Jan Riggenbach: Beware the dreaded black knot - Omaha.com
Published Thursday, February 14, 2013 at 1:00 am / Updated at 5:49 pm
Jan Riggenbach: Beware the dreaded black knot

A thicket of wild chokecherries at the back of our lot is looking very strange this winter. The shrubs have numerous branches with long, bumpy, black growths. My husband Don says the swellings look like “burned marshmallows on a stick.”

It’s the worst case I’ve seen of black knot, a fungus disease that can infect any tree or shrub in the genus Prunus, including all types of plums and cherries. The hard black galls, which range in size from a couple of inches to more than a foot long, are the trademark of this disease. Wild black cherries, chokecherries and wild plums are particularly susceptible, but plum and cherry trees in a backyard orchard are vulnerable, too.

Black knot is more than just a cosmetic problem. Infected trees and shrubs produce fewer flowers and fruits. If not controlled, the disease gets worse every year and will eventually stunt or kill the plants.

The swellings have been developing since last summer, but they were small and easily overlooked at that time. It’s in winter — after the leaves fall — that the knots are most noticeable. In wet weather in spring and early summer, the mature knots will release fungal spores, which will then be carried by wind to infect other cherries and plums in the vicinity. Winter pruning, before new growth begins, is the best way to break the cycle.

Make pruning cuts at least 3 to 4 inches below the swellings. Sterilize your pruners between cuts; some experts recommend using a solution of 1 part chlorine bleach to 4 parts water. Compost, bury or burn the prunings.

Sometimes a black knot swelling occurs on a large branch you’d rather not sacrifice or even on the main trunk. If that happens, try removing the affected portion by cutting down to the wood. Make your cut at least an inch deeper and wider than the visible infection.

Winter is also the best time to control fire blight, a serious bacterial disease of all pears, including the ornamental type. The disease is easily recognized: infected branches look like they’ve been burned.

Fire blight occasionally infects some other woody plants, too, including apple, crabapple, cotoneaster, flowering quince, hawthorn, mountain ash and serviceberry. Scout your property this winter for branches that look scorched, then tackle the problem with a pruning saw and pruning shears.

Remove blackened branches, cutting at least 10 inches below the visible damage. Sterilize your tools after each cut by dipping the cutting blades in a solution of chlorine bleach.

If you’re planting new pear trees, choose resistant varieties such as Moonglow for eating or Chanticleer for an ornamental tree.

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