Black Knot is a widespread fungal disease that effects plum and cherry trees, and to a lesser extent peach, apricot and related cultivars. It is a progressive disease that worsens with each subsequent season that it is left unchecked.
Black knot is a parasitic fungus that primarily affects small twigs before moving on to larger branches, and fruit spurs. In advanced stages tree trunks also become diseased.
Galls, which are woody, knobby swollen black growths appear along the length of stems and branches.
In late spring and early summer, young galls will appear at the perimeter of older galls. These younger galls are pulp like as opposed to the woody texture of older galls, they are blanketed with mossy olive green spores.
By seasons end these new galls will harden and turn black. The hardened galls become more pronounced and noticeable in the winter when the tree is defoliated. Affected branches are frequently distorted and lop-sided and branches will bend under the uneven distribution of weight produced by the galls.
Once the fungus galls envelope a branch completely, leaves will generally not emerge and those that do emerge will commonly wilt and die off early in the summer.
Larger galls will split and crack and sometimes emit a sticky liquid, which is at times mistaken for sap.
By the second or sometimes third season the black knot fungus generally has run its course and dies off The damage caused by the disease weakens the trees and leaves the door open for secondary opportunistic fungi. The invasion of other fungi is apparent when the galls turn a pinkish white or pale color in warm weather.
Although the exact conditions that produce infection are not 100% certain, apparently only a few hours of rain and temperatures in excess of 55 degrees F are necessary. Longer periods of wet and rainy conditions are required to produce infection at lower temperatures.
Prevention and Treatment
Trees should be inspected periodically for black knot presence.The knots are not always visibly apparent in the first year of infection so close inspection is needed.
Infected twigs should be pruned off and destroyed before bud break. It is best to simply burn them or bag them, don't leave them lying around as the fungus will continue to grow even on dead twigs and branches. By all mean DO NOT mulch them.
Galls on larger branches that can not be simply pruned off sould be removed also. Cut away diseased plant tissue down to healthy wood.
Botanists and horticulturists are not in agreement on the effectiveness of fungicides for black knot disease. Some have warned against relying solely on fungicides to cure black knot disease and advise regular pruning of infected areas. Although the experts are not in agreement on the use of fungicides they do agree that fungicides are not effective unless you also follow proper sanitary practices. Fungicides are most effective when used in conjunction with proper pruning and sanitary practices. They should also be applied before rainy periods if at all possible.
Fungicides containing Captan, Chlorothalonil, Lime sulfur or Thiophanate- methyl [Thiomyl] are registered for use against Black knot.
Captan and chlorothalonil can cause injury to fruit if applied after shuck split. Shuck split occurs on some, but not all fruit trees after pollination. The shucks left over from the flowers split and drop. Application of Captan or related fungicides should be done before this occurs.
Lime sulphur can also be applied when budding initiates or when the shucks are dropping - or both. Lime sulphur used in conjunction with Dormant Oil Sprays is effective if applied post harvest or very early in the spring as soon as or before buds appear.