The Iowa State University Plant Disease Clinic has received several samples this year of spruce with brown, drying needles. In many cases, the cause of the needle browning was Rhizosphaera needle cast, a common fungal disease.
Symptoms of Rhizosphaera needle cast can appear on any age of tree, but are most common on trees that are stressed. Symptoms typically start on the lower branches and move to the upper branches. Second-year needles (those closest to the trunk) turn purplish-brown and fall from the tree. Small black fruiting bodies of the Rhizosphaera fungus appear in straight lines along the length of infected needles; these can be seen with a hand lens. The fruiting bodies may appear on both brown and green needles.
To prevent the disease, avoid planting susceptible trees near infected ones, and inspect trees for the characteristic fruiting bodies before planting. Trees should be kept as unstressed as possible by mulching and watering when needed. Good air circulation should be maintained by proper spacing between trees. Avoid trimming trees when the foliage is wet to prevent spread of the fungal spores.
To treat the disease if symptoms are present, apply Bordeaux mixture, chlorothalonil, or EBDC once in late May or early June and again 4-6 weeks later.
Browning of needles may also be caused by seasonal needle loss, where the oldest needles turn brown and fall off in the autumn, as well as by environmental stresses such as drought. Rhizosphaera needle cast can be distinguished from these other causes by the presence of the fruiting bodies on the needles, visible with a hand lens.
This article originally appeared in the 12/12/2004 issue.