Fruit trees and ‘black knot disease’

by Dale Odorizzi, Lanark County Master Gardeners

Plum trees are absolutely spectacular in full flower.  They bloom early in the spring with blooms that line the branches before the leaves appear.  Many people grow them simply for their spring beauty but there are cultivars that are hardy in Eastern Ontario, although some of the cultivars may not be widely available.  Cherry Trees are also a beautiful addition to the spring landscape and some varieties such as Nanking or Manchu produce a delicious Cherry crop, if you can harvest them before the birds get to them.

As beautiful and productive as these small fruit trees can be, one problem that can gravely impact your trees is a fungal infection, know as Black Knot.  Black Knot appears as unsightly swellings (galls) on twigs and branches.  At first, these galls appear relatively harmless.  They are rather subtle and velvety green in appearance.  Gradually, they increase in size, harden and become black.  The tips of the infected branches often die back.  Severe infections can kill whole limbs, and the tree may become stunted.

Black knot disease spreads in spring.  On rainy days, the fungus releases spores which are carried on wind currents.  If the spores happen to land on new spring growth of a susceptible tree, especially if the tree is damp, the spores germinate and infect the tree.  The source of the disease is usually wild, abandoned or neglected trees.  Finding and destroying the source is important to controlling the black knot tree disease.

In fall or late winter, prune off infected limbs 15-30 cm below the knots, disinfect pruners between cuts with 10% bleach solution (1-part bleach to 9-parts water).  Destroy the prunings.  Do not compost.   If possible, remove any wild plums or cherry trees, nearby.

For persistent infections, apply two sprays of lime-sulfur, 7 days apart before the buds begin to grow in spring.  Spraying can help to limit the spread of the disease but must be combined with conscientious removal of galls as they are identified.