This week the majority of questions and samples received in the Plant Disease Clinic were of a fungal disease, Rhizosphaera needlecast. Blue spruce trees are particularly susceptible to this disease. While this needle disease is common in Iowa and doesn't usually kill the tree, it can cause significant aesthetic losses in the landscape.
Symptoms are first observed as a yellowing of mature needles in late summer and fall. The next spring implications become obvious as affected needles turn a purple-brown color and drop from the tree. Symptoms appear initially on lower branches then gradually affect higher and higher branches. Affected branches lose the older inner needles first, leaving a tuft of young green needles at the ends. Eventually the entire branch can be killed. A close look at discolored needles shows orderly lines of black spots (a hand lens may be helpful). These are fruiting bodies of the fungus that have developed in the stomata (natural openings) of the spruce needles. Unless control measures are taken this disease will spread and worsen year after year and may eventually kill the entire tree.
- Trees stressed by environmental factors are more susceptible. Choose a suitable site for planting blue spruce. Blue spruce trees thrive in full sun and well-drained, moderately moist soil. Before planting, check for signs of black spots on needles.
- Remove competing trees, shrubs, and weeds. This will increase air flow around the spruce and reduce the high humidity levels that favor development of the disease.
- Don't trim trees when the needles are wet. This will help prevent the spread of the disease on trimming tools.
- If the disease is already present, protect newly emerging needles with fungicides. Fungicides labeled for Rhizosphaera needlecast control include Daconil, Bravo, Manicure, Twosome, Junction, Protect T/O, and Bordeaux mixture. Check your garden center for labeled products available for control of Rhizosphaera needle cast. These should be first applied in mid-May when needles are 1/2 - 2 inches long. A second application should be made 4-6 weeks later.
This article originally appeared in the April 21, 2000 issue, p. 40.