A fungus threatens valuable old chestnut groves. To control it, research and practice have joined forces and are testing new methods of treatment.
Chestnut trees are widespread in Canton Ticino, but they also grow on the north side of the Alps. Until the 1950s, the chestnut served as the ‘bread tree of the poor’, but was later forgotten. Today, however, it is experiencing a revival. For example, over the past twelve years, an association promoting chestnuts in Central Switzerland (‘IG Pro Kastanie Zentralschweiz’) has restored many previously abandoned chestnut groves around Lake Lucerne, such as that in Chestenenweid near Weggis (Canton Lucerne). The trees, which are up to 150 years old, were pruned, new trees planted and the area cleared of bushes.
But the chestnuts on the north side of the Alps are threatened by chestnut blight, which is a lethal disease. It is caused by the fungus Cryphonectria parasitica, which infects the bark and kills branches or even entire trees. “Even in Chestenenweid, 40 percent of the chestnuts are already infected,” estimates Emanuel Helfenstein, a forest scientist and the project manager of the local chestnut association. There is currently only one effective method to save the trees: biological control of the fungus. This involves using a virus that naturally infects the fungus and weakens it. As a result, the diseased bark areas – the so-called cankers – heal and the trees survive. While on the south side of the Alps the virus has spread on its own, north of the Alps it requires help.
“The biological control works well,” says Simone Prospero, a researcher in the phytopathology group at WSL. Together with his colleague Francesca Dennert, he carries out treatments on behalf of various cantons. For the researchers this involves first isolating the respective fungal strain from the bark of an infected chestnut tree, and then introducing the virus into the fungus in several steps in WSL’s Plant Protection Lab. The next step is to produce a virus-infected fungal paste, before applying it as follows: small holes are made around the infected area on the stem or branches of the original chestnut tree and filled with the fungal paste. The virus is transmitted to the fungus in the tree, and the bark canker should then heal within months or years.
Costly and time-consuming treatment
“Applying the paste is not difficult, but it is very time-consuming,” says Emanuel, who himself carries out treatments in the field. He often needs more than an hour to treat a tree. To make it easier and quicker to apply, WSL is testing new methods, including a fungal spray. In addition, a research project initiated by the association Pro Kastanie is currently underway in Chestenenweid. The WSL researchers, in cooperation with Emanuel, are investigating how effective it is simply tying pieces of wood on which virus-infected fungal spores are present to the branches or the stem of a chestnut tree. “The idea behind this is that the rain washes out the spores, which then get into the bark and transmit the beneficial virus,” Simone explains.
The tests so far have been very promising. “But only the experiments in the Chestenenweid site will show whether the method really works,” says Simone. Both sides benefit from these tests: WSL, because it depends on suitable test sites for its research, and the association Pro Kastanie because the research results are useful for combating chestnut blight and can thus contribute to the rescue of the chestnut trees on the north side of the Alps.
(Claudia Hoffmann, Diagonal 19/2)