Trees Downed by Trade
Humans have long traded commodities across continents with no thought of unnoticed passengers who wreak havoc in consequence. The fungi that caused chestnut blight and Dutch elm disease in North America are exemplary of a perennial problem.
The American chestnut is a magnificent tree, with mature trunks over 3 meters in diameter and crowns ascending to 60 meters. The chestnut was once the dominant tree in the Appalachian Mountains.
Disaster came to the chestnut in 1904. A micro-fungus from Japanese nursery stock, imported to an American zoological garden, rapidly spread. By 1950, American chestnut trees were wiped out.
In its native land of China, the same fungus benignly infects chestnut trees, causing little or no damage. A few European chestnut trees suffer when infected. But American chestnuts were tragically vulnerable.
Another, similar fungus causes Dutch elm disease. It also originated in Asia. Dutch elm disease was introduced into Europe during WW1 before making passage to the United States in the late 1920s. Infested elm timber was imported into Ohio, from where the pandemic slowly spread. Over half of the elms in the disease’s path succumbed.
Dutch elm disease is spread by bark beetles, who carry the fungus but are unaffected by it. Trees within 15 meters can directly catch the disease by root transmission.
In the 1940s, a virus infected the Dutch elm disease fungus in Europe, crippling the fungus’ ability to cause disease. The spread of the fungal disease virtually came to a halt in Europe. As with chestnuts, Chinese elm trees have good resistance to the native fungus.