Small-spored spruce-Labrador tea rusts
- French disease name: Rouille à petites spores des aiguilles de l'épinette et du thé du Labrador
- Other disease names: Spruce needle rust
-
Pathogen names:
- Chrysomyxa nagodhii P.E. Crane
- Chrysomyxa neoglandulosi P.E. Crane
- Kingdom: Fungi
- Phylum: Basidiomycota
- Class: Pucciniomycetes
- Order: Pucciniales
- Family: Coleosporiaceae
General information and importance
Chrysomyxa nagodhii and C. neoglandulosi are two closely related North American species of rust fungi that have small spores compared to those of C. ledicola (large-spored spruce-Labrador tea rust). Microscopic examination is necessary to distinguish between the two species because their distribution and host range overlap. Both rusts may be present on the same leaf of a host plant. The rusts are pathogenic to both spruce (Picea) and Labrador tea (Rhododendron section Ledum: several species of a small evergreen shrub). Prior to 2000, both rust species were named as varieties of C. ledi. However, more recent studies have shown that C. ledi is a complex of multiple species. Subsequently, many of its former varieties, including both of these rusts, have been elevated to species level. Chrysomyxa ledi in its current, strictly defined sense has a distribution range limited to Eurasia. It is microscopically distinct from endemic North American species. Small-spored spruce-Labrador tea rusts in Canada cause foliar discoloration and premature needle loss on spruce, and spots on the undersurface of Labrador tea leaves.
Distribution and hosts
Endemic to North America, small-spored spruce Labrador tea rusts are found wherever their Labrador tea hosts occur, regardless of whether their spruce host also occurs nearby, as they are perennial on this host. Labrador tea species are members of the genus Rhododendron, subsection Ledum (formerly considered a separate genus, Ledum). They are small shrubs with evergreen leaves, common in peatlands and bogs. On conifers, both rust species occur only on species of spruce, with infections requiring the presence of Labrador tea nearby.
The more widespread of the two species is C. nagodhii. Its primary host is Labrador tea (Rhododendron groenlandicum, formerly Ledum groenlandicum). It also occurs on marsh Labrador tea (R. tomentosum ssp. decumbens, formerly L. decumbens). On spruce it is known to occur on Engelmann spruce (Picea engelmannii), white spruce (P. glauca), black spruce (P. mariana), and red spruce (P. rubens). Its distribution range follows its Labrador tea hosts, only overlapping with C. neoglandulosi in western Canada on Engelmann spruce. It occurs in all Canadian provinces and territories, and in the United States in Alaska and the north central and eastern states.
C. neoglandulosi is host-specific to western Labrador tea (R. neoglandulosum, formerly L. glandulosum), and its distribution range is likely to follow the range of this host. Western Labrador tea occurs in western Canada in montane to subalpine areas of southern British Columbia (east of the Coast-Cascade mountains) and southwestern Alberta. In the western United States, it occurs in Montana, Idaho, Wyoming, and northern California. Engelmann spruce is its only known conifer host. It occurs in similar habitats in western Canada and the United States.
Tree parts affected
Foliage of both spruce and Labrador tea, occasionally leaf stems of Labrador tea.
Symptoms and signs
Both rust species cause yellowing of infected current year needles of spruce, resulting in premature casting. They produce yellow or orange leaf spots on Labrador tea. They are distinguishable from each other by microscopic features of spore morphology.
Chrysomyxa nagodhii: Spermogonia and aecia form on rusty yellow bands on infected spruce needles. Spermogonia occur on all sides of the needle. They are globose, 0.7 to 2.0 millimetres wide, prominent, and rusty brown. Spermatia are ellipsoidal to globose, colourless, and 3 to 4.5 micrometres × 2 to 3.5 micrometres. Aecia also occur on all sides of the needle, measuring 0.3 to 2 millimetres wide. They are tubular and covered with a thin colourless layer (peridium), which eventually weathers away. Aeciospores are deep orange and vary in shape from globose to ellipsoidal to egg-shaped. They are 15 to 31 micrometres long × 14 to 24 micrometres wide and ornamented with closely packed tapered warts that are partially covered, with a smooth cap along one side. Uredinia are produced in yellow-orange mottled spots on the undersurface of the Labrador tea leaf and measure 0.3 millimetres wide. Urediniospores are yellow-orange and irregular in shape, ranging from ellipsoidal to subglobose to egg-shaped. They measure 15 to 36 micrometres × 12 to 22 micrometres and are ornamented with closely shaped flat-topped warts (except for smooth patches at both ends). They stick to each other and form small columns that ascend from the uredinia if left undisturbed. Telia are deep orange, gelatinous, and 0.1 to 0.2 millimetres wide. They produce teliospores that are initially cubical and become elliptical as they mature, measuring 12 to 23 micrometres × 15 to 28 micrometres. Basidiospores are colourless, globose to subglobose, and measure 6 to 13 micrometres × 6 to 12 micrometres.
