Large-spored spruce – Labrador tea rust

Written by B.E.Callan Revision 2024

• French disease name: Rouille des aiguilles de l'épinette et du thé du Labrador
• Other disease names: Spruce needle rust
• Pathogen name: Chrysomyxa ledicola (Peck) Lagerh.
• Kingdom: Fungi
• Phylum: Basidiomycota
• Class: Pucciniomycetes
• Order: Pucciniales
• Family: Coleosporiaceae
• Partial list of synonyms:
  • Melampsoropsis ledicola
  • Uredo ledicola

General information and importance

Chrysomyxa ledicola is the most common and destructive spruce needle rust fungus in North America. It causes yellow discolouration and premature needle death on spruce (Picea). It also causes lesions and produces abundant spores on the underleaf surface of its alternate host, Labrador tea (Rhododendron subsection Ledum, formerly Ledum).

Distribution and hosts

Endemic to North America, Chrysomyxa ledicola is found wherever its Labrador tea hosts occur. In Canada, it is found in every province and territory, and ranges as far north as the tree line. In the United States, it is found in the northwest (including Alaska), northeast, and central states. It also is found in Greenland. In Japan and Russia, it only occurs on Rhododendron.

On conifers, the rust is specific to spruce. In Canada, it infects Engelmann spruce (P. engelmannii), white spruce (P. glauca), black spruce (P. mariana), red spruce (P. rubens), and Sitka spruce (P. sitchensis). It also occurs on blue spruce (P. pungens) in the United States (where it grows naturally) and in Canada (where it is widely planted as an ornamental).

The rust causes leaf spots on its alternate hosts, species of Labrador tea, which are members of the genus Rhododendron, subsection Ledum. Labrador tea is a small shrub with evergreen leaves, common in peatlands and bogs. In Canada and elsewhere in North America, alternate host species include Labrador tea (R. groenlandicum, formerly Ledum groenlandicum), western Labrador tea (R. neoglandulosum, formerly L. glandulosum), and marsh Labrador tea (R. tomentosum ssp. decumbens, formerly L. decumbens). In Eurasia, the flowering shrubs R. calostrotum and R. griersonianum are also hosts.

Tree parts affected

On Labrador tea: leaves and occasionally flower stalks and seed capsules.

On spruce: current-year needles and occasionally cone scales.

Symptoms and signs

Infected trees have a yellow-orange cast to the foliage. In areas with heavy infections on numerous trees, the discharged rust spores accumulate as orange dust on understory plants. This also commonly leaves an orange film on the shorelines of nearby bodies of water. Individual infections on spruce needles are first evident as thin yellow bands across the width of otherwise green needles. Advanced infections result in the yellowing of entire needles. Spermogonia are numerous and produced on all surfaces of infected spruce needles. They are present on every spruce host except Sitka spruce. They are globose and measure 100 to 200 micrometres wide × 90 to 150 micrometres high. They are visible as tiny dots with the aid of a hand lens. They are pale at first and darken to a reddish brown to black colour when dry. Spermatia (spores produced in spermogonia) are colourless, 2 to 4 micrometres long × 1.6 to 3.3 micrometres wide. Aecia are produced on all sides of the spruce needles, and erupt through the epidermis, forming blisters up to 4 millimetres long. They are covered by a thin, colourless tissue layer (peridium) that quickly wears away, leaving a small fringe around the aeciospore mass inside. Aeciospores are orange en masse, globose to subglobose, and sometimes elongated. They range from 23 to 60 micrometres long × 18 to 54 micrometres wide. Larger spores are more common in samples collected from western Canada. They have very thick cell walls (up to 2.5 micrometres), which are covered in large warts that are star-shaped in surface view. The combined thickness of the spore wall and warts ranges from 1.6 to 5.7 micrometres. Aecia and spermogonia are also occasionally produced on cone scales.

On Labrador tea, uredinia and telia develop in irregular reddish spots on the upper surface of 1-year old (or older) leaves. They rarely develop on the midvein of the leaf undersurface. On plants occurring on the west coast of Canada, they also occasionally develop on flower stems and seed capsules. Uredinia are round and 0.2 to 0.6 micrometres wide. They either are single and scattered, or form in circular groups, erupting through the host epidermis. The urediniospores they produce are globose to ovoid or ellipsoidal, sometimes with a shallow longitudinal indentation that is ornamented with warts. They are 21 to 44 micrometres long × 14 to 37 micrometres wide. Telia erupt through the host epidermis. They are gelatinous at first and later become crust-like and orange to reddish brown in colour. Teliospores are produced in chains. They are cube-shaped to oblong and measure 18 to 24 micrometres × 12 to 16 micrometres. The teliospores produce colourless, smooth-walled basidiospores, which are round to slightly pear-shaped, and measure 7.5 to 12 micrometres × 6.5 to 11 micrometres.

