Gremmeniella abietina (Scleroderris Canker) - Fact Sheet

Background

Scleroderris canker was first noticed in North America around 1950, but the causative agent was not identified until 1966. The disease was first reported from Michigan where it was wreaking havoc in young red pine plantations (trees under 2 metres in height). In 1964 the first cases where reported from plantations in Quebec. In 1974 a new race, subsequently identified as the "European race" was found to be killing mature red and Scots pine in the Adirondack Mountain region of New York State. Mortality was over 90% on 20-30 year old trees in some areas.

Hosts

There are two distinct races of scleroderris canker in North America; the North American race which infects mainly pine (Pinus spp.) and occasionally spruce (Picea spp.) and the more virulent European race which infects pine (Pinus spp.), larch (Larix spp.), spruce (Picea spp.) and fir (Abies spp.).

Distribution

  • Asia: Georgia and Japan
  • Europe: All countries.
  • North America: Canada ( ON, NB, NFLD, PQ.); USA ( NH, NY, MN, MI*, VT).

* Only the North American race of Scleroderris Canker has been found in Michigan.

Biology

The chief mode of infection for this fungus is by windblown ascospores which takes place under moist conditions. The mature spores are disseminated from sexual structures call apothecia (fig. 1.) from May to October with the peak period of infection occurring in June and July. Hosts become infected through wounded buds, shoots and needles. The infection starts at the tip of the branch (fig. 4.), growing down the length to the main stem where a canker may be formed (fig. 5.). Infection occurs during the active growing period of the tree but the disease can not progress aggressively until winter when the tree is dormant and the environmental conditions are right. The fungus grows well in cool moist conditions, the ideal environment which exists under snow cover (fig. 7.). As a result, the greatest damage can be seen on the lower branches and shoots of infected trees (fig. 3.). Infected branches are usually dead the following spring. Throughout the year on infected trees small black pycnidia (fig. 2.) appear at the base of dead needles or on dead shoots. Infective asexual spores (conidia) which ooze from these structures are transported to neighbouring shoots or branches by rain splash. The sexual fruiting structures (apothecia) occur in the same place as pycnidia but appear one year after the branch has died.

Three strains of the fungus have been identified; European, North American and Asian. The European strain is the most virulent and has a wider host range (fig. 6.). The North American strain attacks young trees but rarely damages tree over 2 metres. It has been observed that the European strain produces few apothecia and ascospores in the field. In contrast, the North American strain produces numerous ascospores.

Detection and Identification

Symptoms:

Infected trees will not show clear symptoms until the following spring. In the spring look for brown or orange coloured needles. The killed needles are loose and easily fall off the branch. Pycnidia will be present in the empty needle fascicles. On infected shoots a green stain can often be observed beneath the bark. On small or stressed trees the fungus will form a canker on the stem.

Identification:

Pycnidia occur in clusters or singly on stems and needles. They are dark brown to black in colour, stromatic, multilocular , without openings and up to 1 mm wide. Apothecia appear in clusters on the surface of stems and in the axes of needles. They are borne on short stalks, are brown to black in colour and up to 1 mm in diameter.

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Text: S. Wallace, Plant Health Survey Unit.
Photos: Canadian Forest Service, Laurentian Forestry Centre, St. Foy, Quebec.

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