The fungus Nectria coccinea is a canker that attacks Beech Fagus spp. It has mainly been important in North America, and is still nowadays. It is called Beech Bark Disease. It provokes some bad consequences like the diminishing of foliar area, and can cause mortality. The value of timber is also decreased.
Therefore we have to prevent this disease from spreading, especially in our European countries. Indeed, in France, Beech is the second main species of hardwood, in term of production. Its value is rather important, so, such a disease could have some awful economical consequences.
But, this fungus is not the only culprit : it acts in correlation with an insect, Cryptococcus fagisuga, which allows the fungus to enter a tree. So, we will see that these two organisms are linked.
II Susceptible species
As I said, this disease concerns the whole genus Fagus that gathers all the species of beech. So, in North America, it attacks Fagus grandifolia, the American Beech. In European countries, especially in France, we can find Fagus sylvatica, which is very valuable and important for production of hardwood.
Although the fungus is not always lethal for trees, it decreases the vitality and facilitates penetration of other pathogens. So, some fungus like Armillaria mellea can enter and kill the trees, and some insects can also cause damages after infection by Nectria.
On the other hand, trees are weakened by these attacks, and it can cause a lot of windthrows.
The disease is mainly located in North America. Some regions and stated are touched by this disease, especially in Canada and in the USA. For example, we can find it in the Eastern part of Canada (Quebec), in Ohio, Maine, Pennsylvania, Massachusetts, West Virginia, or even in New Jersey.
In France, the disease could spread where we can find a lot of beeches : that means, Normandy, Pyrenees and even in North-East (region of “Les Vosges”).
This disease seems to develop in maritime zones. That is why it could easily develop in Normandy, which is close to the sea. Besides, in mountain regions in France, moisture is higher than in other regions, so it could improve the spread too.
The insect and the species Nectria coccinea are native to Europe. But, it first appears in the USA in 1932.
We even know that, before 1914, human beings thought that death of trees was due to the insect. The correlation between an insect and a fungus was unknown.
The fungus is a canker belonging to the Order of Hypocreales, and the family of Ascomycota.
The first step of infection is due to the insect. In fact, females put their eggs under the bark. They provoke some wounds on the barks and open it into small holes. It permits spores to enter the bark, and infest the tree.
However, some different spores exist : first of all, there are some fruiting bodies, that are red, and that we can find easily on the bark. They are connected with some little pockets, which contain eight spores. It is called the sexual stage of the fungus. Finally, there are also some white spores that are asexual.
The fruiting bodies, called Perithecia, are still dangerous after the death of trees. Indeed, they continue production of spores, and can still scatter them in a forest stand. Therefore it is necessary to remove infested trees.
Epidemiology is a great problem with this disease because there are two causal agents. So, if we want to prevent an epidemic, we have to control these two agents : the insect and the fungus.
As usual, spores of the fungus are disseminated by wind, rain. If a stand is too dense, it will also facilitate contacts between individuals. If there were an infested tree, it would easily transmit it to its neighbours.
Besides, fungus and insect can be transported by animals, especially birds. It can scatter spores and larvae to many kilometres far from the origin. Therefore an epidemic can extend to a large region.
Before noticing the presence of this fungus in a stand, we have to see some insects, already installed under barks. In fact, it is the first visible sign of the disease. Insects are quite easy to identify, as they produce a little cocoon, often qualified as a white wax.
After this stage, if Nectria comes in a stand and infests some trees, we will see some dark points on the bark. These dead spots are characteristic of this pathogen. A slime flux runs along the bark from these points. It is the first sign of infection by the fungus. After that, we can observe some red fruiting bodies.
Some different control methods can be applied to prevent the disease from appearing. In fact, it is better to fight against the insect. Indeed, if no insects attack trees, Nectria cannot infest. For this, some chemical products can act as insecticides, once some populations of insects are detected.
However, it is quite difficult to apply such a control in forests. Indeed, using insecticides is mostly used in arboretums and for ornamental trees. Besides, a preventive method is not really efficient. It is easier to act after infection. The only preventive way that could be efficient would be to institute some resistant trees in stands, as we will see in the next part.
Once the fungus has colonized trees, the best way to cure the stand is to remove infested individuals. Cutting stems permits to cut dead trees and prevent the spread.
IX Research fronts
Some trees seem to be resistant against the insect Cryptococcus fagisuga. We know that without this insect, Nectria cannot attack trees. The proportion of resistant trees is quite small (1 to 5%), but, a work on it could be useful.
Therefore, Research aims at developing some resistant trees in nurseries, to obtain more resistant individuals. Indeed, as it could concern a genetic aspect, development of a resistance could be transmitted naturally to the next generations of beech. If they are planted in a stand, this one could be unaffected by Nectria, which would be a great progress for forestry, for beech management and production.
• 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