Black knot caused by fungus

Horticulture Hints

Recent client inquiries included several questions on cherry trees that were exhibiting hard, dark swollen areas appearing on the twigs and limbs. This is a disease called black knot, which is caused by a fungus called Apiosporina morbosa and typically affects both wild and cultivated cherry, chokecherry, and plum species.

These swellings (or galls) are more noticeable before the tree fully leafs out. A tree once covered with these galls will eventually die, as the vascular system is interrupted. To help recognize the symptoms of black knot, it has commonly been referred to “poop on a stick”.

As soon as you notice an infected twig, immediately prune it out to prevent disease spores from spreading. Cuts should be made at least 2 to 3 inches below the swelling, as the fungus may extend beyond the swollen area.

Prevention of this disease is primarily through good sanitation measures. It is important to check stonefruit trees regularly, so that any galls appearing may be pruned out and promptly destroyed, preferably when the tree is dormant. Fungicides may be used, but without removing the galls and proper sanitation measures, fungicides alone will not control black knot disease.

Did you know? Robins love cherries. If you would rather not share your cherries with the robins, apply a tree netting before the fruit ripens. Without netting, on the day you notice your cherries are about ready to be picked, you may return to find the robins have harvested the fruit for you.

Questions? Contact McCormick at yvonne@iastate.edu for information or advice.

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