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 blue spruce samples that are submitted to the Plant Disease Clinic. 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 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.)
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. Fungicides labeled for Rhizosphaera needle cast include Daconil 2787, Daconil Ultrex, Terranil 90, Thalonil 4L, Thalonil 90, Manicure Flowable, and Twosome Flowable.
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.
Rhizosphaera needle cast is pictured and discussed in Pm-1528 "Common Diseases of Conifers in Iowa.
This article originally appeared in the April 26, 1996 issue, p. 61.