The rust fungus Chrysomyxa abietis (Wallr.) is really common on the genus Picea, which regroups species of spruce. As it is an important species in a country like France, it is necessary to fight this disease. In Finland, during the last epidemics in 1998, 63% of spruce trees and 86% of spruce seedlings were infested by this rust. It shows that its spread is rather fast and not really limited. So, in a Scandinavia, where Norway spruce is one of the three main species of production, the problem of the rust is important.
But it is developed in other continents too, as there were great damages in the USA, like in the state of Minnesota, for example. That is why research is mainly done in the whole world to find solutions against this disease. For example, some researches are done in Canada, Finland or even New-Zealand.
II Susceptible species
As already mentioned, it affects all kinds of spruces : Norway spruce Picea abies, Sitka spruce Picea sitchensis, and other species like Picea rubens, Picea pungens, and Picea engelmannii. However, concerning France, it is only the two first species that are concerned by the disease, as we do not find the other ones in forests.
Besides, this rust has no need of an alternative host to achieve its development (Peterson 1963 ; Grill et al., 1978). Contrary to Chrysomyxa ledicola, which develops some cells on Labrador tea Ledum groenlandicum, this species only grows on spruce. Moreover, some species of rusts can develop on angiosperms, and especially heathers, which belong to the family of Ericaceae.
Some rusts of this kind had a great impact in countries like Finland or the USA (state of Minnesota). In France, this rust is really common in forests. In fact, as there is no alternative host, it can develop entirely on spruces. Sitka spruce is present along the west coast of the country, as it needs a lot of humidity to grow. Hence, some rusts develop there.
Norway spruce is really disseminated in the country. A rust infection can appear in all the regions where there are some spruces.
This disease was studied by Unger in 1840, in his book entitled Beiträge zur Vergleichenden Pathologie 24. It is obviously a fungal origin, which means a biological one that belongs to the fifth kingdom. As a lot of spruces were brought by human beings in France, it was the better way to develop the disease. In fact, Norway spruce is natural in this country, but only in mountains. So, in plains and hills, it was planted voluntarily. Besides, Sitka spruce which comes from the west coast of USA is totally artificial in France. So, the large development of this disease in this country has a human origin.
This fungus, as a rust, belongs to the order of Uredinales, and the class of Basidiomycota. However, no alternative host is necessary for this species of rust ; therefore, there is a distinction with other species like Chrysomyxa ledicola or Chrysomyxa rhododendri.
Its development is the following one : the hyphae penetrated the needles through stomata. At the end of summer, telia appear. However, they do not develop until the following spring. Once this season arrives, teliospores that were produced germinate and produce some promycelia. That is in this organism that basidiospores are formed, and they are the agents responsible of the infection. Thus, they develop on underside of young needles.
The spread of this disease is commonly due to abiotic factors. In fact, wind, rainfall and water splash are the main ones. Besides, like other fungi, some climatic conditions favour the spread of the rust. For example, relative humidity and temperature are the main ones. With a low temperature and a high humidity, their will be a higher risk of epidemics. It is written that air moisture which is almost 85% represents an ideal rate for the development. Furthermore, a low radiation is favourable too.
A great epidemic happened at the end of the last century in Finland. As this country has a lot of Norway spruce, damages were important. However, a great infection one year does not mean that it will be repeated the year after.
This pathogen is quite easy to recognize. In fact, some orange pustules appear on needles of the tree, due to teliospores. As it scatters quickly, when basidiospores are released, a lot of needles are affected, and it is easy to see it, as all the ramification becomes orange or yellow. Needles die and fall after infection by these basidiospores.
The direct consequence of it is a loss of growth on trees. In fact, young trees are weaker than bigger ones, so, it can also be lethal for these seedlings. Besides, infection on big trees is also a problem as it weakens trees which can be attacked easily by other pathogens.
There are two kinds of control that we can do. Before the infection, we have to prevent it from coming in our stands. We are aware of the abiotic factors that influence the spread of the disease. So, we can see if a stand is sensible or not, according to climatic conditions. In like manner, some plantations must not be implanted where there are favourable conditions for the development of the rust.
Actually, we must not plant spruces with a high density. In fact, with such a density, there is less light that reach the stand, and, temperature is obviously lesser. In the middle of the stand, moisture can also be higher than outside of it. Some all the conditions are gathered to allow the appearance and rapid spread of the fungus.
After infection, we have to remove infected trees. Use of fungicides is not advised as they are not very efficient. However, we can use it for special cases, but we have to do it before the real infection. In fact, once a tree is infected, fungicides cannot cure and remove the fungus from the tree.
IX Research fronts
A lot of studies permitted to discover and learn lots of things about this pathogen. In fact, nowadays, some species of this rust are re-examined, with new methods that are more recent (inoculation experiment, light and scanning electron microscopy...).
However, knowledge about species delineations, relationships among species on different continents, and life cycles lacks. Eight species of the rust are well described, but there are approximately 30 species that have been discovered nowadays. So, there is a lot of work remaining on discovering each one better.
• CRANE E. Patricia, (2001) Morphology, taxonomy, and nomenclature of the Chrysomyxa ledi complex and related rust fungi on spruce and Ericaceae in North America and Europe, Canadian Journal of Botany.
• 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