Rhizosphaera needle cast on blue spruce, caused by the fungus Rhizosphaera kalkhoffii, has been the most common plant disease sample received in the Plant Disease Clinic this spring. This disease typically causes second-year needles to turn a purple-brown color and fall from the tree. Small black fruiting structures of the fungus can be seen in the small, pore-like openings of infected needles. These fruiting bodies appear in rows along the length of the needle. A hand lens or magnifying glass is helpful in viewing these structures.
Symptoms most commonly start on the lower branches of the tree and work upward. The symptoms, however, may first appear and spread on one side of the tree. Trees of any age may be affected, especially those that are stressed. The fungus can overwinter in infected needles. In the spring spores are released during wet weather and infect newly emerging needles.
If the disease is present, control consists of using a fungicide in the spring to protect these newly emerging needles. The tree should be sprayed with a product such as chlorothalonil (trade name Daconil 2787) or Bordeaux mixture in the last two weeks of May and again 4 to 6 weeks later. Good coverage and correct timing of the applications are important. Be sure to read the label for rates.
Other control measures include proper tree spacing and weed control to promote good air circulation, improving tree vigor through mulching and watering when needed, avoiding shearing trees when the foliage is wet, inspecting trees for disease before planting, and avoiding planting susceptible trees next to infected trees.
This article originally appeared in the April 14, 1995 issue, p. 46.