Chrysomyxa neoglandulosi: Spermogonia and aecia are amphigenous and produced in yellowed bands of current year spruce needles. Spermogonia are pale brown and elliptical, measuring 90 to 170 micrometres high × 114 to 144 micrometres wide. Spermatia are colourless and elliptical to ovoid, measuring 2.5 to 3.7 micrometres long × 1.6 to 2.5 micrometres wide. Aecia are similar in appearance to those of C. nagodhii. Aeciospores are dark orange and globose to subglobose. They are 18 to 25 micrometres × 14 to 22 micrometres, with a cell wall that is 1.6 to 2.5 micrometres thick and ornamented with tiny spine-like warts. Unlike C. nagodhii, the aeciospores lack smooth caps. Uredinia and telia are produced on the stems and underside of yellow spots on Labrador tea leaves. Uredinia are less than 0.3 millimetres in diameter, scattered or in circular groups, and cone-shaped before they push through the epidermis. Urediniospores are orange and globose to subglobose. They are 14 to 28 micrometres × 11 to 23 micrometres, with finely pointed, evenly spaced warts. Telia are apricot yellow and rounded. They are more irregularly shaped than uredinia, measuring 0.3 to 0.8 millimetres in diameter. Teliospores are yellowish and measure 12 to 15 micrometres × 14 to 22 micrometres. Basidiospores are colourless, subglobose or irregular in shape. They are 6 to 10 micrometres × 6 to 9 micrometres.
Other Chrysomyxa species will also produce aecia on spruce or uredinia and telia on Labrador tea. More than one species may occur intermingled on the foliage of a single host plant.
Chrysomyxa cassandrae also produces small aeciospores on spruce and is found in all Canadian provinces and territories. Like C. nagodhii, it has a longitudinal cap on its aeciospores. However, the cap has a distinctive broken edge and is ornamented with wide, shallow warts. It also has a different telial host: leatherleaf (Chamaedaphne calyculata), which is a small shrub often found in wetlands and black spruce peatlands.
Chrysomyxa reticulata (formerly known as C. ledi var. rhododendri) occurs on Labrador tea species (R. groenlandicum and R. decumbens) in British Columbia, Alberta, and Nova Scotia. It is also pathogenic to cultivated rhododendrons, through spread from nearby native species, but has not been found in nature on spruce. Aecia have only been produced on white spruce by artificial inoculation. On its Labrador tea hosts, it is identified by having very small urediniospores (less than 20 micrometres long). This is smaller than any other species in this complex. The urediniospores have a distinctive reticulated (netted) patch of ornamentation.
Large-spored spruce-Labrador tea rust is found throughout Canada and produces similar fruiting bodies on its hosts. However, it produces uredinia on the upper leaf surface of Labrador tea and distinctly larger spores on both hosts.
Disease cycle
The life cycles of both rusts are very similar. They are obligate parasites (require a living host to grow and reproduce). They are macrocyclic (have five different spore types: spermatia, aeciospores, urediniospores, teliospores, and basidiospores), and heteroecious (require alternation between two unrelated host species at different stages of their life cycle to fully complete it). Spermogonia and aecia develop and produce spermatia and aeciospores on spruce, and 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 in late spring, releasing urediniospores, which become windborne and land on and infect adjacent Labrador tea plants. New uredinia form throughout the growing season, increasing inoculum and numbers of Labrador tea infections. Both rusts can persist perennially by overwintering in leaf spots on their Labrador tea host in the absence of an alternate spruce host. The urediniospores of C. nagodhii are different from other Chrysomyxa species because their spread is facilitated by oribatid mites. The spores are sticky and adhere to the bodies of the mites.
On spruce, only current year needles are susceptible to new infections from basidiospores produced on Labrador tea. The basidiospores germinate, and the rust mycelium colonizes the needle 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
These rusts cause premature loss of current year needles on spruce. However, they do not cause as much damage and have much lower levels of incidence than large-spored spruce-Labrador tea rust. Disease levels on spruce are highest in boggy areas where Labrador tea is prolific.
Prevention and management
Control of the rusts in natural forests is not practical. However, elimination of nearby Labrador tea in the vicinity of spruce reforestation nurseries would reduce the levels of infected seedlings. Likewise, establishing new nurseries in areas distant from bogs hosting Labrador tea would also be beneficial. Ornamental Rhododendron species and cultivars are also susceptible to the rusts, so planting them close to areas with Labrador tea is not recommended.
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.
Photos
Aecia and aeciospores of small-spored spruce-Labrador tea rusts on a black spruce needle.
Gaston Laflamme
Rust-affected needles on Colorado spruce.
André Carpentier
Rust-affected needles on black spruce caused by small-spored spruce-Labrador tea rusts.
Marc Bolduc
Closer image of rust-affected needles on black spruce.
Gaston Laflamme
Labrador tea, an alternate host of the small-spored spruce-Labrador tea rusts.
Small-spored spruce-Labrador tea rusts on white spruce.
Selected references
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.
Crane, P.E. 2001. Morphology, taxonomy, and nomenclature of the Chrysomyxa ledi complex and related rust fungi on spruce and Ericaceae in North America and Europe. Canadian Journal of Botany 79(8): 957–982. https://doi.org/10.1139/b01-071
Feau, N.; Vialle, A.; Allaire, M.; 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
Savile, D.B.O. 1950. North American species of Chrysomyxa. Canadian Journal of Research, 28c(3): 318–330. https://doi.org/10.1139/cjr50c-018
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.
Weir, J.R. 1923. The genus Chrysomyxa. Mycologia 15(4): 183–187.
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.