Some Spruce Susceptible to Rhizosphaera Needle Cast
by Don Janssen, Extension Educator
Colorado blue spruce trees are susceptible to an infectious needle disease caused by the fungus Rhizosphaera. The disease, referred to as Rhizosphaera needle cast, is the most common problem seen on Colorado spruce. White spruce are classified as intermediate in susceptibility to the disease and Norway spruce are relatively resistant.
Symptoms - 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 on 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. Fruiting structures of the fungus are not evident on these trees.
Spread - Rhizosphaera overwinters in infected needles on the tree and on needles that have fallen to the ground. The fungus is spread by splashing and dripping water beginning in spring and continuing into the fall. Newly emerging needles can become infected during wet spring weather.
Control - If symptoms appear, diseased trees should be sprayed with a fungicide in the last 2 weeks of May and again 4 to 6 weeks later. Be sure to read the product label for specific rate and timing instructions. Good coverage and proper timing of applications are critical for successful disease control. Treatment requires the use of a preventative fungicide called chlorothalonil.
In addition to fungicide sprays, other control measures include spacing trees adequately to promote good air circulation, improving tree vigor through mulching and watering when needed, and not shearing trees when the foliage is wet.
University of Nebraska-Lincoln Extension in Lancaster County is your on-line yard and garden educational resource. The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office