Rhadocline needle cast is caused by the fungus Rhabdocline weirii.
Rhabdocline needlecast affects only Douglas-fir (Pseudotsuga menziesii).
Symptoms & Disease Cycle
This disease is found throughout North America and outbreaks can occur in both landscape settings and Christmas tree plantations. The fungus causes needle browning and premature defoliation, leading to decreased vigor and exposure to secondary insect pests and pathogens. The needle loss also significantly reduces aesthetics and for Christmas tree growers, infected trees are often unsalable. Because of the Rhabdocline needle cast (and Swiss needle cast), many Christmas tree growers in New England do not grow Douglas-fir. Symptoms first appear in late summer to autumn as yellow/brown spots or blotches on both the upper and lower surface of current year needles. The lesions enlarge and become reddish-brown as the disease progresses. Infected needles may have one to several small lesions, or entire needles may be blighted in heavily infected trees. The disease often initiates and becomes most severe in the lower canopy due to increased shade and moisture and less air flow. In late spring, the lesions begin to swell as the fungus produces fruiting structures below the epidermis. The epidermis splits along the length of the needle, exposing a mass of spores which are spread by wind and rain to newly emerged needles. The exact timing of spore dispersal is often dependent on the weather, but typically it occurs just after budbreak. There is one infection cycle per year.
In locations where the disease is known to occur, or sites less than ideal for Douglas-fir, select cultivars with partial resistance to the disease. Among the most resistant varieties are Shuswap and Pillar Lake. Moderately resistant varieties are Santa Fe, Silver Creek, and Coville, while San Isabel, Lincoln, Apache, Cibola, Kaibob and Coconino are the most susceptible to the disease. Begin scouting for disease symptoms in early April or just before bud break when the needle lesions are most conspicuous. Symptoms are most easily observed on an overcast day, when needle discoloration and lesions can be readily observed. Prune out heavily infected shoots and remove severely infected trees to prevent spread of disease to healthy trees. Do not prune when foliage is wet. Douglas-fir is shade intolerant and requires full sun to thrive. Trees planted in even partial shade are likely to develop needle blight problems over time. If the planting location is exposed to any shade during the day, do not plant Douglas-fir. If trees are planted in rows, maintain proper spacing and prune the lowest canopy branches to promote air flow. Avoid establishing ground cover plantings and remove weeds around the base, which can decrease airflow and allow moisture to linger. Fungicides can be used in an attempt to control Rhabdocline in landscapes and Christmas tree farms. Thorough coverage of all canopy parts is required for effective use. Fungicide applications should commence once 50% of buds have broken and new growth has reached 0.5" inch in length. Subsequent applications should be made at labeled intervals until conditions no longer favor disease development. Three fungicide applications are usually required to achieve good control. Fungicides labeled for use include: copper hydroxide, copper sulfate, copper salts of fatty and/or rosin acids, mancozeb and thiophanate-methyl.