The fungus Heterobasidion annosum (Fr.) Bref., which used to be called Fomes annosus (Fr.) Cooke, is considered as the most dangerous fungus in coniferous stands. We can find it almost everywhere, especially in the Northern hemisphere, and it causes some great losses of production. Indeed, it attacks heartwood in the stem, and root systems. Although it does not often kill directly trees, it destroys timber that we expected to harvest.
Besides, it is quite difficult to see it, and to notice what the infected trees are. However, as stems and root systems are weakened, infected are very weak against wind. That is why we often have a lot of windthrows, due to this pathogen. Finally, there are often some bark beetle attacks in infested stands.
As it concerns some valuable species, we have to prevent this fungus from spreading, even if it is rather impossible. Some methods exist to limit damages, but, nowadays, it does not seem possible to eradicate this pathogenic agent.
II Susceptible species
The quantity of species that is affected by this fungus is quite impressive. It would be too long to describe all. However, it mainly touches conifers, especially Pine species Pinus spp. For example, some Loblolly pines can be infected. But Douglas-fir, Sitka and Norway spruces, western red-cedar... are also victims of this pathogen. As these species are really important for wood production, it is evidence that we have to protect them.
Some broadleaves are also concerned, even if it is less frequent. Some damages were observed on Alder Alnus spp., Chestnut Castanea spp., Poplar Populus spp. or even Oak Quercus spp.
As it touches a lot of different species, we can find the fungus almost everywhere. Either in Europe or in North America, all kinds of stands are affected, but the temperate coniferous stands are the weakest one against this pathogen.
In fact, we can find it everywhere in Canada, and, in the Unites States, it is mainly based in the South East, along the Eastern coast. However, even in Europe, countries like Great Britain, France, or Germany... have the same problem. The main problem is obviously in Scandinavia, where Scots Pine and Norway spruce are very common. In Finland, these species are the two principal ones. So, when it touches stands, it can have some serious damages, and it causes some great economical losses. In this country where the forest cover is bigger than 60%, we have to prevent epidemics.
This disease is caused by a biological agent, which is well-known as it exists on a big scale. But we do not know when it was discovered for the first time.
This fungus belongs to the family of Basidiomycota and the Order of Aphyllophorales.
Once spores are on a freshly cut stump, it will develop and infect the whole stand. It can also infect a tree which is wounded.
However, one interesting thing is that there are two types of Heterobasidion annosum. The s-type will affect only some coniferous species. However, for example, it will not develop on hardwood. The other one is the p-type, which will develop in all the hosts that are described in the part “Susceptible species”.
This fungus also produces some small spores in great quantity. It can be transported by wind and, along coasts, water can even transport them. Therefore, the spread can happen on long distances. Thus we can find it almost everywhere in the Northern hemisphere.
Spores colonise freshly cut stumps surfaces, and it can develop on them and infest root systems after. Besides, wounded trees can easily be infected as openings are a good way of entrance for the fungus.
However, there is one particular way of transmission of this fungus. In fact, in a stand, root systems are often connected each other. There are always some links between several root systems. For the fungus, it is a good way to infect healthy trees.
It is not easy to notice this fungus on a tree. In fact, roots are buried and we cannot see wood that is destroyed by the pathogen. However, some symptoms can be visible. On some young trees, the crown is not well developed. Besides, the foliage often becomes chlorotic, which implies a change of colour. But, a lot of infected trees do not show these symptoms.
Therefore we have to notice some other symptoms. The biggest one is presence of fruiting bodies. In fact, some carpophores appear at the underside of roots, or at the basis of the stem. These big mushrooms have a dark-brown to black upper surface, while the lower surface is white and porous.
Finally, we can observe that heartwood has a change of colour before being destroyed by the fungus. Some zones become dark. Besides, some roots have some black stain, which is evidence of the presence of the Annosus root rot.
We can prevent the spread of this disease by using several methods. The first one is a physical one. In fact, as we know that root-to-root contacts are a cause of this spread, we can decide to dig a trench around infected trees. It will cut links between root systems, and it will block the fungus, as the hole will constitute a barrier for it.
Besides, we have to prevent wounds on healthy trees. Indeed, if bark is removed, it will allow spores to infect this wounded stem. So, during thinnings and silvicultural management, foresters have to be extra careful.
Another method is chemical. In fact, it consists in using Borax. This product is often used as a powder that we have to apply on freshly cut surfaces. If the concentration is sufficient, it is really efficient as it totally prevents stumps from being infested.
Finally, there is a third method, which is biological. In fact, the fungus Peniophora gigantea has been discovered in the 50’s, and it also colonizes freshly cut stumps. However, its presence prevents Heterobasidion annosum from infesting these stumps. Besides, this biological agent is not dangerous for healthy trees. Therefore, this agent is commercialised in small sachets, and can be used on freshly cut surfaces, as well as the chemical treatment.
IX Research fronts
Some researches are led on controls of this disease. Research wants to improve their efficiency. In fact, for the biological agent, some changes have to be applied, like the technical application, to be more efficient. Besides, it is perfectly adapted to Pine stumps, but, concerning spruce, this fungal agent was not often present.
Furthermore, it was shown that there were some differences depending on varieties. In fact, for Norway spruce Picea abies, different clones exist, and their resistance against the fungus is not the same. Therefore it is important to make some studies on these resistances.
• PRATT J.E, January 1997, Fomes Stump Treatment - An update, Quarterly Journal of Forestry, Vol.91, n°1, 3p.
• 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