Biology Reference
In-Depth Information
Black Knot
The term black knot is used to designate a disease
with black knotty excrescences.
with an olive green, velvety layer made up of
brownish conidiophores and one-celled hyaline
conidia of the anamorph
Hormodendron
state.
Conidia are spread by wind.
In late summer black stromata cover the
affected tissues, and the galls become hard. Asci
are formed during the winter in cavities in the
stroma; ascospores are discharged and germinate
in early spring, completing the 2-year cycle.
Knots are produced from primary infection by
ascospores or from secondary infection from
mycelium formed in old knots and growing out
to invade new tissue. Limbs may be girdled and
killed; trees are stunted and dwarfed, nearly
worthless after a few years. Old knots may be
riddled with insects or covered with a pink fungus
growing on the
Apiosporina
mycelium.
Control
Cut out infected twigs and branches, 3
or 4 inches beyond the knot, to include advancing
perennial mycelium. Do this in winter or before
April 1. Eradicate or thoroughly clean up wild
plums and cherries in the vicinity. Spray at
delayed dormant stage in spring (just as buds
break) with bordeaux mixture or with liquid
lime sulfur. The latter is preferable unless oil
is combined in the spray as an insecticide.
Spray with lime sulfur at full bloom. Dibotryon
morbosum (see
Apiosporina morbosa
).
Black
Knot
of plum and cherry,
Prunus Black Knot
,
Apiosporina
Ascomycetes, Pleosporales
Asci are in locules, without well-marked perithe-
cial walls, immersed in a massive, carbonaceous
stroma, erumpent and superficial at maturity.
Spores are hyaline, unequally two-celled.
Apiosporina morbosa (formerly
Dibotryon
morbosum
).
Black Knot
of plum and cherry,
Prunus Black Knot
,
Plum Wart
, widespread and
serious on garden plums, also present on sweet
and sour cherries, chokecherry, and apricot.
Apparently a native disease, destructive in Mas-
sachusetts by 1811 and the pathogen described
from Pennsylvania in 1821, black knot has been
reported on peach, long thought to be immune.
The chief symptoms are black, rough, cylin-
drical or spindle-shaped enlargements of twigs
into knots two to four times their thickness and
several inches long (see
Fig. 1
). Infection takes
place in spring, but swelling is not evident until
growth starts the following spring, at which time
the bark ruptures, and a light yellowish growth
fills the crevices. In late spring this is covered
