American
chestnut, Castanea dentata, once stood tall among the forest trees
in the Appalachain Mountains. This was a major species of tree growing from
Maine to Georgia. In the early 1900's as many as one in four forest trees
was an American chestnut. This rot-resistant tree was very important in the
lumber industry. The average height for this tree was 80 to 100 ft. and the
average diameter was 4-5 ft (though some measured 8-10 ft in diameter). In
1904 these magnificent trees began to wilt and die. Herman Merkel, chief forester
of the New York Zoological Park, took action as he sprayed the trees and made
a report to the Department of Agriculture in Washington. The forest pathologist
determined that Merkel would only have to remove the the dead trees and replant.
Unfortunately this was not the solution.....the following year there were
reports of the same thing happening outside the park. In 1907 the culprit
of the disease was determined, it was a fungus, Endothia parasitica. The
fungus now called Cryphonectria parasitica kills the tree by invading
the bark of twigs through small wounds and begins to invade the vascular cambium
of the growing twig. Eventually the twig is girdled and dies. By 1923, the
disease had invaded 80% of the chestnut's range and by 1950 80% of the trees
were dead. North America was not the only area affected, in 1917 the fungus
spread to Europe on some exported wood. New shoots of the tree still come
up year after year, but quickly succumb to the fungus. Control of the fungus
on one tree can be done by pruning out infected parts. Fungicides are useless.
Both are not pratical when dealing with an entire forest.
Robert Frost, famous American Poet, wrote the poem "Evil Tendencies Cancel"
Will the blight end the chestnut?
The farmers rather guess not.
It keeps smoldering at the roots
And sending up new shoots
Till another parasite
Shall come to end the blight.
He wrote this in reference to the belief that since the shoots came up year after year that the trees could overcome the pathogen. In 1950, some orchard chestnuts were found to be surviving despite the cankers in Italy. It was determined that this strain of the fungus was much weaker and was termed hypovirulent. These hypovirulent strains also had the ability to spread the factor that caused them to be hypovirulent to virulent strains to weaken them. Hypovirulence seems to be a disease of the fungus in which an extra piece of double-stranded RNA in the mycelium of the hypovirulent strians replicates and spreads into the mycelium of the virulent strains, weaking them. In Europe the hypovirulent strains are added to stop the active cankers. Due to complications, this does not seem very practical in the U. S. The hypovirulent strains usually grow slower and produce less spores therefore reducing the ability to spread in nature. But many groups are working at trying to create resisitant trees. But for now we must wait to one see the king of trees return.
