Different species of Armillaria exist, but their behaviour is almost the same. They are very common in forest stands, and we can find it almost everywhere, especially in Northern hemisphere, in temperate forests.
Besides, damages caused by this fungus are always serious. Indeed, it causes growth reduction, and often mortality of trees. In fact, it provokes a root rot, and the wood is also altered by the fungus. Therefore, trees are weakened, and even if they do not die, they become sensitive to insect attacks (e.g. bark beetles) and wind. Obviously, there are some economical losses. With Heterobasidion annosum, it is probably the most famous root disease.
Armillaria mellea (Vahl :Fr.) Kummer is one of the most relevant in France, as it touches a lot of species that we can find there. Hence, foresters have to be aware of this pathological problem, as it is always possible to find it in a forest stand.
II Susceptible species
It is said that this pathogen attacks hundreds of species. Contrary to Armillaria ostoyae which attacks a lot of conifers, this one is more relevant on broadleaves.
Therefore, it can be present on species like maple Acer spp., birch Betula spp., walnut Juglans spp., poplar Populus spp., oak Quercus spp., willow Salix spp. or even elm Ulmus spp. So, it affects some important species of production that we can find in France and even elsewhere.
However, some conifers are also affected by this species of Armillaria. For example, we can notice infections on fir Abies spp., Spruce Picea spp., Pine Pinus spp. or even Thuya Thuja spp.
As it touches a lot of different species, this fungus is distributed on a big scale. The Northern hemisphere is especially concerned, as this fungus is frequent in temperate forests. So, we can find it in North America (Eastern part of Canada and almost in each state of the USA) and in Europe. For example, countries like France, Germany, Poland or Scandinavian countries are affected by this pathogen.
However, a specific species Armillaria luteobubalina is present and relevant in Australasia. So, the distribution can be extended to this region too. Besides, this genus is not only present in temperate forests, as we can also find it in tropical conditions. It is also the evidence of its importance in term of pathology.
This fungus is well-known, but we do not know where it appeared for the first time. However, scientists began to be interested by it at the beginning of the nineteenth century. Indeed, it has been classified in 1821 for the first time by Fries.
This fungus belongs to the family of Basidiomycota, while its Order is Agaricales.
It will develop some carpophores that are visible at the basis of an infected stem. Their colour is yellow, that is why we often call this fungus “Honey mushroom”. They are the reproductive stage of the development. Hence, they produce millions of spores, yellow to white-coloured.
The fungus lives as a parasite in living tissues, but it can also develop on dead stumps as a saprophyte. These stumps are “food sources” for the pathogen which can live and survive during several decades. A mycelium develops and infests root systems.
The fungus also develops rhizomorphs, which can grow on the soil. However, there is a need of a food source to bring some nutrients. But, if the development is good and vigorous, these rhizomorphs can penetrate the bark of roots and infest the whole root system. For this, there are two phenomena that are a mechanical pressure and an enzyme action.
Once there is an infection on a tree, it can easily grow and develop, before infesting new trees. There are two main ways of infection.
The first one is due to mycelium which develops in roots. In high density stands, there are a lot of connections between several root systems. So, a mycelium can infect healthy roots by this way. It is the same way of propagation than the Fomes annosus for example.
However, there is a second way of contamination, which is much more specific to this disease. Rhizomorphs that are produced by the fungus grow on the soil and can penetrate healthy roots, as it is explained before.
There are several methods to identify this fungus. The first one is the fruiting bodies that are carpophores that appear at the basis of boles. They are yellow and quite easy to recognize. Besides, there is often a cluster of mushrooms, therefore it is rather noticeable.
Rhizomorphs are also visible, even if it is very small. But, one of the most visible symptoms is under the bark. In fact, there is a white mycelium that is present, and it is very characteristic.
Furthermore, once there is an infection, trees have behaviour and some specific symptoms. For conifers, there is often a lot of resin that is outside the bark. It is a natural reaction to limit infection by the fungus. Some other symptoms are :
• A change of colour of leaves and needles.
• Overproduction of cones for conifers, due to stress.
• Mortality of trees.
• Some windthrows in the plot, as trees are weakened.
• Root system and wood are altered, and spongy for broadleaves.
There are several methods of control. Before infection, the best thing to do is to plant adapted species according to soils and climatic conditions. So, it is not a recommended to plant when the soil is full of water, as it will favour development of pathogens. Furthermore, infection directly depends on vigour of trees. Besides, it is better when stands are mixed and not monospecific.
Concerning mechanical operations, we have to prevent wounds of roots, of bark, as it is a way of infection. Once there is an infection, we can dig a deep trench to break root systems connections. However, it is not enough as rhizomorphs can still cause problems. So, it is recommended to remove dead stumps, and to burn them after. Although it is expansive, it is necessary to limit risks of infection of new stands.
There are no real chemical treatments for this fungus. However, we can use some chemical fumigants (chloropicrin, methyl bromide, carbon disulfide) to reduce infection. But, it is mainly used for high valuable trees.
IX Research fronts
The main front consists in finding some resistance against this fungus. So, for Douglas fir and western larch, which are touched by Armillaria ostoyae, some resistances were found by scientists. Indeed, they produce some necrophylactic periderm to limit infection. However, they also found that its production depended on the age of individuals. Some natural resistances would also be a great solution to prevent this fungus from causing great damages.
• 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