Introduction

Poplar Populus spp. is a species that is commonly used in Central Europe. In fact, it is really appreciated for its profit, as its annual growth often is very important. Even if its quality is not applicable for the best products, a lot of owners decide to plant it to get money. However, like other tree species, it suffers from a lot of diseases that could be lethal for the whole stand. Therefore the weaknesses against pathogens must be fought to reach the final cutting in due form.

Among these diseases, a rust called Melampsora larici-populina exists. It causes some problems for the poplar stands and can decimate them if we do not act. This disease is really serious, thus we have to take it in consideration from the plantation to the final harvesting.

On the one hand, this paper will describe how the disease attacks and hurts the poplar stands. The first part concerns some general knowledge about the illness. On the second hand, we will see the bad consequences on stands, especially as it touches a very important species in our countries. Finally, we will try to find some solutions to prevent the disease from having consequences on stands.

This rust is a well-known disease

As populiculture is quite a recent activity, its growth is important, in terms of surface and volume. As we rapidly discovered the economical advantages of this, a lot of studies were led to prevent appearances of diseases. So, nowadays, this pathogen is well-known but it still continues to attack.

It needs two different species to develop

A correlation between larch and poplar

This disease is caused by a fungus, whose family is Basidiomycota. The particularity of rusts is that they need two hosts to achieve their development. (VANDERWEYEN) Concerning this one, the development is due to a correlation between poplars and larches Larix spp. So, if a larch stand is close from a poplar plantation, it obviously increases the risk of appearance and spread of the fungus.

However, as the spores are really small and very light, wind can disseminate them for a few kilometres (more than 2 kilometres according to PINON).

The cycle of development

The fungus is scattered by spores. However, they are subjected to evolution. So, we can detail the disease transmission into five different stages (VANDERWEYEN) :

   On an infectious tree, basidiospores will produce some small spores called spermaties. It happens on poplars’ leaves.
   A meeting between two different myceliums will lead to appearance of ecidiospores. These one will grow only on a different host, which means that they will be displaced by the wind. Once on the larch, they will be able to develop themselves.
   Uredospores appear now, and their role is to scatter the disease on the second host.
   After that, there are some compound teliospores, whose cellular nucleus becomes diploid.
   Finally, they will germinate and some basidiospores appear. They are not infectious for larch, but only for poplar.

With this whole process, the disease can fulfil the entire disease cycle : penetration, colonization, and then be harmful for the host. Besides, the propagation goes fast as it has been estimated that less than 5 years have been necessary for the rust to conquer Europe (LEFEVRE).

It perpetually evolves

One of the problems with this kind of disease is that it evolves. In fact, it means that solutions can be found to fight the disease, but, it does not prevent a new and stronger race of the fungus from appearing.

Some cultivars are resistant...

Diversity is the main strength of poplars. In fact, this species is subjected to a lot of studies, and genetic improvement. Thus a lot of clones exist, due to combination between different kinds of poplar (e.g. Populus nigra ; Populus trichocarpa ; Populus deltoides...) With these studies, a lot of cultivars are created, with their own characteristics (e.g. of cultivars : I214 ; Dorskamp ; Koster ; Flevo ; Ghoy...)

Some cultivars were developed and are resistant to the rust (e.g. Hazendans ; Hoogvoort). But, as we will see, they are not obviously the best ones nowadays.

...but new races of the rust appear

Concerning these two last cultivars, they are resistant, but only to 4 races of the rust. It means that a disease can have different races, and it is the case for Melampsora. For the moment, 5 races exist : E1 ; E2 ; E3 ; E4 ; and E5, which is the most recent. Indeed, E4 has appeared in 1994, while E5 has appeared three years later (DUPLESSIS, 2001).

So, the new races are always more virulent than the previous ones. It means that their capacity to infect is more important. It is different from the aggressiveness, which represents the consequences on trees and stands. However, the most aggressive rust on a precise cultivar is not always the most aggressive on another one. (FREY 2002, PINON 1999)

That is why a cultivar like Beaupré is resistant to E1, E2 and E3, but not the two last races. Besides, Hazendans is resistant to E4, but not to E5, which is stronger. As a result, this cultivar can be attacked by the rust like the other ones (For more details on resistances of cultivars : PINON and FREY in 1999 in Forêt-Entreprise n°126, p.56).

Consequences on our poplar stands

The disease has some consequences that can be observed on trees. Beside, we can find some indirect effects, due to the fungus, which explains the behaviour of trees.

It is characterized by some small orange pustules that appear on the inferior face of leaves. These leaves become orange, get dry and fall because of the disease. Some young trees can die because of it, if the majority of leaves is infected. Moreover, although trees do not always die, there is a loss of production : obviously, as leaves catch the light to produce organic matter, production depends on quantity of functional leaves. So, with the fall of leaves, the area to catch the light is decreased, and the photosynthesis is less important.

