I. Introduction: Black knot is a
very destructive fungal disease on susceptible cultivars of plum and prune. Losses in
commercial orchards are sporadic; the disease is often found in poorly managed orchards,
home plantings, or on abandoned and wild trees. This fungal pathogen is also occasionally
found on apricot, peach, sweet and tart cherry, and ornamental Prunus spp. First described
in 1821 in Pennsylvania, black knot is now generally distributed throughout North America.
It was one of the most destructive diseases of plum and tart cherry in northern and
eastern fruit regions during the late 1800s, but today is considered of less importance in
most commercial orchards, except in blocks where it is well established.
II. Symptoms: The disease occurs only on the woody
parts of trees, primarily on twigs and branches, and sometimes on trunks and scaffold
limbs. The warty swellings first become visible on new shoots in late summer or the
following spring. At first the knots are olive-green and corky, but with age turn black
and become hard and brittle (photo 2-74). The knots vary in length from one inch to nearly
one foot (2.5-30 cm). Many times they do not completely encircle the branch. Knots one
year or older may become covered with a pinkish-white mold of another fungus and riddled
with insects, especially the lesser Peachtree borer.
III. Disease Cycle: Infections occur on new shoot
growth, mainly from ascospores during periods of measurable rainfall of six hours or more
at 72 F (22 C). Ascospores of the fungus are discharged from tiny sacs in the surface of
the knots. Germination is very low at 45 to 50 F (7-I0 C), but increases significantly
from 55 to 75 F (13-24 C). Very little infection is known to occur from conidia. Unwounded
susceptible twigs may become infected soon after bud-break throughout the active shoot
elongation period. Following infection, excessive production of parenchyma cells is pushed
outward, forming the base of the knot. The first symptoms of infection are visible by
early autumn, but further development continues the following spring. The knots develop
rapidly the second summer, and the layer in which the ascospores are formed develops
during the second winter after infection is initiated. The fungus in the woody tissues
continues to grow in the spring and fall, increasing the length of the knots. Their
eventual size depends greatly on the species and cultivar of the host plant.
IV. Monitoring: During the dormant period, monitor
in a block that includes a susceptible cultivar, such as 'Stanley'. Observe ten vegetative
terminal shoots on each sample tree for small, developing knots (photo 2-74). Presence of
any black knots represents high risk on susceptible cultivars. Awareness of black knot
inoculum from adjacent commercial orchards or wild hosts may affect control decisions.
Continue to monitor for black knots (photo 2-74) during the
bud-break to bloom period by examining ten vegetative terminal shoots on each sample tree.
V. Management: Control of black knot
requires a combination of cultural and chemical methods. Cultural practices should
include removing wild plum and cherry seedlings from fence rows, woodlots, and along
orchard perimeters; inspect orchards and surrounding areas each winter for black knots and
prune out infected shoots and limbs; remove pruned knots from the orchard and bury or burn
them before budbreak in the spring. When pruning infected material in the dormant
season, always make the cut 3 to 4 inches below the margin of each knot, since the fungus
grows in the tissue beyond the visible swellings. Consult your local Extension
Service for fungicide recommendations since there appears to be some variation in
effectiveness from one area to the next. Sprays should be applied from white bud
through shuck split (green tip through second cover in problem orchards). Spraying
by itself, without implementing the recommended cultural practices, may not provide
adequate control of the disease. There is considerable variation in cultivar susceptibility to black knot.
Chemical control -
Chemical control - home
orchardists (pdf file - Acrobat Reader required)
Text prepared by K.D. Hickey and A.R. Biggs
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