Ophiostoma ulmi (Buisman) Nannf, which is well-known as Dutch Elm Disease, belongs to great diseases that touch forest stands. It is used to killing trees and has done great ravages in the past. It is also called Graphium ulmi or Ceratocystis ulmi.
Nowadays, this disease is well-known, and it permits to prevent it from appearing when it is possible. However, a lot of foresters really do not want to plant elms as, they fear that the illness could come and kill the whole stands, as it did in the past. Indeed, this organism is almost always lethal for trees, as it provokes a drying up.
So, the wood of elm species is rare and obviously valuable.
II Susceptible species
Only one genus is damages by this disease, but it was so important that it causes great losses. It is Ulmus spp., which gathers all the elms. We consider that 28 species of elms are affected by this pathogen. So, even if there is only one genus, it attacks a lot of species, which do not have great differences in term of resistance.
These species were really appreciated a few decades ago, but it was before the appearance of the fungus. Therefore we could find a lot of elm stands, in America or in Europe, as its wood has a good quality too.
But, when it arrived, it decimated all the stands, and only a few trees survived. It was considered as an economical and ecological catastrophe. It really was an epidemic, which was really serious for forestry.
This fungus could have been identified almost everywhere in the Northern hemisphere. Either in Europe or in North America, elm was a main species in forest. Therefore it was not really hard for the pathogen to colonize these two continents. It provoked like a world epidemic. The southern hemisphere is not really concerned, as elms do not grow in this part of the world.
Nowadays, elms are not very common as they are scattered. Production of elms is still risky therefore we do not find big monocultural stands.
This fungus was first discovered in 1918 in Northern France, and in the Netherlands. Therefore its name is Dutch elm disease. However, this fungal disease may find its origin in Asia.
Since 1918, it has made great damages in Europe. Besides, it was involuntarily introduced in the USA in 1928. Thus, it began to attack American elm, which was rather present on this continent. Besides, the first introduction in Canada was registered in 1940.
By the way, a new variety of this fungus appeared in 1970. It was more virulent than the previous one, and a new epidemic occurred and caused also some great damages on elms.
This fungal agent belongs to the Order of Ophiostomatales, and its family is Ascomycota. By the way, there is also another agent that is necessary for transmission of this fungus. It is an insect of the genus Scolytus spp.
Once the insect brings spores in a healthy elm, the fungus can develop. It will affect sapwood, and functional vessels. Because of its development, vessels will wilt, and sap can not circulate between roots and tops of trees. That is why Dutch elm disease is considered as a vascular wilt.
After that, leaves will dry up and the tree will die, because he is like deprived of sap, which is essential for its life.
The transmission of this fungus is really specific. In fact, epidemics are due to a vector which is an insect. It is a Scolytus which carries spores and moves from trees to trees. Indeed, this insect will feed with bark, and even some xylem vessels. It wounds the tree and spores can easily penetrate, as they are on body of the insect.
Besides, insects will dig some little galleries that can be observed easily, and females will lay their eggs there. So, the new generation that gets born here will be new vectors of the spread of this disease.
When a tree is standing up, it is impossible to see the damages of wood caused by the fungus. However, we can easily detect its presence in a stand, and we can also notice presence of the insects, which often belong to a large population.
So, if we notice these insects, we have to act, because it often means that the fungus is or can be there too.
By the way, if an elm is affected by Dutch elm disease, we can observe that its foliage is dry. Leaves are not green anymore, and wilt. This symptom is the most visible one.
Another symptom is a dark brown coloration of vessels. It forms a ring, and it is visible on dead branches for example.
The particularity of this disease is that there is no curative method. In fact, once a tree gets ill because of Ophiostoma ulmi, it is impossible to cure it. Once one detects an infection, the main objective is to prevent the spread.
For this, we can act on the two different agents. If we attack the insect, we have to use some insecticides. Moreover, we must not let dead trees on the ground, because insects often colonize dead trees, and their bark. So, removal of the bark is also conceivable.
Against the fungus, some fungicides can be applied, while it is not the most efficient way. We obviously remove dead trees, or infested ones. If there are some wound and openings in the bark, we should apply fungicide mastic to limit penetration by spores.
However, in plantation, the best solution is to plant some resistant species, while it is more expansive and not always efficient. Indeed, the fungus could evolve and some new varieties, stronger and more virulent, could appear. So, biodiversity should be also a good solution. The mix of species would prevent a fast spread.
IX Research fronts
Against this very dangerous pathogen, a lot of researches were led to find how to continue production of elms. For this, we had to find how to prevent appearance of the fungus in such stands.
So, the main research front consists in finding and developing a resistant elm. It means that we have to “create” a genetic resistance, which would be transmitted to other generations. Some previous researches on this topic were not always successful : for example, French scientists developed an elm called “orme resista” which was resistant in laboratory. However, it was not the case in forest, after plantation.
• 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