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Phytopathology
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Article

Author(s): Ursula Heiniger
Editorial office: WSL, Switzerland
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The risks of introduced fungal diseases for forest trees

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.

Ulmenwelke
Fig. 1 - Dutch elm disease (elm wilt) is caused by the fungi Ophiostoma ulmi and O.novo-ulmi.
Photo: Swiss Forest Protection (WSL)
 
Stroben-Blasenrost
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.

New diseases in Switzerland

  • 1986: the appearance of the chestnut blight in many regions north of the Swiss Alps.
  • Fire blight on pome fruit trees, a dangerous bacterial disease (Erwinia amylovora) was first observed in Switzerland in 1989. Since 1994 it has been spreading and has, up until the end of 2005, infected around 45'000 standard apple and pear trees in the German speaking part of Switzerland .
  • Ceratocystis fimbriata f. sp. platani, which was first discovered in the Canton of Tessin in 1986 is considered especially dangerous. C. fimbriata f. sp. platani – which has been well known in the USA for a long time – started to spread after 1945 in southern France. In Marseille between 1960 und 1972 13% of plane trees fell victim to this disease. Spreading out from France the fungi reached the Canton of Geneva in 2001, where up to now three areas have been affected.
  • Considered less of a danger is the powdery mildew on horse chestnut trees (Erysiphe flexuosa), which was first recorded in Europe in 1999 with findings in Germany and Western Switzerland. It quickly spread to the Swiss Plateau.
  • Scirrhia acicola Dearn (Siggers), the Lecanosticta-brown spot needle blight on pine trees, which is listed in the plant protection act as a quarantine organism, was discovered in central Switzerland on mountain pines in a garden in 2001. 
  • Sudden Oak Death (SOD): Since 1995 in the coastal area around San Francisco (California, USA) there has been a rapid spread of oak tree deaths. An area of over 300 km in length has been affected and in Oregon there have also been new findings. The leaves and shoots of various oak species are affected. The affected trees sometimes die very quickly which has led to the disease being given the name Sudden Oak Death. In Switzerland the pathogen Phytophthora ramorum was first diagnosed in 2004. Luckily it has so far only been diagnosed on Viburnum and Rhododendron in nurseries and gardens.

Since the publication of this article, other diseases have been introduced to Switzerland, for example the ash dieback.

Diseases which have not yet been detected in Switzerland

  • Ceratocystis fagacearum (Oak wilt): Since 1982 the American oak wilt has spread in the USA from Minnesota to Texas and Tennessee. Because the section of white oaks (Quercus) to which the Quercus petraea and Q. robur belong are quite resistant in the USA, it was hoped for a while that European oaks would not be threatened. It appears, however, that European oaks are just as susceptible as American oaks.
  • Alder-Phytophthora: Since 1993 many alder trees have been killed by an unknown species of Phytophthora. The new Phytophthora species was also detected in 1996 in France and seems to be present along the course of all important rivers. The pathogen has also been identified in Austria, Denmark, Germany, the Netherlands, Sweden, Belgium, Hungary and Italy. In 2008 it was also detected in the Swiss plateau.
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.
Pathogen Disease Affected Tree Species
Country
Date of importation
Cryphonectria parasitica Chestnut blight
Castanea dentata

C. sativa

USA

Europe

1904

1938

Ophiostoma ulmi Dutch elm disease
Ulmus Europe ca. 1910
Ophiostoma novo-ulmi New Dutch elm disease
Ulmus
Europe ca. 1975
Ceratocystis fimbriata f. sp. platani Plane tree wilt
Platanus Frannce ca. 1960
Cronartium ribicola White pine blister rust
Pinus strobus USA
1906

Outlook

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.


  • Translation: Dawn Meister, Stallikon

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