Update, January 4, 2015 Thousand Canker Disease has been found in Marion County, TN, breaking out of the quarantine zone to the west. We will continue posting updates and updating the maps as the disease spreads.
A horrible disease is killing the most valuable timber tree in North America. The story of this disease, its origin and its movement across North America is a fascinating biological story. It is also an important economic story because of the careless way we have allowed one industry, ornamental horticulture, to threaten another industry, forestry. And it is an important element in our understanding of the impacts of climate change on the lives of trees.
The story is complex, and requires us to follow the lives of a tree, a beetle, a fungus, and some other players. It also requires us to understand something about the odd behavior of humans.
The Tree. Black walnut is valued for its rich, dark wood, often seen in furniture or guns stocks. Although walnut has been over-harvested and is much less common than it was in the last century, it is still an important timber tree. Some walnut logs are sawn into boards, but the best walnut is thinly sliced into veneer, glued to cheaper wood and used for furniture and paneling. Walnut is also an ecologically important tree, especially for wildlife.
Black walnut is found in the eastern half of North America, but is most abundant in the Midwest, where most of the walnut timber is harvested. A closely related tree, butternut, also called white walnut, Juglans cinerea, overlaps the range of black walnut (see maps below).
Black walnut became popular as an ornamental in the western US in the last century, and has been planted throughout the western states. This is a key factor in the disaster that is now unfolding.
The beetle. You would think that an insect with an innocuous name like walnut twig beetle, and reaching a maximum size of the head of a pin, would not cause a serious problem. And indeed, for most if its history it probably did not. Like other bark beetles, the walnut twig beetle, Pityophthorus juglandis, finds a suitable walnut branch, tunnels in and settles down to procreate. OK, it’s a big more complicated – the males make a tunnel and then release aggregation and sex pheromones to attract females and other males. If that was all that was going on, walnut trees might lose a few branches, and the beetle would receive little attention. Until recently, the beetle didn’t even feed on black walnut, but on western walnut species: the beetle is native to Arizona, California, New Mexico, and northern Mexico, where it happily fed on Arizona walnut, California walnut and other species. It is only in the last few decades that horticulture put black walnut into the reach of the walnut twig beetle.
The fungus and the disease. Beginning in 2001, foresters and arborists began to see severe decline and death in black walnuts in Colorado, and then in other states. Affected trees had small holes with dead black tissue around the holes – cankers. Although the cankers were small, there were thousands of them. A fungus was soon identified that had never been seen before, and given the name Geosmithia morbida. The disease caused by G. morbida was given the ominous name thousand cankers disease. The holes were caused by the walnut twig beetle, which carried the fungus into black walnut trees, and the fungus killed the tree. The beetle went from being a minor pest to the vector of a major disease outbreak.
We know the beetle was western, and only came in contact with black walnut recently. Where did the fungus come from? It is possible that the beetle has been carrying the fungus around for millenia, but that western walnuts are fairly resistant. Or the fungus could have evolved recently from a less virulent ancestor, though other Geosmithia species don’t cause plant diseases.
Whatever its origin, the combination of black walnut, walnut twig beetle and the fungus has been devastating for black walnuts planted in the west.
The people. So far, we have a story of a tree carried by the horticulture trade to the western US where it encountered a beetle and fungus that proved fatal. And that would be the end of the story, but for the behavior of people. People love to carry wood around, even when they have been warned it’s a bad idea. A westerner going camping in the east carries firewood from Colorado to Tennessee to save a little money. A woodworker finds a nice walnut log in California and brings it back home to Virginia to make a cabinet. However it happened, it is almost certain that the insect and fungus were carried back east by people and did not move that far without assistance.
Suddenly, beginning in 2010 in Tennessee, black walnut trees began dying in the east. By the middle of 2014, black walnut trees were dying in Indiana, North Carolina, Ohio, Pennsylvania, Tennessee and Virginia. Quarantines have been placed around infested counties to forbid the transportation of wood out of the infested areas. (see the maps, below).
It is not possible to know how fast thousand cankers disease will spread in the east. The disease threatens billions of dollars worth of timber, as well as the ecosystem services provided by this important tree.
The climate. Thousand cankers disease has arrived in the east just as the impacts of climate change are accelerating. As temperatures climb and the frequency of drought increases, it is likely that the southern part of the range of black walnut will become unsuitable for it to reproduce. As I described in a previous column, trees can acclimate to climate change – a mature tree can withstand quite a lot of variation in temperature and rainfall, and black walnut is fairly drought tolerant. But now we have overlaid a new disease on top of the effects of climate change. The likely outcome is to accelerate the loss of black walnut.
Climate change is not taking place in isolation. We are repeatedly introducing new stresses to our forests, as commerce brings pests and pathogens into contact with new host trees. Black walnut is only one example of this interaction, with eastern hemlock as another important example.
How do we keep from wiping out a species like black walnut? Our capacity to slow the spread of thousand cankers disease is limited. But our capacity to deal with climate change is limitless, if we only can accumulate the political will to take action.
The map below shows the current range of all walnuts, the current range of thousand cankers disease, and the quarantined areas in the east. Whether butternut is susceptible to thousand cankers disease is not known, but it is already in steep decline due to another canker disease, butternut canker. Click for a larger map.
The maps below show the current and potential future habitat suitable for black walnut. Current habitat is indicated by the importance value, a relative measure of the abundance of a species within a forest area. Potential habitat is the area expected to be suitable for the growth of black walnut in the year 2100 based on climate models. Data are from the US Forest Service Climate Change Tree Atlas.
For more information on Thousand Cankers Disease, and to monitor its spread, visit the Thousand Cankers Disease web site.
Images: Table from US Department of the Treasury; Beetles on coin by Whitney Cranshaw, Colorado State University, bugwood.org; Adult walnut twig beetle by Steven Valley, Oregon Department of Agriculture, bugwood.org; Twig with canker by Troy Kimoto, Canadian Food Inspection Agency, bugwood.org; Map by Venerable Trees; All images used by permission.