This dramatic increase can be attributed to a number of factors including the weather, increased planting of Douglas fir and associated seed sources, and environmental stress. The primary damage associated with this important disease is defoliation which leads to suppressed growth and to value loss in Christmas trees. Symptoms of Rhabdocline first become apparent in late fall or early winter as yellow spots on one or both surfaces of first-year needles. A distinctive diagnostic characteristic is the sharp border between the healthy green tissue and the infected brown tissue. Discolored needles are most conspicuous in early spring.
Symptoms are often most severe on the lower portion of the tree where air circulation is poor.
Although some of the heavily infected needles drop before or during budbreak, most will persist for several months. In late spring, fruiting structures of the fungus develop beneath the epidermis on the lower surface of the needle. The epidermis eventually ruptures and splits open, usually in two longitudinal lines, and exposes the spores of the fungus. These spores are carried by rain and wind to newly expanding needles. When the spores land on immature needles they germinate, penetrate the cuticle, and begin to grow within the needle.