Deadly Walnut Tree Disease Difficult to Control

Walnut trees and other related species suffering from a deadly fungal disease may not have hope of combating its lethal effects after all, as new research shows that the fungus is difficult to control due to its genetic diversity.

Thousand cankers disease, caused by the fungus Geosmithia morbida, has been found in trees throughout the western United States and in several eastern states, including Tennessee, Virginia, Pennsylvania and Ohio. Similar to the emerald ash borer, which continues to invade NY counties, the disease is spread primarily through the transportation of infested wood.

A black walnut tree, or its relatives such as the butternut and wingnut tree, first becomes infected with the disease when walnut twig beetles - natural carriers of G. morbida - burrow deep into its bark, carving out finger-shaped "galleries." The fungus then spreads into these galleries, forming multiple cankers in the wood and eventually leading to the tree's death.

However, the fact that G. morbida is so genetically diverse suggests that it mutates readily, according to study leader Keith Woeste, making it difficult for scientists to help save black walnuts.

"The high mutability of this fungus means we can expect the unexpected," Woeste, a hardwood specialist with the US Forest Service, said in a statement. "We can't count on the fungus' genes to be the same year after year, which certainly makes it harder to control. It will also be harder to breed trees resistant to this disease."

After analyzing the genes of 209 samples of G. morbida from 17 regions of the United States, Woeste and researchers from Purdue and Colorado State universities identified 57 distinct genetic races (haplotypes) among the samples. Such diversity surprised the researchers, given that the organism reproduces by cloning itself.

"This is an example of evolution that surprised us," added Woeste.

"There probably isn't going to be a way to get thousand cankers disease out of our forests in the East," he explained further, "but there might be ways to control it. Sanitation, attentiveness and care are just going to become part of the everyday routine for forest landowners."

The results were published in the journal PLoS ONE.