To combat the diseases caused by Heterobasidion fungi, decision-makers must act urgently: these fungi threaten European forestry and are already causing considerable financial losses.
Those responsible for taking decisions must clearly recognise the need to combat Heterobasidion fungi, which threaten pine and spruce stands.
"If we want to halt the spread of Heterobasidion fungi, the EU and national governments need to start thinking about regulation and support measures," stresses Lauri Lapalainen of Lallemand Plant Care, a company that produces a biological fungicide for controlling Heterobasidion.
Heterobasidion species cause a number of diseases, mainly in pine and spruce species: decay, heart rot and Heterobasidion root disease. The financial losses they cause in Europe reach 800 million euros a year. In Finland the figure is 50-80 million euros.
Heterobasidion prevention should always be part of harvesting operations, yet for financial reasons, and because the damage appears very slowly, it is often neglected. Forest owners must also be kept informed. Heterobasidion can be tackled by treating the tops of stumps with a biological fungicide or a urea solution. If the stumps are left untreated, the fungus spreads through the root system and later on to neighbouring healthy trees as well.
Heterobasidion is causing a very serious epidemic in forests
Finland's largest newspaper Helsingin Sanomat has called Heterobasidion an even worse scourge of commercial forests than bark beetles. The fungus damages the roots and trunks of spruce and pine, often to a height of several metres. It destroys valuable roundwood, but decades can pass before the damage comes to light.
"The damage only becomes visible after some time, and there is not always an interest in tackling the treatment right now," notes L. Lapalainen. Diseased trees are less able to capture carbon dioxide, which has a negative effect on the fight against climate change.
Heterobasidion reduces the availability of raw material for the forest sector. Damaged trees are also more vulnerable to storm and insect damage. A study carried out in Finland last year showed that three out of four trees affected by Heterobasidion had also been invaded by bark beetles.
How can Heterobasidion be tackled?
In Finland two different species can attack trees: Heterobasidion parviporum, which "specialises" in Norway spruce, and Heterobasidion annosum, which mainly damages the roots of Scots pine. Both fungi can also attack other tree species, although broadleaved trees are more resistant to them. Once Heterobasidion has reached a site, it keeps that site infected for decades to come.
According to the Natural Resources Institute Finland, providing for Heterobasidion prevention is an investment in tree growth and in the yield potential of the future.
The best way to prevent the spread of Heterobasidion is to schedule felling for a time when the air temperature is below zero, when the fungus does not produce spores. In the European Union this is usually only possible in the northern parts of Finland and Sweden. The second-best option is to use a fungicide to protect the exposed upper surfaces of stumps when harvesting takes place in warmer weather.
Using a biological fungicide is also an effective method; it contains spores of the fungus Phlebiopsis gigantea, a natural competitor of Heterobasidion. During harvesting, the fungicide is applied to the upper surface of the stump.
Finnish legislation stipulates that from the spring season until late autumn the spread of Heterobasidion must be prevented during harvesting operations in all conifer-dominated forests located within a defined risk zone. Elsewhere in Europe the situation is less favourable. In the Baltic States and Poland, the spread of Heterobasidion is in practice only prevented in state-owned forests.
"If sufficient investment is not made and every effort is not put into combating root rot, the coming generations of trees will suffer more and more, jeopardising the future of growing conifer crops," stresses Lauri Lapalainen.
Another way to combat Heterobasidion is to use specially bred planting stock. Swedish and Finnish researchers have observed that some spruce clones are resistant to the spread of Heterobasidion. In these trees, the B allele of the PaLAR3 gene held back the spread of Heterobasidion by up to 27 per cent. By using planting stock carrying this allele, a forest owner will gain a considerable benefit when the trees are felled, because the high-value roundwood will not be damaged. The allele does not, however, protect against existing infection.
Sources: Natural Resources Institute Finland, Tapio, Helsingin Sanomat, Verde/Lallemand Finland
https://forest.fi/article/fungal-diseases-caused-by-heterobasidion-specie/