Pathogen
Weir's cushion rust, also known as spruce needle rust, is caused by the fungal pathogen Ceropsora weirii (formerly Chrysomyxa) (Aime and McTaggart 2020). This species is autoecious, meaning it requires only one host genus (Picea) to complete its life cycle. This was a unique feature in genus Chrysomyxa, as most species are heteroecious (require two unrelated plant hosts to complete their life cycles). Phylogenetic analysis proved the species was genetically distinct and it was reclassified in genus Ceropsora (Aime and McTaggart 2020). The fungus is widely distributed across North America and Asia (Sinclair and Lyon 2005).
Hosts
Many spruce (Picea) species are host to the pathogen, but Colorado blue spruce (P. pungens) is particularly susceptible. Other hosts in southern New England include white spruce (P. glauca), black spruce (P. mariana), Serbian spruce (P. omorika), Oriental spruce (P. orientalis), and red spruce (P. rubens).
Symptoms & Disease Cycle
In the northeast, C. weirii is native to boreal forests on white, red, and black spruce. The first documented occurrences of C. weirii on blue spruce in New England were from New Hampshire in 1969 (Pawuk 1970) and 1982 in Vermont (Bergdahl 1983). Based on submissions to the UMass Plant Diagnostic Laboratory, there was an increase in disease incidence on blue spruce across southern New England and New York State in the late 2000s and early 2010s, but since that time the disease has been less common. Overall, it appears that there can be long periods where the disease is uncommon but will flare up for relatively short periods when environmental conditions are ideal.
In late winter to early spring, orange to yellow spots or bands appear on infected one year old needles (Sinclair and Lyon 2005). 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 (Crane et al. 2000). Prolonged wet weather during this time will increase the spore production period and severity of disease the following year. Newly infected needles remain free of symptoms until the following winter or spring, approximately 10 months after infection. After the pathogen sporulates on one year old needles, they may become rusty brown and drop during the summer. As with most needle diseases of conifers, needle rust severity is often highest on lower canopy branches. Severely infected trees can experience reduced growth and degraded aesthetics but the disease does not cause serious dieback.
Management
Cultural management is difficult, since rust fungi are obligate parasites and they do not survive in dead needles. Removing infected one year old needles to reduce inoculum in late winter or early spring prior to the release of spores is not practical, especially given the disease does not cause major injury. Fungicide application to newly developing needles shortly after budbreak can suppress new infections. Copper-based fungicides, such as copper hydroxide and copper octanoate, should be effective against C. weirii. Spores are disseminated only in the spring and outside of this time, chemical control will not be effective. Because the disease is primarily an aesthetic issue, chemical management is not recommended in most cases.
Literature Cited
Aime MC and McTaggart AR. 2021. A higher-rank classification for rust fungi, with notes on genera. Fungal Systematics and Evolution 7(1): 21–47. https://doi.org/10.3114/fuse.2021.07.02
Bergdahl DR. 1983. Chrysomyxa weirii on Picea pungens in Vermont. Plant Disease 67(8): 918. https://doi.org/10.1094/PD-67-918
Crane PE, Hiratsuka Y, and Currah RS. 2000. Reproductive biology and evidence for water dispersal of teliospores in Chrysomyxa weirii, a microcyclic spruce needle rust. Mycologia 92(4): 754–763. https://doi.org/10.1080/00275514.2000.12061215
Pawuk, WH. 1970. Chrysomyxa weirii found on blue spruce (Picea pungens). Phytopathology 61(2): 131. https://doi.org/10.1094/Phyto-61-127
Sinclair WA and Lyon HH. 2005. Diseases of Trees and Shrubs, 2nd edn. Cornell University Press, Ithaca, NY.