Hypovirulence-Infecting the infection

The story behind the blight's virus

Hypo vs. Virulent

Primary image: (white) A virus-containing isolate (CHV1) of the chestnut blight fungus. Roll-over image: (orange) A virulent (virus-free) colony of the chestnut blight fungus Photo and caption courtesy of Dr. Mark Double (WVU) .

What is hypovirulence? The term hypovirulence refers to an infection of the blight that is less virulent, or less damaging to the tree. The tree is still infected, but it can fight back. The result is not pretty--a lumpy, cruddy bark with knotty calluses and sores. But the tree survives.

So what makes the blight weaker? An infection of its own. Basically, the fungus gets a virus, just as a person can get influenza. Once infected, it is too debilitated to attack the tree with force. The tree walls off the fungus with layers of callus tissue and builds a cork like barrier to infection called a periderm.

Callus Tissue

Callus tissue is produced by the tree in an effort to ward off infection by the chestnut blight fungus. When a tree is infected by a virus-free isolate, the fungus grows too fast for the tree to produce much callus tissue. When the fungus is virus-infected, often the virus reduces the ability of the fungus to grow and the callus production can wall off the invading fungus. The callus tissue will eventually close completely Photo and caption courtesy of Dr. Mark Double (WVU)

Why is this important to TACF? Some think that the wild American chestnuts that we use in our breeding program, or Large Surviving Americans (LSA), are a product of a virus infected fungus. The cankers on these trees would then be hypovirulent, explaining their survival. TACF selects LSAs with the hopes that they have survived because of an intrinsic resistance to the blight. The existence of a hypovirulent fungus may mean that this resistance is an illusion.

What happened in Europe? Just like American chestnuts, European chestnuts suffer from infection by the blight fungus. The first infected trees were found in 1938. But early in the 1960's a French scientist, Dr. Jean Grente, discovered a chestnut tree in Switzerland with healing cankers. He isolated the fungus from its bark and noticed that it was different in color. When applied to the cankers of other European chestnut trees, they began to heal too. European chestnuts are alive today because of a mass treatment with hypovirulence.

Why won't it work here? The problem is dispersing the virus. Unlike the flu, the blight doesn't catch the virus from the air or water. In order to be infected, it must actually join with another growing blight--one that is already sick. This joining is called anastomosis. Anastomosis only occurs if they match in vegetative compatibility genes. If all the vegetative compatibility genes are the same, the hyphae will fuse. But when one gene differs, the hyphae can't always fuse, and if two or more differ, fusion is very rare.

In Europe, there are generally many fewer kinds of vegetative compatibility than in America. This means that the virus is easily transmitted between different trees with blight by applying the sick fungus to a preexisting canker. However, in America there are many different kinds of vegetative compatibility,so it is unlikely that a particular fungus will catch the virus from a neighbor. Because there are so many strains, infecting them all is tricky.

Cultural Outcomes

When isolating bark from chestnut blight cankers, quite often they are a mix of isolates--virus-free, virus-containing and other fungi (in this case, Trichoderma, a fast-growing, aggressive saprophyte) Photo and caption courtesy of Dr. Mark Double (WVU)

What research is being done? Much is being done to learn more about the fungus and its virus, as well as more effective ways of spreading the disease. Scientists at doctor William MacDonald's lab at West Virginia University, in Morgantown, are investigating everything from fungal strains to transgenic forms of the blight. There are many variations on the process of infecting the infection. Strains of the blight have different levels of pathogenicity, and strains of the virus have different impacts on the fungus as well as capacities for dispersal.

Some of the scientists in doctor MacDonald's lab are studying transgenic fungi. This artificially engineered blight may create its own infection by copying and expressing introduced viral DNA. If the viral DNA remains in the genome, it would be an effective dispersal mechanism.

Laboratory Procedures with hypovirulence