Spruce samples showing the presence of the fungal disease Rhizosphaera needle cast have been arriving almost daily in the Plant Disease Clinic. Spruce trees infected with the fungus Rhizosphaera tend to show sparse lower branches. The older (inner) needles turn a purplish-brown color and fall prematurely from the tree. If you take a close look at these needles with a hand lens or magnifying glass, small black spots (structures of the fungus) will be visible in rows in the infected needles.
The disease tends to start on lower branches and gradually progress to upper branches, although the disease may start higher on the tree. After several years of early needle loss branches may die. Images of symptomatic trees can be found at the Plant Disease Clinic website .
Once a tree is infected with Rhizosphaera, the primary means of control is the use of fungicide sprays. Bordeaux mixture, chlorothalonil, or EBDC products can be used, with the first spray usually applied in the last 2 weeks of May and a second application 4 to 6 weeks later. (The weather can affect when the new growth emerges in the spring, so be sure to follow label instructions for timing, rates, and safety precautions.)
Refer to Pm-1528 Common Diseases of Conifers in Iowa for various trade names of fungicides that are labeled for disease control. Additional details about Rhizosphaera and other conifer diseases are also included in this bulletin. You can find this publication at your local county extension office or through the Iowa State University Continuing Education and Communication Services (515) 294-5247.
Conifers of all kinds have also been submitted showing symptoms of browning caused by winter-related stresses. Injury in these cases typically appeared over the winter months and is sometimes limited to one side of the tree. Fungicide use will not be useful for treating browning related to winter injury.
The Plant Disease Clinic is available to offer assistance in diagnosing conifer diseases as well as other plant disease problems. You may mail a sample through your local extension office or mail it directly to the Clinic. The form for submitting a sample can be found at the Plant Disease Clinic website .
Plant Disease Clinic
323 Bessey Hall
Ames, IA 50011
This article originally appeared in the May 11, 2001 issue, p. 54.