Black knot could be called the doggie doo disease, since its galls resemble dried-up Fido feces. It is actually a fungus (Apiosporina morbosa or Dibotryon morbosum) that attacks plum and cherry trees, including chokecherry (Prunus virginiana) -- a native species perennial in U.S. Department of Agriculture plant hardiness zones 2 to 9. Infection happens around bloom-time, but only shows up in autumn as corky tan swellings. The following spring those “knots” moss over and convert into brittle black galls that can kill branches. Prevention tactics include the monitoring of related trees and, if necessary, pruning and application of fungicides.
Plant only disease-resistant types of chokecherry, such as Canada Red Select Cherry (Prunus virginiana “Shubert Select”) or Amur chokecherry (Prunus maackii), both also hardy in USDA zones 2 to 9. Check the saplings closely before purchasing them to make sure you see no abnormal swellings on their branches or twigs.
Examine any other fruit trees from the Prunus family that stand within 600 feet of the chokecherries in late winter or early spring. Make sure they don't have corky growths on them, which they can infect the chokecherries with black knot disease — or vice versa. If you find galls or the beginnings of galls on the branches of those trees, prepare to prune them out.
Fill a small bucket with 9 parts water to 1 part bleach. Lop off any branch that shows a gall, cutting the branch at least 4 inches below that swelling. Dip your pruning shears in bleach solution after each snip to disinfect the blades.
Chisel out any galls that appear on the trunks by cutting at least an inch beneath and around each one. Disinfect the chisel between cuts as well. Dispose of the branches and galls in the manner required by your community for diseased wood, keeping in mind that it should be either burned or buried.
Cover any holes you have made in the trunks with shellac and wound dressing. Although these substances are no longer recommended for most pruning, they do help prevent fungal spores from infecting damaged wood.
Spray infected trees and those you wish to protect thoroughly four times in spring with a liquid copper fungicide. Use 3 teaspoons of the copper concentrate per gallon of water, or whatever amount is specified on the label. Wear goggles and a pesticide respirator, and treat the trees once when the leaf buds seem on the verge of opening, once when the flower buds begin to show the tips of petals, once after the blossoms fall and once more two weeks later.
Repeat the pruning and spraying regimen in late winter and early spring of each year.