ONCE upon a time, according to folklore, a squirrel could travel through America’s chestnut forests from Maine to Florida without ever touching the ground. The chestnut population of North America was reckoned then to have been about 4 billion trees. No longer. Axes and chainsaws must take a share of the blame. But the principal culprit is Cryphonectria parasitica, the fungus that causes chestnut blight. In the late 19th century, some infected saplings from Asia brought C. parasitica to North America. By 1950 the chestnut was little more than a memory in most parts of the continent.

American chestnuts may, however, be about to rise again—thanks to genetic engineering. This month three experimental patches will be planted, under the watchful eye of the Department of Agriculture, in Georgia, New York and Virginia. Along with their normal complements of genes, these trees have been fitted with a handful of others that researchers hope will protect them from the fungus.

The project has been organised by the Forest Health Initiative (FHI), a quango set up to look into the idea of using genetic engineering to rescue species of tree whose populations have been devastated by fungal diseases or insect pests. It has sponsored research at several universities, and this month’s trial is the first big field test. If it works, the FHI will ask the government for permission to plant transgenic chestnuts in the wild, with the intention of re-establishing the species in America’s woodlands. And if that goes well, it could provide a model for projects to re-establish elm trees (being devastated by Dutch elm disease, a beetle-born fungal infection), ash trees (threatened in North America by a beetle called the emerald ash borer, and in Europe by a fungal disease called ash dieback) and a fir tree known, confusingly, as the eastern hemlock (which is plagued by the woolly adelgid, a sap-sucking bug).

The search for genetic protection for the chestnut was begun in 1990 by William Powell of the State University of New York, in Syracuse, and Scott Merkle of the University of Georgia, in Athens. Dr Powell knew that many of the symptoms of chestnut blight are caused by the oxalic acid that C. parasitica generates as it grows. He also knew that wheat has an enzyme called oxalate oxidase, which detoxifies oxalic acid. He and his team therefore transferred the gene that encodes oxalate oxidase from wheat to chestnut. Last summer they showed that oxalate oxidase can indeed enhance blight-resistance.

A few years ago the Forest Health Initiative asked Dr Powell and some other researchers doing related studies to look at the work of the American Chestnut Foundation, a group which had been crossbreeding Chinese and American chestnuts. Since C. parasitica is Asian, Asiatic trees have evolved resistance to it. The foundation hoped to make a hybrid sufficiently Chinese to be protected, but sufficiently American to pass muster as local.

With the aid of the genomes of the two species, Dr Powell and his collaborators began testing 27 Chinese chestnut genes in the American tree. The 800 trees to be planted this month will contain various combinations of these genes, the original wheat gene and six further genes from other tree species. And results should come quickly. Field tests for blight-resistance are typically done when trees are a few years old, but Dr Powell’s team have devised a test of the sapling’s leaves that they believe can tell whether a tree is resistant when it is less than a year old.

The trial itself will last three years, and the researchers running it will monitor how the modified chestnuts fit in to the local ecosystem, as well as how healthy they are. If they both do fit and are fit, a decision will then have to be made about whether to release them into the wild. That will be up to the Department of Agriculture, the Environmental Protection Agency and the Food and Drug Administration.

Until now, the genetic modification of trees has had strictly commercial aims: speeding up the growth and extending the environmental tolerance of species intended for plantations. This use of genetic modification has been opposed by environmentalists, who fear that such “supertrees” may escape and damage wild forests. The Forest Health Initiative’s goal, though, is to heal wild forests, not hurt them. If its experiments do produce a strain of chestnut that could do the job, it will be interesting to see how enthusiastically greens embrace it.