Keep cherry trees healthy by blocking black knot

Hauling in six cords of winter’s firewood, piece by piece, affords the mind time to wander. Some work is joyously mindless, isn’t it? Once the garden’s been put to bed for the season, stacking the firewood replaces the delightfully mindless task of weeding, albeit for just a short while.

Some cherry was found among the piles of wood in the yard this autumn. Bulbous knots of diseased wood refused to be split cleanly. Inside the tree stem, swollen and distorted tissue twisted and turned. If trees had nerves, it seems this disease would be a most gruesome, agonizing affliction.

Cherry trees grow wild in the forests of Maine. In spring, pretty pale pink and white flowers bloom. In autumn and winter the trees do not stand out among the other deciduous species in the woods.

One thing that may draw the eye to this otherwise inconspicuous tree is the swollen tissue of black knot. Black knot is a very common fungal disease that occurs on a variety of cherry trees including chokecherry, black cherry, wild cherry and plum. If left unchecked, black knot will cause branches to die back and may even lead eventually to the tree’s death.

The disease is not limited to trees growing in the wild: both cultivated and wild species of these plants are susceptible. Trees severely infected with black knot become an eyesore and are almost worthless as an ornamental plant after several years.

A fungal organism causes black knot. The first sign of the disease is usually the swelling of small twigs. Next, elongated swollen areas appear on limbs as the infection spreads. Affected tissue is greenish and relatively soft in texture at first. Typically, the growths occur on only one side of the limb, yet it is not uncommon for them to girdle the entire limb.

The fungus overwinters in the stem of the infected host and erupts during the spring. As the growing season progresses, spores are produced. Over time, the resulting knots become coal black and very hard. The diseased tissue may range in length from a few inches to more than a foot. At this stage, the appearance has been described as looking like charred marshmallows on a stick.

The fungus, living in these black knots, produces spores that are discharged and spread to other limbs and trees. The spores germinate and produce new infections.

If you spot the disease among your trees, manage black knot infections by pruning branch swellings and knots in January. Prune branches 3 to 4 inches below visible knots and swellings. Burn all infected material. Continue to monitor branches for knots and swellings throughout the growing season, pruning any new infections.

It is not necessary to use wound paint on branches from which you’ve removed diseased tissue. Trees have the ability to wall off this damage and heal the wound site. If practical, wild plum and cherry trees in the infected area should be destroyed. To truly eradicate the disease, encourage your neighbors to join in this control program.

When considering the purchase of cherry species for the home landscape, as always, buy disease-free trees from a reputable nursery.

Never accept trees with swollen branches or knots.

Diana George Chapin is the NEWS garden columnist. Send horticulture questions to Gardening Questions, 512 North Ridge Road, Montville 04941 or e-mail dianagc@midcoast.com. Selected questions will be answered in future columns. Include name, address and telephone number.