The fungus Lachnellula willkommii (Hartig) Dennis is commonly named the Larch canker. As we will see after, it touches different species of Larch. Although this disease is not very relevant on a world scale, it affects a genus which has some advantages (especially in France, where European larch is natural in the South-East). This species is adapted in mountains and can produce timber in spite of high altitudes. That is why it is a good choice in plantations in mountains.
II Susceptible species
This fungus attacks a specific genus : Larix spp. We can also find it on Pseudolarix genus, but it is less important, as this genus is less represented in the world.
However, concerning Larix spp., it gathers some well-known species such as European Larch Larix decidua or Japanese Larch Larix kaempferi. These two species are present in France ; therefore this disease could be relevant one day. Larch is an important species, especially in mountains, that is why a disease could disturb severely this species and the global environment.
Furthermore, for researches, infection was inoculated to a specific species, which is Larix laricina (Du Roi) K.Koch.
It was discovered in North America, and it is still present there. Indeed, some attacks are identified in the state of Maine in the USA. Besides, the region of New Brunswick is concerned by this disease. We can also find it in Nova Scotia.
So, not a lot of regions are affected by this canker. However, their crossing point is that they have a maritime climate. It seems that the fungus prefers this climate. Therefore some maritime regions refuse to plant larches, to prevent any risk of spread of this canker. In France, for example, we can find larches in Western part and in the South-East. These regions are close to the Mediterranean Sea or the Atlantic Ocean. So, maritime conditions can be found there, which could represent a risk of appearance of this canker.
This disease was first found in the state of Massachusetts in the USA. It was in 1927. However, as it was not very significant. Therefore we often consider that the first relevant attack of this canker was identified in 1980 in Eastern part of Canada. This canker is fungal. So, it does not belong to cankers which are caused by bacterium for example.
This fungus is classified in the Order of Leotiales, and it belongs to the family Ascomycota. Some other names exist to speak about the same fungus. Thus, we can find the following names : Trichoscyphella willkommii or even Dasyscypha willkommii.
As it was discovered a few years ago, biology of this fungus cannot be detailed. However, some researches led by INRA (Institut National de la Recherche Agronomique) showed that infection really depended on vitality of organs. The weakest ones will be more sensitive to infection.
The direct consequence of an infection is a loss of growth. In fact, not many trees die from this disease, but they often suffer from it in terms of production. Therefore it is important to prevent this fungus from appearing, as larches can have an interesting production.
Like every fungus, it is mainly propagated by wind. Spores are disseminated each year to colonize new individuals, and wind is the best way to transport these spores.
Besides, a high density can improve the propagation. Indeed, in a high density stand, branches and crowns are too close. Branches of two neighbours can even touch each other. So, if a tree is infected, it can easily transmit it to its neighbours.
Moreover, human beings can transmit the disease involuntarily. In fact, during control which consists in cutting infected branches, some spores can stay on tools. If it is not disinfected, spores can move onto other trees and infect it.
Even if the biological steps cannot be described with a lot of details, this pathogen is easy to recognize. It provokes bulges on the trunk and branches. The bark becomes wider and cracks. Therefore some resin goes out and run along the bark. This flowing out is quite easy to notice and identify.
Moreover, fructifications of the fungus appear on the tree. Although they are rather small (a few millimetres), they are orange and their form represents a saucer, which permit to see them easily.
As it touches essentially branches, the control has to focus on them. In fact, foresters have to remove infected branches, to prevent the spread of the disease. However, it must be done under specific conditions. Indeed, we have to wait winter time and a dry weather : it permits to limit propagation of the fungus, as it prefers high moisture. Besides, tools have to be as clean as possible. Therefore we should clean tools with alcohol, after each cutting of an infested branch. However, it is very long and not always possible.
But, wounds must be recovered by fungicide mastic. Indeed, wounds are openings for the pathogen, and that is why we need to protect trees by “closing” these wounds.
Some infected trees can also die, even if it is quite rare. In fact, it concerns essentially young trees, and the ones which grow in poor conditions. In case of death, we have to remove these trees from the plot.
However, in most cases, trees are damages but do not die. So, we have to improve the vitality of trees, by applying a fertilizer for example.
Finally, a preventive control can be applied during periods of sporulation of the fungus. It is also a good mean to prevent the spread.
IX Research fronts
This disease can be considered as a new one, as it was really discovered at the end of the 20th century. So, research is very useful concerning this fungus.
The main research program concerns the infection and its biological aspects. It is not really well-known for the moment, and a bigger knowledge would permit to find new controls and to limit the spread.
Some researches were led to compare infection in a dormant stage and in a growth stage. It showed that infection of a dormant tree was faster and attacked internal tissues, compared to infection of a growing tree.
• BLANCHETTE Jean-Yves, 2001, Microscopic observations of the early stages of the infection process by Lachnellula willkommii (Hartig) Dennis artificially inoculated on larch (Larix laricina (Du Roi) K. Koch) seedlings. The University of New Brunswick.
• Tree Doctor Software, developed by CFPF, French Agriculture and fishing Minister, Chlorophyl Assistance, Citare, French forest development Institute, Disease and advisory Service forest research, Instituto per le piante de legno e l’ambiente, Alterra, Staatsbosehner, IPC groene ruimte