Rhizosphaera Needle Cast
Blue spruce trees are susceptible to an infectious needle disease caused by the fungus Rhizosphaera. White spruce are classified as intermediate in susceptibility to the disease and Norway spruce are relatively resistant.
The disease is usually first evident on lower branches and then works upward gradually. Second-year needles turn a purple or brown color and eventually fall from the tree. After several successive years of needle loss branches may die. In general, trees appear to die from the bottom upward. In some cases, however, infections start higher on the tree, giving the appearance of scattered dead areas.
The disease can be diagnosed by looking at the discolored needles with a magnifying glass or hand lens. Small black spots (fruiting structures of the fungus) appear in rows in the infected needles. The fungus is actually emerging from the stomata (natural pore-like openings) that occur in lines on all sides of a spruce needle. Green needles may show these small black fruiting structures.
Keep in mind that environmental or site-related stresses can also cause discoloration and loss of needles on spruce trees. The extended wet weather of 1993, for instance, has been responsible for needle browning and even tree death in some cases. (Fruiting structures of the fungus are not evident on these trees.)
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