Rhadocline needle cast is a common disease of Douglas-fir caused by the fungus Rhabdocline weirii. 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, making infected trees unsightly or unsalable. Because of the disease, many Christmas tree growers in New England do not grow Douglas-fir.
Rhabdocline needlecast affects only Douglas-fir (Pseudotsuga menziesii).
Symptoms & Disease Cycle
Symptoms first appear in late summer to autumn as yellow to 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 lower portion of the tree is more severely affected because of increased shade and moisture towards the base. 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.
Always purchase clean planting stock and select varieties 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. Symptoms are most easily observed on an overcast day, when needle discoloration and lesions can be readily observed. Prune out heavily infected branches and remove severely infected trees to prevent spread of disease to healthy trees. Do not prune when foliage is wet and disinfest tools after pruning using bleach or 70% alcohol. Keep trees healthy and maintain good air-flow to reduce needle wetness. If trees are planted in rows, keep them free from weeds around the base, which can decrease airflow and harbor moisture. 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 be initiated once 50% of buds have broken and new growth has reached 1/2 inch in length. Subsequent applications should be made at two to three week intervals until conditions no longer favor disease development. Three fungicide applications are usually required to achieve good control. Chlorothalonil gives the best control but copper based products and mancozeb are also effective materials.
Written by: Susan Scheufele and Nicholas Brazee
Fig 1. Needle lesions caused by Rhabdocline on a young Douglas-fir growing at a Christmas tree farm.