New Disease Threatens Chesapeake Bay’s Black Walnut Trees

Until recently “Thousand Cankers Disease” was only in western states. Maryland State University Extension says that the disease has found its way to Maryland and they don’t know how to stop it. 

Signs of Thousand Cankers Disease in Black Walnut trees
Walnut leaves that have collapsed and wilted, signs of Thousand Cankers Disease hitting Maryland’s Black Walnut trees (Photo courtesy Curtis Utley, Colorado State University Extension)

“The walnut twig beetle was introduced and established in Tennesee a very long time ago. How it got there is still a mystery, but hopes of keeping this disease confined to western states is now not possible,” according to the the College of Agricultural & Natural Resources information center.

They suspect that people brought Tennessee wood here that was infested with the Walnut twig beetle. States constantly warn against transporting firewood between states.

Walnut Twig Beetle
The cat flea (top) is bigger than the Walnut twig beetle (bottom). They have been enlarged to provide more detail. (Photo courtesy D.L.Clement, UME, Home and Garden)

The disease was confirmed in Cecil County, Maryland in October (2014). The tiny beetle lives under the Walnut tree’s bark and carves trenches in the tree, causing a fungus which blocks the tree’s ability to suck up water. It’s hard to tell if your Walnut tree is infested; the beetle’s entrance points are about the size of a pin hole. The beetle is smaller than a grain of rice, about the size and color of a plastic pencil’s broken lead tip.

Walnut twig beetle damage to Walnut tree
Trenches carved under the Black Walnut bark by Walnut twig beetles (Photo courtesy Whitney Cranshaw, Colorado State University)

“To scout for symptoms look first in black walnut trees with existing crown dieback,” says the state extension service. “Then look for individual branches that show flagging with either yellowing leaves remaining attached or leaves that have collapsed and wilted (Fig. 2). The latter is a somewhat stronger possible symptom. Then try to collect a dead or dying limb and look for the minute exit holes.”

ThousandCankers.com says the trees rarely show any external symptoms of the fungi. The leaves at the crown of the tree will turn yellow, wilt and die, then branches will gradually begin to die down the tree. But new sprouts may grow from the tree roots or trunk (away from the infected areas). Trees usually die within three years of the crown dying.

Some hearty Black Walnut trees do survive, but not many. Officials say no effective treatment has been found yet, but they’re working on it. They suggest cutting off the infected branches and warn against keeping the dead wood because the beetles can remain under the bark for years. They say the wood can be milled, but don’t take it to other areas just in case some beetles are still hanging out.

Read more about the Thousand Cankers Disease here.

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