spring and the needle drop in the following summer, the ends of the infected branches appear green and healthy. The branches appear to lose their needles from the trunk outward. A protective fungicide with the active ingredient chlorothalonil, which is sold as Multi-Purpose Fungicide, Daconil 2787 and others, can prevent new growth from becoming infected. The fungicide should be applied when the new growth is one to one-and-a-half inches in length and again one month later. The trees are beginning to make their new growth so it will soon be time to make the first fungicide application. Although Rhizosphaera needle cast may be controlled in one year if the fungicide is applied correctly, severely infected trees usually require two years of fungicide application. Read the fungicide labels carefully and apply only as directed. Since Colorado blue spruce is not native to Minnesota, it often suffers from environmental stresses such as drought, excessive heat, and compacted soils. This makes them more susceptible to infection by the needle cast fungus and they can sustain severe damage. White spruce, Black Hills spruce and Norway spruce should be considered when selecting landscape trees as they show more resistance to the needlecast disease.
