WHAT IS IT?
Black knot is a fungal disease that can affect several different species of trees. It can be fairly easily identified by the knobby black growths that appear on infected branches. The disease is caused by a fungus called Dibotryon morbosa.
Black Knot most commonly affects different varieties of plum and cherry trees.
HOW TO IDENTIFY:
The black growths are the most obvious sign of Black Knot disease. These growths are galled galls and will typically only appear on the woody branches of the tree, though is some severe cases, galls may appear on the trunk of an infected tree. The disease matures in the summer, but the galls are most identifiable during the winter after the tree has lost its leaves and the galls can be seen more clearly.
In some severe cases of Black Knot, leaf wilt or even branch death can occur.
The fungus that causes the disease overwinters in the black galls on the tree. During rainy periods in the spring, fungus spores are released and then blown by the wind to new branches. Young tree growth or damaged branches are the most likely to become infected. The spores enter their new host where they germinate and begin the grow in-between the plant cells. During this period there is no visible sign that the tree has been infected. During this time a chemical is released which causes excessive cell growth in the plant and this is what causes the galls to appear. The galls are made up of both plant tissue and fungal tissue.
IF LEFT UNTREATED:
The galls that appear when a tree has been infected with Black Knot are very unsightly and they won't go away on their own. If the disease is severe enough for a gall to completely encase a branch, it is likely that the leaves on that branch will wilt and the branch itself will die. Once its host branch dies, the gall will die along with it, but not before it has released new spores to germinate on new branches.
In the most severe cases of Black Knot where the host tree is highly susceptible or very young, it is possible that an infected tree could die if it is not treated.
Pruning out the galls is the most important step to take when dealing with Black Knot. Because of the high number of spores produced by a single gall however, it may take multiple years of pruning to completely deal with the problem.
A fungicide spray can be applied in the early spring as a preventative measure against black knot. This would be advisable for highly susceptible species of trees, very young trees, or damaged trees.