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Black walnut trees are prized for wood and walnuts, but fruit and vegetable crops planted near walnut trees can be afflicted with a disorder known as walnut wilt.
Ward Upham is an Extension horticulturist at Kansas State University. He says black walnuts release a naturally-occurring chemical called juglone which is quite toxic for some plant species.
"It can be given off by the leaves if they fall to the ground and then that juglone can be leached into the soil, or it can be given off by the roots," says Upham. "So, anything within probably 50’-60’ of a large walnut tree could be affected, sometimes up to 80’ away."
Tomatoes, potatoes, apples, lilacs, and asparagus are some of the plants affected by juglone. Other plants such as black raspberry, corn, beans, carrots, and zinnias are resistant.
Upham says the good thing is if juglone leaches from the walnut tree leaves, it doesn’t last long in the soil. However, if you take down the walnut tree and juglone is in the roots, it takes time for roots to decompose so you may not be able to grow tomatoes in that area for up to two-years.
Juglone-sensitive plants will show signs that they’re not feeling well.
"For example, on tomatoes, you’ll get those plants looking kind of flacid, in other words they look like they’re wilting. In severe cases it can actually kill them"," he says. So, you’re going to notice a lack of production, those plants just don’t look thrifty, and eventually they may die."
The major uptake of the toxin occurs when tomato roots make contact with roots of the walnut. There is no cure so Upham suggests planting tomatoes and other juglone-sensitive plants in a raised bed lined with landscape fabric.