The closely related Chrysomyxa empetri has a different uredinial/telial host (crowberry, Empetrum nigrum) but also produces aecia on spruce needles. Aecia of C. empetri are more tongue-like and up to 1.4 millimetres tall, with a tougher, more persistent peridium. Their aeciospores are more egg-shaped to ellipsoid, 27 to 50 (up to 60) micrometres long × 16 to 37 micrometres wide. They have much thinner cell walls, tiny warts, and smooth patches on both ends. Two other Chrysomyxa species, C. nagohidii and C. neoglandulosi  also occur on Labrador tea but produce much smaller spores on all their hosts.

Disease cycle

Chrysomyxa ledicola is an obligate parasite (requires a living host to grow and reproduce). It is macrocyclic (has five different spore types: spermatia, aeciospores, urediniospores, teliospores, and basidiospores), and heteroecious (requires alternation between two unrelated host species at different stages of its life cycle to fully complete it). Spermogonia and aecia develop and produce spermatia and aeciospores on spruce. The uredinia, telia, and basidia develop and produce urediniospores, teliospores, and basidiospores on Labrador tea or rhododendron. Except for the urediniospores, which are produced on and re-infect Labrador tea, the other spore stages are not infective to the host they are produced on; they can only infect the alternate host.

In early spring, telia develop from mycelium in overwintered leaf spots on Labrador tea. They produce teliospores, which germinate to produce basidia and basidiospores around the time that new spruce needles are emerging from buds. Basidiospores, which become windborne, land on nearby young spruce needles, germinate, and infect the conifer host. After telia are produced, uredinia start to develop, releasing urediniospores that become windborne and land on and infect adjacent Labrador tea plants. These infections do not become visible as new leaf spots until the fall. By midsummer in northern regions, diseased leaves bearing the current crop of uredinia and telia fall off. The new spots that appear in the fall do not start producing uredinia until the following spring, except for coastal or more southern locations where the longer summers may result in new uredinia and repeat infections in the same growing season. The rust can persist perennially on its Labrador tea host in the absence of a spruce host due to the overwintering mycelium in leaf spots.

Current year spruce needles are the only foliage susceptible to new infections from basidiospores produced on Labrador tea. The basidiospores germinate on the young, soft needle, and the rust mycelium colonizes it and produces spermogonia scattered along the stomatal lines. Spermatia are extruded from the spermogonia in a sweet liquid that is attractive to insects. This causes the spread of spermatia to other spermogonia and facilitates sexual recombination. Aecia develop in late spring. During the summer, they release aeciospores that become windborne and infect nearby Labrador tea plants. Symptoms of needle yellowing start to appear by mid-summer. Infected needles are prematurely shed the following year.

Damage

Large-spored spruce-Labrador tea rust causes premature defoliation and, on rare occasions, cone rust of spruce. It also causes leaf spots on Labrador tea and ornamental rhododendrons. It rarely causes mortality, but in seasons where infections are heavy, the entire crop of current year spruce needles may be lost. Older trees are affected by reduced growth in years with heavy rust. In 2020, more than 10,000 hectares of trees infected with large-spored spruce-Labrador tea rust were mapped aerially in northeastern British Columbia, with 83% percent of the infections rated as moderate in severity. Most of the trees affected were 60 to 80 years old. Severe outbreaks (up to 90 or 100% current-year needle loss) have also been reported from Alaska and Manitoba.

Prevention and management

Young understory spruce trees are most likely to succumb to the stress of repeated defoliation of year-old foliage, so the establishment of spruce nurseries near large populations of Labrador tea should be avoided. Ornamental rhododendrons may be affected by leaf blight if they are grown close to sources of Labrador tea. In some regions, different spruce species may have lower levels of infection if they have a delayed bud break that does not coincide as precisely with basidiospore production on Labrador tea. Observations of spruce trees in Nova Scotia reported that white spruce buds opened earlier than those of red spruce and black spruce and were more affected by large-spored spruce-Labrador tea rust.

