Purple Rain

Why are flowers so difficult to grow and weeds so difficult to eliminate? It is one of the paradoxes of life that the more desirable an item is, the more problematic that thing may be.

Such is the case with the Colorado Spruce. Whether called Blue Spruce, Colorado Blue, or Colorado Green, the species Picea pungent is a much sought after tree for both specimen plantings and, formerly, windbreaks. With its immense size, stately shape, and frequently powder-blue coloration, a healthy Colorado can be an asset in most home landscapes.

And yet, it is arguably the most cantankerous, disease susceptible, and chemically dependent conifer that you can purchase. Besides being susceptible to tip blight, spruce needle borers, and types of root rot, the Colorado is also notoriously intolerant of shade and abhors humidity and wet ground. Alas, these are the least of its problems.

Almost everyone that has these trees in a windbreak or shelter belt has noticed the tendency of the needles, especially on the lower parts and the interior of the tree to turn a bronze-purple color and then fall off, often leaving entire sections of the tree completely denuded.

The culprit is a highly virulent, easily spread fungus called Rhizosphere; the set of symptoms it results in is called needle cast, sometimes purple needle cast. The fungus, which can attack anywhere in the tree is especially fond of young needles, the crucial growing points for a healthy tree. Moreover, the fungal spores which spread the disease are dispersed by rain-splash, which helps spread the fungus from branch to branch, and from tree to tree.

Picture Source: https://www.russelltreeexperts.com/arbor-ed/2019/11/13/rhizosphaera

Although not entirely curable, Needle Cast can be controlled and its spread largely arrested by the timely application of a copper fungicide a minimum of twice per year. A heavily infected tree may never regenerate new growth in the defoliated areas, but treating the tree with fungicide will help protect the new needles, and keep the tree healthy, while eventually regaining a more pleasing appearance.

Now the bad news: for people living in our service area, the Colorado Spruce should be only used as a specimen, or in very small, widely spaced groupings. We recommend a minimum of 30 to 45 feet between trees, and here is why. Rhizosphaera is an endemic fungus in the Midwest; it is constantly here, It thrives in moist ground, mild evenings, and high humidity. The Colorado Spruce prefers low humidity, cool evenings, and well-drained soil; anything else causes the tree to go into stress, and as it does it loses its ability to fight off disease and fungal attacks. It is a tree that is truly homesick for the mountains.

If you insist on planting a Colorado, give it plenty of sunlight and plenty of room. Good air circulation helps discourage Needle Cast by not giving the fungus the moisture it needs to breed, and keeping your specimen away from other trees, conifer or deciduous, helps prevent rain-splash from spreading the fungal spores.

And as always, if you see or suspect that your tree is under fungal attack, contact us. The cost of treatment is far less than the cost of replacement.

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