This is the first part of a series on winter tree diseases and disorders. This article examines American chestnut blight.
During the winter months, many of the fungal pathogens that affect trees enter dormancy. The pathogens overwinter on their hosts or in the soil, awaiting spring’s arrival. Despite the frigid temperatures, trees suffering from fungal diseases may still exhibit infection symptoms in winter, especially if the disease has advanced into its later stages. The following describes some of the most common diseases to overwinter on trees, and how they may be detected.
American Chestnut Blight
Chestnut blight, also called chestnut bark disease or American chestnut blight, is a fungal disease that has significantly reduced the chestnut population in North America. The fungus is spread by infectious spores disseminated by air currents and rainstorms. The root system of the chestnut tree exhibits some resistance to blight infection. The soil organisms react adversely to the fungus, effectively repelling it. Consequently, a large number of small American chestnut trees remain as shoots growing from existing root bases. These regrown shoots seldom reach maturity before being killed by the fungus. Instead, they survive as living stumps, with only a few developing enough shoots to produce seeds.
Chestnut blight occurs most frequently on American chestnut. It also infects numerous other species of chestnut, albeit with less frequency. Some of the more common examples include American chinquapin, European chestnut, and West Asian chestnut.
Symptoms of Infection
The first symptom of chestnut blight is bark discoloration. The infected bark turns orange, and then brown. Distinctive yellow tendrils, also referred to as cirrhi, can be seen extruding from the bark during periods of heavy rainfall. Small fruiting bodies called perithecia also appear on the tree, expelling spores once they mature in fall. The spores are released once the perithecia become sufficiently moist. The fungus spreads to other nearby trees as the spores are disseminated by the wind. Animals and insects also serve as vectors for the spores.
Surviving chestnut trees have been selected for breeding programs, notably by the American Chestnut Foundation. The intention of these breeding programs is to replenish American chestnut populations by introducing blight-resistant chestnuts to their original forest range. Resistant species, particularly Japanese chestnut, Chinese chestnut, Seguin’s chestnut, and Henry’s chestnut, are being hybridized with American chestnut to create new disease-resistant species. Chinese chestnuts vary considerably in their resistance to blight; some individuals are susceptible, while others are essentially immune to the disease. Environmental stressors may reduce a tree’s resistance to blight. At higher elevations in areas exposed to harsh weather, and extreme temperatures, normally resistant Oriental chestnuts can be weakened, or killed by blight.
At the New York College of Environmental Science and Forestry, plant pathologists William Powell and Charles Maynard have developed blight resistant American chestnuts. Full resistance was attained by introducing a wheat gene coding for the enzyme oxalate oxidase into the American chestnut genome. The enzyme converts the oxalic acid secreted by the blight into carbon dioxide and hydrogen peroxide. Transgenic trees produced through this method have been shown to exhibit blight resistance equal to, or surpassing that of the Chinese chestnut. The New York Botanical Garden has planted several of the transgenic trees for public display.
Photograph courtesy of Claudette Hoffman CC-by-3.0