How2Day: Black Tube Fungus on a Cherry Tree

The black knot disease on cherry trees (Prunus sp) is caused by the fungus Dibotryon morbosum. This disease produces black galls that look like tubes, and range in size from ½ inch to 1 foot. Infected limbs and twigs lose vigor and are killed. Eventually the entire tree will die from stress.The fungus will overwinter in the galls, and infect the tree again the following year. Take control measures to stop the disease from becoming progressively more severe each year until it kills the tree.

Tree vigor should be maintained as a cultural method. You should water your cherry trees during drought, keep fertilizer at optimum levels, mulch around the tree, and keep the tree as stress free as possible. When you see the beginning small galls start to form on the trees, they should be pruned away immediately. It is very important not to let the black knot disease overwinter in the galls. The tree should be monitored regularly, and the disease should be pruned out. If there are other infected trees near your cherry trees, then they should be pruned as well. The galls can produce spores that will infect your trees after removal from the tree. Therefore, infected branches should be burned, buried or removed from your property. If you are planting new cherry trees, you should not plant them near an orchard that has had previous problems with black knot disease.

Lime sulfur should be applied to the pruned areas to maintain dormancy on the wounds. The fungicide triophanate-methyl should be sprayed on the trees four times per year. You should apply it while the tree is dormant during the winter. Then, apply it when the flowers have reached pink bud. After that, apply it when the petals fall. Finally, apply it 3 weeks after the petals fall. You may need to apply fungicides for two consecutive years to completely eradicate the disease. Fungicides will be most effective if applied before rainy periods, and when temperatures are greater than 55 degrees F.

For planting new cherry trees, purchase healthy varieties that are resistant to black knot disease from reputable nurseries. Unfortunately, there are only a couple of cherry tree varieties that are resistant to black knot. These are Amur cherry (Prunus maackii) and Goldrush cherry (P. maackii 'Jefree').

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