Hypovirulence

A Possible Solution

A hypovirulent culture of Cryphonectria parasitica.

One discovery that has been very successful for controlling Chestnut blight in Europe was a biological control method called hypovirulence. Hypovirulence refers to a virus that infects Cryphonectria parasitica, the fungal pathogen that causes Chestnut blight, and reduces its ability to cause disease. The use of fungal viruses to control fungal plant pathogens is a relatively new strategy for biological control of plant diseases that was discovered in Europe after the European chestnut (Castanea sativa) was destroyed in a similar manner as American chestnut was destroyed in North America.

The Arner Tree
However, in Europe, isolates of the pathogen that contained fungal viruses naturally spread throughout the populations of European chestnut and reduced the ability of the fungal pathogen to cause disease. This, in turn, allowed the European chestnuts to naturally regenerate in many regions to the point where extensive stands of European chestnut are present once again.
In North America, hypovirulence has also been found and several sites containing hypovirulent isolates of Chestnut blight have been reported in southern Ontario (see Publications). However, hypovirulence has not spread as effectively in North America as it has in Europe. There are several possible reasons for this and experiments being conducted in southern Ontario have focused on trying to determine how these isolates could be used more effectively to protect American chestnut.

A healed canker on
the Arner tree

The "Arner Tree" represents one of the most impressive examples of naturally-occurring hypovirulence in Ontario. This is a mature tree that has been severely infected with Chestnut blight. However, this tree keeps recovering from the blight and large cankers eventually become healed over and the tree continues to grow vigorously. Chestnut blight is still present on the tree but many of the isolates recovered from this tree are hypovirulent. Recently, scientists discovered that these hypovirulent isolates are different from other hypovirulent isolates because these isolates do not contain the fungal viruses that are found in other isolates.

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Various reports on the status of restoration efforts, tree population surveys, DNA analysis, breeding and propagation programs, etc.