Pest management strategies for a particular pest vary depending on several factors. These include:

• the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
• the expected damage or other negative consequences of the pest’s activity and population level (either to the host, property, or the environment);
• an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
• how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
• the value of the host(s) versus the costs of pest management approaches; and
• consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.

Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against any particular pest.

Pesticides registered for use against Chrysomyxa ledicola under specific situations may change from year to year. Therefore, please search Health Canada’s Pesticide Product Information Database for currently registered pesticides and product information for use against C. ledicola. The application of any registered product should be based on population size and applied only when necessary and against the approved life stage. It is also recommended to consult a local tree care professional. Pesticides may be toxic to humans, animals, birds, fish, and other beneficial insects. Apply registered products only as necessary and follow all directions and precautions noted on the manufacturer’s label. In some jurisdictions and situations, only a licensed professional can apply pesticides. Consulting relevant local authorities to determine local regulations that are in place is recommended.

Photos

Aecia of Chrysomyxa ledicola on spruce needles.

Uredinia of the fungus Chrysomyxa ledicola on the upper surface of leaves of Labrador tea.

FIDS

Aecia of Chrysomyxa ledicola on spruce needles.

John Vallentgoed

Aecia of Chrysomyxa ledicola on a spruce needle. Note the orange colour of the spore mass and the torn pustules, indicating aeciospores are being released. 

An infected leaf of Labrador tea with uredinia of the fungus Chrysomyxa ledicola. Labrador tea is an alternate host of C. ledicola, which causes large-spored spruce-Labrador tea rust on spruce.

Selected references

Allen, E.A.; Morrison, D.J.; Wallis, G.W. 1996. Common tree diseases of British Columbia. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Victoria, British Columbia. 178 p.

Crane, P.E. 2000. Systematics and biology of the genus Chrysomyxa (Uredinales). Ph.D. thesis, Department of Biological Sciences, University of Alberta. Edmonton, Alberta. 249 p.

Fraser, W.P. 1911. Cultures of some heteroecious rusts. Mycologia 3: 67–74.

Fraser, W.P. 1912. Cultures of heteroecious rusts. Mycologia 4: l75–193.

Feau, N.; Vialle, A.; Maier, W.; Hamelin, R.C. 2011. DNA barcoding in the rust genus Chrysomyxa and its implications for the phylogeny of the genus. Mycologia 103(6): 1250–1266. https://doi.org/10.3852/10-426

Hiratsuka, Y.; Langor, D.W.; Crane, P.E. 1995. A field guide to forest insects and diseases of the prairie provinces. Natural Resources Canada, Canadian Forestry Service. Northwest Region, Northern Forestry Centre. Edmonton, Alberta. Special Report No. 3. 297 p.

Khasa, D.P.; Hiratsuka, Y. 1995. Chrysomyxa ledicola Lagerh. and its aecial hosts in British Columbia. Canadian Journal of Forest Research 25(9): 1555–1563. 

Khasa, D.P.; Hiratsuka, Y. 1996. Chrysomyxa ledicola Lagerh. and its telial hosts in British Columbia. Canadian Journal of Forest Research 26(3): 465–472. 

Myren, D.T.; Laflamme, G.; Singh, P.; Magasi, L.P.; Lachance, D. (editors). 1994. Tree diseases of eastern Canada. Natural Resources Canada, Canadian Forest Service, Science and Sustainable Development Directorate. Canada Communication Group Publishing. Ottawa, Ontario. 159 p.

Sinclair, W.A.; Lyon, H.H. 2005. Diseases of trees and shrubs. Second edition. Comstock Publishing Associates, Cornell University Press. Ithaca, New York. 660 p.

Westfall, J.; Ebata, T.; Bains, B. 2020. 2020 Summary of forest health conditions in British Columbia. British Columbia Ministry of Forests, Lands, Natural Resource Operations and Rural Development, Resources Practices Branch. Pest Management Report Number 15. 86 p.

Yukon Forest Health. 2009. Spruce needle rust. Yukon Energy, Mines and Resources, Forest Management Branch. Forest insect and disease brochure 21. 16 p.

Ziller, W.G. 1974. The tree rusts of western Canada. Environment Canada, Canadian Forestry Service, Pacific Forest Research Centre. Victoria, British Columbia. Publication 1329. 272 p.

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