Spruce needle rust is caused by the fungus Chrysomyxa weirii. Spruce needle rust is different from other landscape rust diseases (e.g. cedar-apple rust) in that the pathogen is autoecious; it occurs only on spruce and has no known alternate host. Trees are rarely killed but extensive needle discoloration and premature needle drop occur, reducing the aesthetic appearance and compounding other stresses. There are several species in genus Chrysomyxa causing a variety of symptoms on spruce in eastern North America, but C. weirii is the primary causal agent of spruce needle rust (also known as Weir's spruce cushion rust) in New England.
Most spruce (Picea) species are susceptible but Colorado blue spruce (P. pungens), especially trees growing in tight hedgerows or in deep shade, is particularly susceptible to the disease. Other common hosts include white spruce (P. glauca), black spruce (P. mariana), red spruce (P. rubens), and Sitka spruce (P. sitchensis).
Symptoms & Disease Cycle
In late winter to early spring, orange to yellow spots or bands appear on infected needles. At first, they may be hard to detect, but when conditions warm and bud break approaches, the bands swell to produce waxy, yellow-orange pustules containing masses of spores. At bud break, the pustules erupt and the yellow-orange spores are dispersed by wind and rain to newly emerging needles on adjacent branches or nearby trees. Wet weather will prolong the spore production period. Newly infected needles often do not show symptoms until the following winter or spring. After the pathogen sporulates on needles infected the previous year, the needles may become rusty brown and drop during the summer. As with most needle diseases, spruce needle rust tends to affect the lower branches more severely but, because C. weirii spores are wind dispersed, the upper foliage may also be infected over time. Severely infected trees can have a orange-colored appearance from the lesions and repeated defoliation can retard growth and reduced aesthetic appearance.
Disease Cycle & Management
If detected early, infected branches can be pruned to slow or potentially stop the spread of the disease. Removal of severely infected trees will reduce spread of wind-blown spores to nearby healthy trees. Fungicides may be necessary to protect new growth and stop spread of the disease. Chlorothalonil is effective and is labeled for use on spruce. The first application should be made just before bud break and subsequent applications made on a 7-10 day spray interval until needles mature (three to five applications). Trees infected with spruce needle rust, especially blue spruce, are often harboring additional needle and stem diseases (e.g. Rhizosphaera needle cast and Cytospora canker).
Written by: Susan Scheufele and Nicholas Brazee