Test Your Knowledge - December

Black knot gall
Black knot gall
Terry Price, Georgia Forestry Commission, www.forestryimages.org

Yes, this swollen, woody gall is commonly referred to as “black knot”.

In the winter when a lot of trees lose their leaves, it’s easy to see the large, black galls that are often found on wild cherry and plum trees in wooded areas. Black knot disease is caused by the fungus Apiosporina morbosa (syn. Dibotryon morbosum). After a warm, wet period in early spring, the fungus releases spores (reproductive structures) that are carried by wind and splashing water to infect woody tissue of twigs, branches and sometimes main stems of susceptible trees. Initial symptoms of infection are light brown swellings which are easily overlooked. By the next growing season, the young galls are covered with an olive-green, velvety fungal growth. The young galls turn darker over the summer, and by fall they harden into rough, black knots. The galls enlarge each year and can eventually girdle twigs, branches and even main stems. When this happens, water is cut off and death of the tissue beyond the knot occurs.

Black knot gall on black cherry stem
Black knot gall on stem of black cherry tree
Joseph O'Brien, USDA Forest Service, www.forestryimages.org

Fungal spores from infected trees in the wild can be a source of infection to susceptible fruit and ornamental trees in the Prunus genus (especially cherries and plums, and occasionally apricots and peaches). Control measures include pruning out and destroying black knots (by burning or burying). It is important that cuts are made 6-8 inches below the swellings and that pruning tools are cleaned with a 10% bleach solution after every cut. When feasible, remove wild cherry and plum trees within 500 feet of susceptible trees. Some resistant cultivars are available and when possible, should be planted. It is usually not necessary for trees in home orchards to be sprayed with fungicides.

Janet McLeod Scott
HGIC Extension Agent

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