Black Knot – Fungal disease of Prunus species

(Trees affected: Mayday, Shubert, Chokecherry, Pincherry, Plum and Apricot)

Yellowhead County’s Agricultural Services Department has received numerous calls regarding black growths on residents’cherry trees. These tar-like black swellings are an inevitable sign of Black Knot and will distort and stunt the affected tree. This disease reduces the aesthetic value of affected trees and if left long enough may result in eventual death of the tree. The disease spreads in the spring attacking new growth on branches, and spores released by the fungi can be carried by wind and splashing rain. The only reliable method of controlling Black Knot is pruning. This should be done between Nov-March and not in the spring or in wet, warm weather.

Pruning guidelines:

1. Cleanly cutoff at least 12 inches from the knot, better to prune back to a healthy collar, then leaving a stub.

2. For knots on branches or trunks that can’t be removed, use a knife to cut away diseased tissue down to good wood and then ½ inch beyond the edge of the knot.

3. Cleaning your equipment properly is another good way to prevent the spread, Pruning shears should be dipped in a 1 per cent bleach/water solution between cuts and once you are finished pruning ensure you run your tools under water for 10 minutes and dry thoroughly.

4. Treat pruned branches like hazardous waste, they can release spores for up to 4 months. Burn, bury, or wrap them in plastic and throw them in the trash. Do not compost.

If you are unsure when it comes to pruning or you require more information it is recommended you call an Arborist or the Agricultural Services Department at Yellowhead County Wildwood Office 1800-814-3935 or 780-3253782.