Strange, bad things continue to happen as we blend the biota of the world, even within a continent. Thousand cankers disease is one of the most recent developments in North America. It was discovered after widespread mortality of black walnut began in the Colorado Front Range in 2001 .
The host of primary concern in North America is eastern black walnut (Juglans nigra), whose native range is from the eastern Great Plains to the Atlantic Seaboard. It is widely planted in towns and cities in the western United States. Four western black walnut species are found primarily in the southwestern states and Mexico. Juglans nigra and J. regia (English walnut) have been found with the disease in Italy .
The pathogen, Geosmithia morbida (Hypocreales: Bionectriaceae), was unknown before the disease was investigated. There is no known sexual fruiting.
The pathogen is vectored by the walnut twig beetle, Pityophthorus juglandis. The two organisms together damage the phloem and lead to dieback and mortality.
Other than the presence of both the fungus and the beetle, there do not appear to be significant environmental constraints on disease development. The beetle and fungus are capable of dispersing and causing damage under a wide range of conditions.
The tiny twig beetle, < 2 mm long, burrows in the phloem of branches and stems. It inoculates the fungus, which creates numerous small cankers. The cankers enlarge and coalesce, eventually girdling the branch or stem. Generally, branches are attacked first, then the main stem.
Beetles overwinter as adults under the bark. They emerge in the spring, fly to branches of black walnut, and begin tunneling and laying eggs. The beetle may complete 2-3 generations in a growing season.
The cankers and galleries can only be seen when cutting into the phloem. As phloem is killed, foliage turns yellow and then brown and wilted. Branches die. Mortality can occur within three years of initial symptoms.
Distribution and Origin
The disease was first discovered in Colorado, but it soon became clear that it occurred in most of the western United States where walnuts grew. Since 2010, it began to appear in the eastern United States. Concern is high for the native range of eastern black walnut, a very valuable timber species.
Genetic evidence suggests that both the beetle and the fungus are native to southwestern North America, where native walnuts are not greatly affected by the disease [2, 5]. Therefore, a likely explanation for the emergence of the disease is range expansion by the pair, bringing them into contact with eastern black walnut, which is highly susceptible. This apparently occurred throughout the western United States .