However, even if the attack of the fungus is not always lethal for trees, it weakens them a lot. Hence, they are perfect preys for insects. So, there is a relationship between the fungus and insects. The two pathogens that are linked fulfil to the death of poplars.

It touches one of the most important species in our country

Poplar is an important species if the main objective of a forest owner is to make money. For example, the rate of profit can reach 10%, which is the hugest rate that we can obtain in forestry.

Some figures about Poplar in France

The following figures must be considered carefully, as they are not the same following to the authors. As the sources of information are numerous, there are some differences.

Figures change according to the reference

France is the first producer of poplars in Europe. It is the evidence that this species is relevant in this country. In term of surface, it represents 250000 hectares, that is to say 4,5% of the national area (LEFEVRE). It is the second broadleaves species after oaks Quercus spp., according to this reference, in term of volume. With another reference, it only represents 1,5% of the territory, but is the first hardwood species in term of volume, even before oaks (BARTHOT 1999 cited by SIROIS in 2000). It is also said that production of saw timber is almost 3 million tons per year.

Moreover, the average annual growth is almost 8-12 m3/ha/year, but on very fertile zones, it can reach 15-20 m3/ha/year. According to the productivity, we can decide to harvest poplars between 10 and 25 years after the plantation (PERINET, 1999).

So, figures are really numerous and it is difficult to know which ones are right. Therefore we have to decide which organism we can trust, and the IFN* seems to be the best one as it is its role. * IFN : Inventaire Forestier National

We may trust our national Inventory IFN

According to IFN, there are approximately 8,6 million cubic metres of poplars in France. Concerning surface, it only represents less than 75200 hectares, while the production of poplar wood is only 513750 cubic metres per year. As the annual growth is close to 500000 cubic metres per year, it represents an average of 6,6 m3/ha/year.

Why is this species so appreciated and so special ?

The economical aspect is really attractive for forest owners. In fact, there is not only the income for the owner, as populiculture is also important for employment : if one plants 10 hectares of poplars, it creates a job in the same time (LECLERC 1999 cited by SIROIS in 2000). Concerning its management, it is really easy, and not very expansive : in fact, we plant almost 200 plants/ha, and, after the rotation time, we can harvest. Even if there are some silvicultural works that are necessary (pruning, weed control...) there is no thinning, and the costs of management are really cheap. As the rotation time is really short, the profitability is better.

In spite of bad mechanical properties, poplars are commonly used. In fact, the best quality that we can obtain is used for veneer-making (especially for plywood) or sawmilling.

Besides, poplar is one of the rare species to adapt on wet soils. It is the best alternative on this kind of sites, as it permits to obtain a good productivity. It is a good way to favour this kind of soils and grounds.

The disease decreases production

Growth of the stand is limited and obviously lower

We saw that consequences of the disease have an effect on production. Indeed, it is a foliar disease, so it touches the part which is necessary to make photosynthesis process. Obviously, with a lesser photosynthesis, trees cannot produce as much timber as usual, thus production is lower.

The fungus can diminish the capacity of growth by 50% (DOWKIW, 2003). It shows that we have to do something against it, as with such a damage, it is no worth planting poplars anymore. All the management is based on the incredible growth of poplars, so, if it is altered, this species loses all its advantages.

It generates some economic losses

It is logical to say that if production gets lesser, the economical aspect also diminishes. As profitability is the main advantage of populiculture, consequences of the disease are really bad for economics.

An example will be clearer to explain this problem : in fact, a hectare of poplars produces almost 200 m3 during a rotation, in normal conditions (PERINET, 1999). With a loss of half of production, it only produces 100 m3/ha. According to tables of prices of wood in France, the best quality for poplars can be sold 65 euros/m3 (Prices in February 2004, published in “Forêts de France”). It is the maximum price and it only concerns veneer-making quality, which is the most valuable for this species. So, in this example, the owner can lose 6500 euros/ha, which is a really important loss. Even with a less valuable quality, whose price is 30 euros in average, the economical loss would be 3000 euros/ha.

A disease that must be fought

The best way is to know perfectly the disease

Our knowledge concerning the disease is the best way to prevent its spread. Besides, if we continue these investigations, to discover how cultivars react against the rust, it will be a good way to know how to manage without favouring the spread.

The exterior factors that influence it

The climate is the first abiotic factor. In fact, we cannot have an influence on it, but we can estimate the risk of planting in precise climatic conditions. Furthermore, with the greenhouse effect, the average temperature increases on the planet. This increment favours some diseases’ development, as well as the diminishing of some plant’s resistances (LEPOIVRE, 2001). However, it is the microclimate that is most important. For example, the spread is favoured by a wet and fresh environment, so, in shaded zones, it is unadvised to plant.

