What’s going on with blue spruce?
A combination of factors is likely causing the decline of blue spruce trees throughout Michigan
It seems that everywhere you look you see Colorado blue spruce is showing needle loss, dead branches, browning foliage and even large trees dying across Michigan’s landscapes (Photo 1). This article looks at the likely combination of factors causing this decline.
Colorado blue spruce performs best on moist, well-drained loam to sandy loam soils with a pH of 6.0 – 7.5 and full sunlight. Often, we see more problems with trees that were planted on either very sandy or very wet soils.
Over the past several years, we have seen environmental conditions that have caused a lot of stress to conifers such as drought, extreme temperature fluctuations, hot summers and late frosts.
Rhizosphaera. With Rhizosphaera, infected needles turn purple or brown and drop off the stems prematurely (Photo 2). If you look at the needles with a hand lens, you will find round, black fruiting bodies sticking out of the tiny pore-like openings (stomata) on both green and brown needles. For more information on Rhizosphaera, view this fact sheet from Ohio State University.
Stigmina.The symptoms caused by Stigmina look identical to those of Rhizosphaera. The only way to tell the difference is with a microscope. The fruiting bodies coming out of the stomata look fuzzy, like tiny black spiders (Photo 3), compared to the fruiting bodies of Rhizosphaera that appear smooth and round (Photo 4). Because Stigmina is so new, we still do not know much about this fungus.
If the needles stay wet, there is a greater chance of Stigmina infecting the needles. To help manage these needle cast diseases, consider better air drainage, greater planting distance between trees and better site selection. Well-timed sprays in the spring can control needle casting diseases if the spruce is currently managed with fungicides after the new growth has expanded to 0.5 to 2.0 inches in length. New growth typically appears in May. Follow-up with additional sprays as indicated on the fungicide label.
Cytospora canker. This canker gradually kills the lower branches and can spread to higher branches of the tree. Cankers may occur anywhere on the branch; however, they are more prone to occur on branch segments near the trunk. The cankers are somewhat inconspicuous except for the presence of resin where the fungus has attacked the branch. Trees 15 to 20 years old (or older) suffering from drought, compaction or poor site conditions are prime targets. Trees are not usually killed outright, but the loss of major branches destroys the ornamental value of the tree.
There are no effective chemical controls for Cytospora canker. Avoid wounding lower branches with lawn mowers. A very important management tool is to prune out and destroy cankers as soon as they are observed. Prune only during dry weather to avoid spreading spores to healthy branches. For more information on Cytospora canker, view this fact sheet from MSU Diagnostic Services.
Mites. Feeding by mites causes the chlorotic spots usually on older needles, giving the interior of the tree a bronze look. Even though it is winter, you can look on the oldest needles near the stem of the tree for old webbing, yellow, bronzing or grayish, dirty-looking foliage that is indicative of spider mite feeding. Eventually the damaged needles may turn brown and drop off. Predatory mites are the most important predators of spider mites, but are very difficult to see without a microscope.
If you see some mites in the spring, but damage does not appear to be heavy, give it another seven to ten days and check the trees again. Several predators feed on spider mites, and you want to give them a chance to do their thing. If levels are high, you might want to consider using an approved miticide. Using broad-spectrum insecticides can enhance mite populations by destroying their natural enemies. For more information on mites, view this fact sheet from Penn State University.