Fir-polypody rust
- French disease name: Milesina leviuscula
- Pathogen name: Milesina leviuscula (Dietel & Holw.) Hirats.
- Kingdom: Fungi
- Phylum: Basidiomycota
- Class: Pucciniomycetes
- Order: Pucciniales
- Family: Milesinaceae
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Partial list of synonyms:
- Hyalopsora leviuscula (Dietel & Holw.) Arthur
- Milesia leviuscula (Dietel & Holw.) Faull
- Uredo leviuscula Dietel & Holw.
General information and importance
Milesina leviuscula is a rust fungus that causes a minor infection of fir (Abies) needles and leaf blight of licorice fern (Polypodium glycyrrhiza) (also known as polypody fern).
Distribution and hosts
Fir-polypody rust is native and endemic to coastal western North America where its hosts, grand fir (A. grandis) and licorice fern are found. It is rarely collected in the field on its fir host, and then only from Vancouver Island, British Columbia. It is hard to detect on fir and may be more widespread than reported because it occurs sporadically and does not cause readily observable levels of foliar discolouration or needle loss. On licorice fern, M. leviuscula has a much broader distribution, occurring in coastal areas of British Columbia, north to southern Alaska, and south to northern California.
Tree parts affected
Needles of fir, and foliage of licorice fern
Symptoms and signs
Fir needles infected with the rust appear paler in colour and turn brown and die at the end of the growing season. Spermogonia are colourless, subglobose, develop under the fir needle epidermis, and measure 150 to 190 micrometres × 150 to 230 micrometres. Spermatia, the spores that they produce, are colourless, smooth-walled, oblong, and measure 5.5 to 6.5 micrometres × 2.0 to 2.5 micrometres. Multiple aecia develop in rows on either side of the midrib of individual, infected current-year fir needles, which become slightly discoloured. Aecia are tube-shaped, measuring 0.3 to 0.5 millimetres in diameter, and up to 1.5 millimetres tall, initially covered with a fragile white peridium, which ruptures to expose the white aeciospores that are produced in long chains within. Aeciospores are colourless, single-celled, subglobose, measuring 20 to 40 micrometres × 18 to 26 micrometres, and covered with rod-like warts that are star-shaped in optical section.
Uredinia, produced beneath the epidermis of browned areas on the undersurface of fern leaves, are blister-like, round, measuring 0.2 to 0.5 millimetres in diameter, and covered with a peridium that ruptures to form a central pore or slit. Peridial cells of the uredinia are angular and thick-walled. Urediniospores are colourless, single-celled, smooth-walled, irregularly obovoid to subglobose, and measure 23 to 48 micrometres × 14 to 22 micrometres. Telia develop within epidermal cells of overwintered leaf undersurfaces, often completely filling the leaf cell with 1 to 40 teliospores. Teliospores are colourless, single-celled, smooth, thin-walled, and irregularly polygonal in shape, measuring 10 to 20 micrometres × 12 to 25 micrometres, each germinating to form a tube-like four-celled basidium. Each basidial cell produces a single-celled, colourless globose to ellipsoidal basidiospore measuring 7 to 11 micrometres × 7 to 8 micrometres.
Two other species of Milesina, which alternate from grand fir to sword fern (Polystichum munitum) have very rarely been observed on fir needles in western Canada (Vancouver Island, British Columbia). Further collections to confirm these records, as well as genomic and inoculation studies, would be required to fully understand their distribution and host associations. Milesina vogesiaca has tube-like aecia that are very similar in appearance to M. leviuscula, and its aeciospore size range partially overlaps with this species. However, they average slightly larger, measuring 32 to 46 micrometres × 24 to 30 micrometres. On sword fern, its urediniospores are smooth. Milesina winelandi is distinguishable from M. vogesiaca on its sword fern host by its spiny urediniospores.
Tube-like aecia are produced on fir needles by three other rust genera: Hyalopsora, Pucciniastrum, and Uredinopsis (see Table 1). The first two genera are readily separated from Milesina because they produce orange aeciospores. On fir, Uredinopsis species produce white tube-like aecia but much smaller spermogonia (80–150 micrometres wide) on the needles. None have Polypodium as alternate hosts; they occur on other fern genera. The fir-needle rusts are usually easier to identify on their alternate non-conifer hosts where they produce more variable and distinctive uredinia and telia. Additionally, almost every fir needle rust species is uniquely host-specific to a different non-conifer plant species or family. Firs are also susceptible to foliar rusts caused by Melampsora and Melampsorella, but these genera do not produce tubular aecia.
| Genus | Aeciospore colour | Needle age when aecia first develop | Needle age at aecia death | Spermogonial features |
|---|---|---|---|---|
| Hyalopsora | Orange | 2 y | Third growing season | Conspicuous, yellowish; 300–500 µm in diameter |
| Milesina | White | 1 y | The following spring | Subepidermal; 150–230 µm in diameter |
| Pucciniastrum | Orange | Current year or 1 y in early summer | The year aecia are produced | Inconspicuous or not present |
| Uredinopsis | White | Current year | Annual or some aecia regrowing each spring from the same spot for 1–4 y | Subcuticular or intraepidermal; ˂160 µm in diameter |
Disease cycle
The disease cycle of fir-polypody rust requires one year for completion. Basidiospores produced on the fern are released in spring, become airborne and, if they land on wet, newly flushed fir needles, germinate and establish an infection. Spermogonia are produced in late summer. Sexual recombination (mating) occurs when spermatia ooze out in a sweet sticky liquid and are spread (usually by mites or insects) to receptive hyphae on another nearby spermogonium. Aecia develop in September. Aeciospores are released in late autumn and early winter. They cannot re-infect fir, but if they land on wet fern foliage, they germinate and infect. Uredinia are produced the following spring on overwintered fern foliage. Urediniospores infect new fern foliage throughout the growing season; they are not capable of infecting fir. Telia also develop on the fern foliage during the summer and overwinter in the leaves. The telia remain in place in the leaves, germinating to form basidia the following spring to complete the cycle. Overwintering uredinia ensure that the rust can be perpetuated on its fern host in the absence of nearby fir trees.
Damage
Damage to forest trees is negligible, as the fungus has sparse and rare occurrence on its grand fir host in British Columbia.
Prevention and management
Prevention and management are generally not warranted because damage is negligible and population levels are usually low.
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.
Selected references
Aime, M.C.; McTaggart, A.R. 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
Bubner, B.; Buchheit, R.; Friedrich, F.; Kummer, V.; Scholler, M. 2019. Species identification of European forest pathogens of the genus Milesina (Pucciniales) using urediniospore morphology and molecular barcoding including M. woodwardiana sp. nov. MycoKeys 48: 1–40. https://doi.org/10.3897/mycokeys.48.30350
Faull, J.H. 1932. Taxonomy and geographical distribution of the genus Milesia. Contributions from the Arnold Arboretum of Harvard University. 2: 1–138. https://www.biodiversitylibrary.org/item/256973 [Accessed August 2024]
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.
Wegwitz, E. 1993. Needle and broom rusts of true firs. Forestry Canada, Pacific Forestry Centre. Victoria, British Columbia. Forest Pest Leaflet 45. 8 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.