April 18, 2014

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Picea pungens – Colorado Blue Spruce

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Colorado blue spruce is an introduced plant to the Minnesota landscape and this tree is very popular. Unfortunately for this species, popularity leads to stress related problems. Colorado blue spruce’s native range is in the Rocky Mountains between 6,000 to 10,000 feet – a cool climate. While this species is said to be somewhat drought tolerant, Minnesota’s drier, warmer continental climate places this species under a fair amount of stress.
Medium to largetree height: 30-60′
Canopy spreads: 10-20′, Pyramidal crown
Drought Tolerance: Intermediate
Shade Tolerance: Intolerant (some sources say tolerant)
Soil pH Tolerance: Tolerant to high pH
Poor Soil Drainage: Intolerant to flooding
Salt Tolerance: Tolerant to salt spray and Sensitive to soil salts.
Spruces bear single needles that are typically 4-sided or square-ish in cross-section. The needles will roll between your fingers. Typically, the needles are pointed at the end – compare the stiff, sharply pointed needles of Colorado blue spruce to the dull or blunt points of white spruce and black spruce. Needle length on the Colorado blue spruce is fairly long in comparison to white and black spruce. Colorado blue spruce needles are typically 3/4 to 1 1/4 inches long. If the needles are crushed a strong resinous odor is released (hence, the latin name – pungens).

The twigs on Colorado blue spruce are light colored (yellow brown), hairless (compare to black spruce) and stout. The bud scales are usually reflexed (curved back) and are typically golden brown overlapping scales. Colorado blue spruce bears its winged seeds in cones that are roughly 2 1/2 to 4 inches long. The cones are constructed of oblong overlapping, papery scales with scale margins that are irregularly toothed.

Colorado blue spruce is extremely popular landscape tree and is widely planted as a Christmas tree in Minnesota. There are numerous varieties available in the nursery trade. As stated above this popularity places the tree in many unsuitable sites, thus creating stressed trees that become susceptible to insect and disease problems.

Disease Management
Cytospora canker, Leucostoma kunzei Fungi Branch dieback in the lower crown with copious amounts of oozing sap. Cankers spread from infection site on branch toward trunk. Needles turn purple or brown then drop. This disease attacks stressed spruce, esp. mature Colorado blue spruce. Plant on good sites. Avoid wounds and crowding. Maintain plant health. Remove and destroy infected branches in late winter.

CHEMICAL: None.Lirula needle blight, Lirula sp.FungiInner needles turn a light tan and remain on the tree. Black, slightly raised, elongate fruiting bodies develop on the lower surface of infected needles.Plant on good sites. Avoid wounds and crowding.

CHEMICAL: Make a single application of chlorothalonil by July 1.Rhizosphaera needle cast, Rhizosphaera kalkhoffiiFungiInfection occurs on new needles which discolor and shed the next spring and summer. Needles discolor first yellow then purple or brown. Tiny black fruiting bodies replace the white stomata on individual green or discolored needles and can easily be seen with a hand lens. Trees are predisposed by stress, especially drought.Avoid stress, especially from drought. Increase air circulation by removing lower branches and mowing grass or weeds. Avoid overhead watering.

CHEMICAL: Two spring applications of chlorothalonil will protect new needles from infection.