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. The spores released by the fungi can be carried by wind and splashing rain. The only reliable method of controlling Black Knot is pruning. Pruning should only be done between November and March and not in the spring or in wet, warm weather.

Black knot (Dibotryon morbosum) is a naturally occurring disease throughout Canadian prairies. Trees and shrubs of Cherry ( Prunus) genus are mostly affected. Various species of ornamental or edible cherries (Maydays, Shubert chokecherry, chokecherry, Nanking cherry, pin cherry, sand cherry, sour cherry, etc.), Saskatoon’s, plums, flowering almond are mostly inflected by this disease.

Unless you are in the fruit production business, where this disease can greatly reduce the crop yield, this disease on individual trees will take years and sometimes decades to entirely kill the tree. Black knot is not one of the quick and deadly diseases that will kill your tree within a year or two. One of the biggest spreaders of black knot is improper pruning and the time when people prune trees and shrubs. Avoiding improper pruning mistakes and choosing appropriate time is the best way to not spread this disease.

This Guide on Black Knot Management from the ATTS Group covers:

• Disease and Symptoms Identification for Black Knot
• Management of Black Knot (When and how to remove Black Knot)

Black Knot – Pruning

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 1800-814-3935 or 780-325-3782.

Only prune between November and March.