Colorado blue spruce and white spruce are susceptible to a fungal disease which causes needles to discolor and drop from the tree. The primary disease  is one called Rhizosphaera needle cast.

The fungus (Rhizosphaera kalkhoffii) attacks the older needles found toward the inside of the tree. The tips of the branches are usually not affected.

Infected needles will turn brown or purple and will eventually drop from the tree. During the color change, tiny black dots may appear on the needles. These are the fruiting bodies of the fungus. Spores will be released from these dots during wet weather. Rain splashing will spread the infection to other needles.

The lower branches on the tree are usually affected first. The infections will gradually move up the tree leaving only the newer needles at the tips of the branches. Severely infected branches lower on the tree may be killed.

Spread of the disease may be encouraged by prolonged periods of rain or irrigation systems that spray onto the trees frequently. Although infection may occur at any time in the year, most tend to get started in the spring.

Plant resistant species of trees, if possible. Space them with adequate room for their mature size so that there is plenty of breeze moving through them. This will help keep the foliage as dry as possible and discourage the fungus.

Cultural control would consist of planting trees with proper spacing to allow enough room for the tree's mature size. This also allows for more air movement which may help tree foliage to dry out more quickly after rains or irrigation.

Chemical fungicides may also help inhibit the spread of the disease. Fungicides labeled for control should be applied to the tree every 3 or 4 weeks from mid-April to early July are recommended.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

 

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