White County
 Gardening
Podcast
February 20, 2009
Black Knot of Plum and Cherry (2:11 minutes)
Audio/Video Script:
Sherri Sanders
County Extension Agent - Agriculture
With a few warm days here and there - it always stirs the interest of
gardeners in anticipation of spring’s arrival. Lately several people have been
asking about a peculiar substance on their plum tree limbs. Hello this Sherri
Sanders in White county.
The common name "black knot" describes the main symptoms of the disease.
However, the first symptoms are small, light brown swellings usually located at
the base of the leaf petiole or on the fruit spur. These appear usually during
the summer and first year after infection. Young knots may have an olive-green
color, but later become hard, brittle and black in color. Older knots are
coal-black in color (hence the name "black knot") and hard in texture. The knots
often protrude more on one side of the affected branch.
The disease black knot is caused by a fungus. Black knot can occur on sweet
and sour cherry, American, European, and Japanese varieties of cultivated plums
and prunes as well as wild cherries and plums. The disease is common throughout
much of Arkansas occurring on many wild plum and cherry trees. These serve as an
annual source of infection for cultivated varieties.
For control, destroy sources of infection. During fall and winter, or before
new growth starts in the spring, prune out and destroy all visible knots. On
large, main branches and trunks, knots should be cut out with a knife or chisel.
One inch of healthy bark around the knot should also be removed. Obtain
disease-free stock, if possible. Never purchase nursery stock showing visible
knots or abnormal swellings on the twigs and branches. When pruning branches
with knots, the cut should be made at least 2-4 inches below the lowest part of
the knot. Because spores can develop and spread from knots left on the ground or
in brush piles for several weeks after removal from the tree, they should be
burned or buried.
Fungicide sprays applied in a timely, regular manner can prevent most new
infections. There are some resistant varieties available to try as well. For
additional horticulture information, contact your county Extension office. This
has been Sherri Sanders in Searcy.
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