Colorado Blue Spruce (Picea pungens var. glauca) is one of the most commonly used conifers in landscaping. The symmetric pyramidal form and shiny blue foliage makes it a beacon in your yard. Most often used as a specimen plant, it also functions as a background plant, screen, or windbreak. Blue Spruce is native to the central Rocky Mountains from southern Montana and eastern Idaho south to New Mexico, where it is typically found growing in moist locations from 6000 to 11,000 feet in elevation.
Though it likes moist soil, the Blue Spruce does not like high humidity. In such situations, it is susceptible to a host of fungus-caused diseases, including Rhizosphaera needle cast, Diplodia tip blight and Cytospora canker. Rhizosphaera and Cytospora are the most commonly seen problems in Wisconsin. Cytospora canker usually affects trees that are at least 15 years old. Symptoms typically start low and progress upward, causing a gradual decline of the tree. Needles change from green or blue to purple, then brown, and eventually drop off. A white pitch often covers the cankered portion of the trunk. Fungal spores spread easily through splashing rain. Rhizosphaera generally affects first year needles on the lower branches and again progresses upward. Branches die but you do not see the white pitch on the trunk. In both cases, the tree may not die for many years, or at all, but you usually wish it would die because of the dramatic decrease in aesthetic value.
Cytospora canker and Rhizosphaera needle cast normally affect trees that are under some type of stress. To avoid these diseases, plant on a site that is not prone to drought and where the tree is not overcrowded. Though they don’t require much pruning, if you do, do it only when the weather is dry. Mulch the soil and do not injure the roots by planting underneath or rototilling the soil.
Fungicides containing chlorothalonil, copper hydroxide, or mancozeb are used to chemically control these fungi. Apply in spring when the new needles have grown to one-half their mature length, and then again about three to four weeks later. Because of the large size of the tree, this is a job for a professional, not the homeowner.
Better yet, don’t plant Colorado blue spruce. There are alternatives that are far less susceptible to disease and just as attractive. My top choice would be White Fir, Abies concolor. It has similar form and color, has good soil tolerance, and is more accepting of part shade. Many people would not know the difference between the two. A good way to tell the difference is to shake hands with the foliage. Blue spruce needles are pointed and will feel sharp while White fir will not because the needles have rounded tips and are more flexible. A native alternative is White Spruce, Picea glauca. Though it is not immune to Cytospora canker or Rhizosphaera needle cast, it is less susceptible than Colorado spruce and more tolerant of Midwest climatic conditions.
Properly sited dwarf cultivars of Picea pugens seem to be less disease susceptible than full-sized plants. I have three of them in my garden. ‘Blue Pearl’ is a mini blue globe that grows just one inch per year. ‘Ruby Teardrops’ is a dwarf that grows two to four inches per year. New cones have a ruby-red color and persist for over a month. ‘Zafiro’ is a pyramidal blue dwarf that has a landscape size of eight feet tall by six feet wide after about 10 years. It puts out three to six inches of growth per year.
Cytospora canker and Rhizosphaera needle cast are serious diseases of Colorado Blue Spruce. If you must plant it, be sure it is full sun and the site has excellent air circulation. It may look great for 15 to 20 years, but will decline after that. Limit your Picea pungens planting to the dwarf cultivars and choose an alternative for the full-sized variety.
The Cottage Gardener 