Concerning the conditions of the ground, the spread is easier if there is grass covering the area. Besides, if there is a lack of potassium in the soil, or an excess of nitrogen, it is dangerous to plant poplars. All these factors are evocated by PINON and FREY (1999)

The natural evolution of the fungus is inevitable. Indeed, there are always some mutations, which lead to appearance of new races. Although they belong to the same disease, their genetic characteristics are not the same.

Finally, the last factor is Mankind, which increases the risk of development by exchanging products. With transport of logs in the whole world, it favours the potential transport of pathogens, and so, their scattering.

What we can ameliorate

One of the greatest problems is homogeneity of stands. In fact, when a cultivar has a lot of advantages (rapid growth, resistance...), it is commonly used by owners, and on a big scale. So, although there are a lot of available clones, only a few are used, because we only plant the best ones. However, the matter with cultivars is that they have the same genotype. Thus, if the rust succeeds in attacking one tree, nothing can prevent it from attacking “another same one”, as all trees are clones.

So, we have to limit the areas with the same clone. Diversity is a solution and we have to increase its application. That is the main advantage in wild poplar stands, where trees do not come from a nursery. With a sexual reproduction, new genotypes are created and trees are totally different each other.

Furthermore, it would be a good thing to reduce greenhouse gas emission. Indeed, development of pathogens is favoured by the increment of temperature. So, it is one additional bad consequence of greenhouse effect, therefore, we shall take it in consideration.

Some silvicultural methods to prevent its development

The biggest improvement that we can do lies in management. In fact, some common methods can limit appearance and spread of the rust. For example, as grass is a vector of the spread, some weed controls have to be applied. It permits to prevent the appearance of a plant cover on the ground.

Besides, lopping is really necessary. In fact, it not only consists in improving the quality of timber, but also in preventing spread. Like grass, the lowest branches favour infection and development. Finally, high density must be avoided. For poplars, it is not really a problem as they are always planted with a small density. The good one is almost 200 stems/ha, which represents a space 7x7 metres. Indeed, high density creates a shaded and moist environment. Besides, crowns touch each other, so, if a tree gets ill because of the rust, it can easily transmit it to its neighbours by the leaves.

Resistance is a key word

It is necessary...

Some clones are developed by different research organisms. In France, INRA is the most famous. It permits to obtain some multiple genotypes, and resistance against rust is one of the main aims of developing a new one.

There are two types of resistances : the vertical one and the horizontal one (Conseil du Peuplier du Canada, 1998). The vertical resistance is only due to one gene, while the other one is polygenic. The vertical resistance is easier to develop, but it is also easier for the pathogen to thwart it. Contrary to this, the polygenic resistance is difficult for scientists, but more efficient against the fungus.

...but it is also an utopia

A cultivar like Hazendans is perfect against races of the rust, except the new one, E5. In fact, this race is more harmful, but is not really developed in France yet. So, to prevent its spread, we must not plant such a cultivar. Hazendans is one of the rare cultivar to be sensitive to this race, so, even if it is a new clone and has a lot of advantages, we cannot use it. It is the best way to get sure that E5 will not develop itself on a big scale (PINON).

We saw the consequences of such a development with Beaupré. This cultivar is resistant to E1, E2, E3, so, until 1994, it was really appreciated and everybody planted this clone. But, when E4 appeared, it was a real catastrophe. E4 could develop as Beaupré was planted everywhere in France.

Other solutions

The greatest solution lies in diversity of poplars in stands. In fact, we saw that planting the same clone at a large scale had awful consequences. Now, we have to repair our mistakes. For example, Koster is a tolerant cultivar. Even if it is not resistant, it is a good alternative to replace Beaupré, in moderation.

Besides, research centres work on developing new varieties and clones, to improve resistances (FREY, PINON). It permits to diversify, which is really the key word. As each clone has its own tolerance and resistance, one race of the rust cannot develop in a large scale if we mix clones.

Conclusion

If poplars had been less important in France, there would have been no worth doing so many studies and investigations to fight Melampsora larici-populina. However, even if we know this pathogen, we cannot stop it definitely. There is no perfect solution, as too many factors influence the disease. When we try to prevent it by developing new resistant clones, the fungus always evolve to become more virulent.

However, the best solution is to diversify stands, to obtain different genotypes more or less resistant to each race of the rust. By this way, the fungus cannot develop at a large scale. So, even if it is impossible to eradicate such a disease, some methods must be applied to prevent its spread. At a local scale, it is obviously easier to fight against the pathogen, and to limit damage.

Thibaud SURINI

BIBLIOGRAPHY

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