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Biological control of chestnut blight

 

We investigate the Cryphonectria - hypovirus - Castanea pathosystem with the aim to develop successful management strategies against chestnut blight. Laboratory experiments and field studies combined with population genomic analysis are being used to understand the biology and epidemiology of C. parasitica and hypovirulence. 

 

Chestnut blight is caused by the ascomycete fungus Cryphonectria parasitica. An infection with C. parasitica is typically associated with extensive necrosis (cankers) of the bark on stems or branches. On the susceptible American chestnut (Castanea dentata) and European chestnut (C. sativa), the cankers may enlarge rapidly and girdle the affected stem or branch, resulting in the subsequent death of the plant part distal to the infection point. 

Cryphonectria parasitica is native to China and Japan and during the 20th century it was accidentally introduced into North America and Europe. In contrast to North America, where the fungus almost caused the extinction of the American chestnut, consequences of the disease were less dramatic in Europe because of the emergence of a virus-infection within the C. parasitica population. The so-called hypovirus reduces virulence of the infected fungal strain so that cankers are no longer lethal for the chestnut trees. This phenomenon (called hypovirulence) provides the basis for biological control of chestnut blight.

 

Study goals

Specifically, we aim to:

  • reconstruct the invasion history of C. parasitica and its hypovirus in Switzerland and Europe
  • understand how the C. parasitica – hypovirus interaction evolves in different environments
  • identify factors that are important for the success of biological control of chestnut blight using hypovirulence
 

Hypovirus genomics

RNA viruses are known to exist as a diverse population and due to high mutation rates. This can lead to rapid evolution and high differentiation across hosts in natural infections. This may affect infection dynamics and the biocontrol success of mycoviruses. Here we are using long-read next generation sequencing to characterise intra-host CHV-1 diversity within a naturally infected chestnut forest system. By exploring how CHV-1 diversity varies across space, we will get insight into its population dynamics and the spread of CHV-1 to new infections. We will also examine differences in CHV-1 diversity across active and healed infections, to explore how diversity varies through the biocontrol process.

 

The nanopore MinION sequencer is revolutionizing sequencing, providing almost the genome of viruses and microorganisms in food safety, medical and animal diagnostics. However, so far there are few reports of its application in forest health diagnostics or research. This project works on optimizing the MinION-based sequencing technology for forest diagnostics. Notably, we recently used CHL-1 RNA and cDNA in the WSL plant protection lab.

 

Application of our research

Biological control with hypovirulence represents one of the few hopes for a successful management of chestnut blight both in chestnut forests and orchards. In Switzerland, natural hypovirulence is only present in chestnut stands south of the Alps (Ticino and Grisons). On the contrary, north of the Alps and in Valais only the virulent form of C. parasitica occurs causing severe decline of the local chestnut stands. In this project, we aim to artificially introduce and promote the spread of hypovirulence in hypovirus- free chestnut stands in Switzerland. For this, we treat virulent cankers with a hypo- virulent C. parasitica isolate containing a selected hypovirus from the Ticino. Biological control is considered successful if cankers become virus-infected and cease expansion. We are using molecular markers to identify the hypovirus, which allows us to assess persistence and spread of the biological control agent. Additionally, we are testing new, less time-consuming and more effective methods for canker treatment.

 

FURTHER INFORMATION