During the course of evolution a balance has been established between indigenous species and their diseases. However, if pathogens spread into new areas with potential new host plants then epidemics can develop. Some of these epidemics have made history.
Fig. 1 - Dutch elm disease (elm
wilt) is caused by the fungi Ophiostoma ulmi
Photo: Swiss Forest Protection (WSL)
Fig. 2 - The white pine blister rust
fungus (Cronartium ribicola) needs
two hosts to complete its life cycle. Intermediate hosts are often cultivated
blackcurrants or gooseberries.
Photo: Phytopathology WSL
Wild plants as well as cultivated plants have a large number of fungal pathogens. An outbreak of a disease amongst wild plants mainly takes a mild course, because during the course of evolution a balance has been established between indigenous species and their diseases.
If, however, pathogens spread into new areas with potential new host plants then epidemics cannot be ruled out. Some of these epidemics have made history. For instance, the potato late blight caused by Phytophthora infestans, which was imported into Ireland in 1845 and which led within a very short time to a famine and a wave of emigration. Even trees are not immune to serious epidemics. (See Table 1).
The result of imported tree diseases can be immense changes in the landscape. Within the last 30 years chestnut blight (Cryphonectria parasitica) has largely decimated American sweet chestnut tree stands (Castanea dentata) in the Appalachians (USA) where they once made up a quarter of tree stands. In Great Britain around 30 million and in North America hundred of millions of elm trees have fallen victim to the Dutch elm disease (Ophiostoma ulmi und O. novo-ulmi, Fig. 1), not only were forest trees affected, but also trees in fields and cities.
The white pine blister rust (Cronartium ribicola, Fig. 2) – brought into North America with seedlings from Europe, where the rust is endemic on indigenous five needled pine trees (Pinus cembra and P. sibirica) – destroyed the widely spread white pine stands (P. strobus) and even today it is hindering the reforestation of white pines. In the south of France Ceratocystis fimbriata f. sp. platani (plane tree canker or wilt) has destroyed many plane trees since 1970 and has in the mean time spread into the Cantons of Tessin and Geneva.
Once established fungal diseases are nearly impossible to wipe out. In agriculture diseases can be controlled with chemicals and with measures such as crop rotation or the cultivation of more resistant cultivars. In forests where trees live much longer such strategies cannot be easily put into practice and are usually less successful. That is why it is important to avoid the importation of diseases.
Since the publication of this article, other diseases have been introduced to Switzerland, for example the ash dieback.
|Table 1: Examples of imported fungal diseases which are greatly threatening forest and park tree species and which have lead to notable changes in the landscape.|
Affected Tree Species
Date of importation
Dutch elm disease
New Dutch elm disease
|Ceratocystis fimbriata f. sp. platani||
Plane tree wilt
White pine blister rust
Plant diseases caused by imported fungal pathogens must be avoided at all costs, because once they are introduced it is nearly impossible to eradicate them. Many fungi are spread by the wind, so even when rigorous phytosanitary measures are taken there is only a limited chance of any disease being confined. Due to an increase in global trade there is a greater danger of importing diseases with living plants, in the soil of container plants, on dirty shoes, with wood products and especially with packaging materials.
Pathogens are not always spotted during goods inspection. The plant could probably be only latently affected and the disease therefore not observable. There is also a threat from, up to date, unknown pathogens which in their country of origin live in balance with their hosts not causing any large amount of damage. In a new area the fungi may find more susceptible trees. This was the case for example with the chestnut blight. It did not cause any substantial damage in Asia, where it originates.
The danger of new tree diseases could also be increased due to changes in climatic conditions. Global warming could encourage warmth loving pathogens and climatic stress, e.g. dryness, water logging and soil acidification, could reduce the resistance of trees.