Black mildew
- French disease name: Fumagine
- Pathogen name: Rasutoria abietis (Dearn.) M.E. Barr
- Kingdom: Fungi
- Phylum: Ascomycota
- Class: Dothideomycetes
- Order: Capnodiales
- Family: Euantennariaceae
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Partial list of synonyms:
- Dimerosporium abietis Dearn.
- Epipolaeum abietis (Dearn.) Shoemaker
General information and importance
Black mildew on fir (Abies) is caused by the fungus Rasutoria abietis, which is native to western Canada and the United States. This fungus is considered weakly parasitic, with its mostly superficial colonies causing minimal damage to fir trees. Although not a major pathogen, black mildew can be an indicator of environmental conditions conducive to other more serious fungal diseases.
Distribution and hosts
Black mildew is found throughout western Canada and the United States, colonizing the needles of various species of fir (Abies). Although common in its native range, black mildew has not been reported as a significant issue outside of western North America.
Tree parts affected
Needles
Symptoms and signs
Infected needles, whether living or dead, develop conspicuous black, soot-like colonies that impart the common name “black mildew”. Needles of all age classes are affected, and severely colonized needles may senesce prematurely.
Rasutoria abietis produces pseudothecia that are dark brown and setose (hairy), with a short yellow beak no more than 20 micrometres high. These structures range from 120 to 180 micrometres in diameter and form clusters on the radiating superficial mycelium on the undersides of needles. Asci are saccate, 50 to 70 micrometres × 20 to 36 micrometres, eight-spored, and bitunicate; paraphyses are absent. Ascospores are ovoid, hyaline to light brown, 1-septate, 24 to 36 micrometres × 6 to 9 micrometres, with older ascospores rarely becoming 3-septate and finely echinulate. In advanced cases, the black colonies may become extensive, covering a significant portion of the needle surface, and leading to increased needle drop.
Disease cycle
Limited research has been done on the complete life cycle of R. abietis. Infections are likely initiated by airborne ascospores, which land on host needles, germinate, and develop dark brown, radiating, superficial mycelia. The fungus enters the needle through stomata and extracts nutrients from adjacent host cells. Pseudothecia develop from the superficial mycelia. Like many needle blights and foliar pathogens, R. abietis tends to be more prevalent during wet or humid springs. The timing of spore release and infection may vary based on local climatic conditions, with higher prevalence in areas experiencing prolonged periods of high humidity.
Damage
The black mildew fungus R. abietis can extensively colonize needles epiphytically and invade through stomata, extracting nutrients from adjacent host cells. Rasutoria abietis is weakly parasitic, causing limited damage, although extensive colonization can accelerate needle senescence. Although needle drop may be accelerated in heavily colonized trees, overall tree health is rarely affected. However, trees experiencing chronic colonization over multiple seasons may show a reduction in photosynthetic capacity due to premature needle loss.
Prevention and management
Treatment of forests for black mildew on fir is not necessary in Canada due to the generally low levels of damage associated with this disease. 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
Funk, A. 1985. Foliar fungi of western trees. Canadian Forestry Service, Pacific Forest Research Centre. Victoria, British Columbia. BC-X-265. 159 p. https://ostrnrcan-dostrncan.canada.ca/entities/publication/f31475f4-c426-46c2-9456-21025c2c419c?fromSearchPage=true
Hagle, S.K.; Gibson, K.E.; Tunnock, S. 2003. Field guide to diseases and insect pests of northern and central Rocky Mountain conifers. Report No. R1-03-08. (Reprinted in 2013 with minor revisions; B.A. Ferguson, Montana DNRC, editor). United States Department of Agriculture, Forest Service, State and Private Forestry, Northern and Intermountain Regions. Missoula, Montana, and Ogden, Utah. 197 p.
Hunt, R.S.; Wood, C.S. 1978. True fir blights. Fisheries and Environment Canada, Canadian Forestry Service, Pacific Forest Research Centre, Victoria, British Columbia. Forest Pest Leaflet 46. 4 p.
Shoemaker, R.A. 1965. Revision of some Dimeriella and Dimerosporium parasites of conifers. Canadian Journal of Botany 43(6):631–639. https://doi.org/10.1139/b65-070
Winton, L.M.; Stone, J.K.; Hansen, E.M.; Shoemaker, R.A. 2007. The systematic position of Phaeocryptopus gaeumannii. Mycologia 99(2): 240–252. https://doi.org/10.1080/15572536.2007.11832584