MEDIA RELEASE
May 27, 2003
Written by Lynn McNaughton, U of M Extension Service Summer Horticulture Intern (507) 389-8146 or 1-888-899-7758
Common Spruce Tree Diseases
The tremendously popular spruce tree is a common sight in yards throughout the area. They are favored for their evergreen habit, relatively fast growth and pyramidal shape and are often used as screens or windbreaks. However, as trees become crowded, shaded, or simply older, they become susceptible to several diseases. Two common spruce diseases are Rhizosphaera needle cast and Cytospora canker. Both cause early needle drop and bare branches, but attack trees differently and require different means of control.
Rhizosphaera needle cast usually starts at the bottom of the tree, with branches losing their needles from the inside out. In summer, the infected needles turn yellowish and then purplish-brown before falling. Diseased branches often appear totally bare except for the current year’s growth at the tips. Branches that lose needles three or four years in a row may die.
Colorado Blue spruce is extremely susceptible to needle cast. White spruce (including Black Hills spruce) is moderately susceptible, while Norway spruce is quite resistant.
Rhizosphaera infects new growth in spring, but these needles do not fall off until the following summer. Last year’s growth may look fine now, but will drop this summer as a result of last year’s infection. During the summer, especially during wet weather, check older needles to see if tiny black dots (the fungus fruiting bodies) appear in rows along the needles. If so, Rhizosphaera is definitely the culprit.
Needle cast can be controlled with properly timed fungicide sprays. To keep new growth from getting infected, spray trees with a fungicide containing chlorothalonil when the new needles are half-elongated (mid-May to early June). Follow up with a second application three to four weeks later. While one year of applications (2 sprayings) can control needle cast, heavily infected trees usually need treatment again the following year.
The other common spruce disease is Cytospora canker, which can seriously disfigure mature trees. Cytospora attacks all spruce, with Colorado Blue Spruce again being most susceptible. Most spruces are about 10 to 15 years old when this canker disease moves in; young trees are rarely attacked.
Cytospora infects the branches, not the needles, and causes elliptical or diamond shaped lesions (cankers) to form. Clear resin drips from these lesions, resulting in a white crust on the plant parts below. Because so much resin is formed, the cankers may not be visible since the bark stay stuck to the infected areas.
Cankers grow larger every year and will eventually kill the entire branch. Needles will turn orange, then brown and eventually fall off. Unlike Rhizosphaera, Cytospora causes random branches to die throughout the tree, although most do occur near the bottom.
There are no chemical controls for Cytospora, although good maintenance practices and selective pruning can help. Drought stressed trees are the most susceptible to Cytospora, so keep spruce well watered. Also, prune off infected branches six to twelve inches below the canker. Avoid cutting into the tree trunk as this can seriously harm the tree.