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. Even though the fungus has infected the needle, no obvious external symptoms are evident until considerably later, by fall or winter.
Infection is favored by cool, moist weather and periods of rain.
Rhabdocline needlecast is most damaging in plantations where weed growth, close spacing of trees or dense foliage induced by shearing impede air circulation and prolong wetness on lower branches. Rogue severely symptomatic trees. Practice good sanitation. San Isabel, Lincoln, Apache, Cibola, Kaibob, Coconino g. Fungicide sprays. Sprays should continue until needles are fully elongated and conditions are no longer favorable for disease.