Black Knot of Plums
Table of Contents
- Introduction
- Symptoms
- Disease Cycle
- Disease Management
Introduction
Black knot is a serious disease of most American, European, and Japanese
cultivated plum and prune varieties as well as wild plum. Black knot is
caused by the fungus Apiosporina morbosa (=Dibotryon morbosum) and affects
only the woody parts of trees, primarily twigs and branches. The disease
is most recognizable as large, black, misshapen growths on branches (Figure
1). The black knots tend to be largest on the very susceptible and
moderately susceptible varieties of European plum and smaller on less
susceptible Japanese plums. A strain of A. morbosa that causes small knots
on sour cherry will not infect plum and plum strains will not infect cherry.
Figure 1. Black
knot on major limb of infected plum tree.
Symptoms
Symptoms of black knot first appear as light brown, warty swellings about
1 cm long on the shoots of the current season's growth in late summer
or the spring following infection (Figure 2). The
small brown swellings often cover only one side of the infected twig.
On some trees the young, warty swellings show up in August or September
(3-4 months after the initial infection) (Figure 3)
while in other cases the disease symptoms may not be expressed until the
following March or April (11-12 months after the initial infection). During
the following year (about 14 months after infection) the warty swellings
begin to expand and gradually become an olive-green or tan coloured knot
with a velvety texture. By autumn of the second year after infection,
the knots mature and turn coal-black and woody.
Figure
2. The first symptom of black knot is corky brown swelling on
current season's growth
Figure 3. Expanding
corky swelling on limb of current season's growth
Figure 4. Badly
neglected plum showing advanced black knot infection with fusing of multiple
knots.
The knots can be 10-15 cm long and may completely encircle the limb.
Two or more knots will often fuse to create knots that are 30 cm or more
in length. The black knots continue to expand in following years until
the branch becomes girdled and dies (Figure 4). Older
knots are frequently invaded by wood-boring insects (especially lesser
peachtree borer), which also contribute to the decline of the tree. Severely
diseased trees will have fewer blooms, poor fruit production, and are
more susceptibility to winter injury. Heavily infected trees become unproductive
after a few years due to the loss of vigour and limb death caused by the
disease.
Disease Cycle
In Ontario, the disease cycle of black knot (the time from initial infection
until a black knot releases spores) normally takes 2 years to complete.
In some situations, the fungus can produce a knot in 1 year, but spores
are not produced until the following spring.
The fungus overwinters in knots or in the infected wood around the knots.
Ascospores capable of spreading the disease are produced in asci (sacs)
inside tiny fruiting bodies at the surface of the mature black knots.
Mature spores are usually present from the last week of March or early
April through the first week of June. The spores are ejected from the
mature black knots during wet periods and are carried by wind to susceptible
twigs and branches. At least 6 hours of continuous wetness and temperatures
above 11°C are required for infection. In an average spring season
with regular rainfall, most spores are released and subsequent infections
can occur from the popcorn stage to petal fall during the bloom period.
However, in a warm, dry spring, spores develop and mature but are not
released until rainfall occurs. During dry springs, it may be necessary
to maintain protective fungicide coverage for a longer period to avoid
later infections.
Very young, tender shoots are most susceptible to infection, particularly
during cool, wet spring weather. Infections will often occur in the axil
of leaves where rainwater collects and tissue remains wet for longer periods.
Blossoms, leaves, and fruit tissue are generally not infected by A. morbosa.
The fungus will grow from infected developing fruit spurs into older limbs
(Figure 5). The fungus colonizes the woody tissue
of infected shoots and limbs and may stimulate the surrounding tissue
to produce tumour-like corky brown growths by the end of the first growing
season or early in the second season. The fungus overwinters in woody
tissue and the corky brown swellings and tissue surrounding them during
the first winter after infection. The following spring, the brown corky
swellings continue to develop and expand, transforming into the familiar
mature black knots by autumn. The fungus overwinters as a mature black
knot during the second year and produces spores for release early the
following spring. Black knots will continue to expand over time and eventually
surround the twig or branch.
Figure 5. Black
knot developing on fruit spur of three-year-old wood.
Disease Management
Pruning and Burning
Management of black knot in plums relies first and foremost on pruning
infected branches and removing them from the orchard. All black knot pruning
should be done well before bloom - preferably in late winter before any
new growth begins. Pruned branches must be removed from the orchard and
promptly burned. Burning the prunings immediately is preferable, but if
burning is not feasible, knots can be buried beneath at least 30 cm of
well-packed soil. Pruned knots left on the orchard floor can produce and
release spores. Eliminating all sources of inoculum from the orchard is
vital to prevent further infections.
Black knots on major scaffolds or trunks can be "chiselled out"
without removing the entire limb or tree. At least 10 cm of tissue surrounding
visible knots on major limbs should be removed because the fungus colonizes
the inner bark beyond the visible swelling. Failure to remove enough tissue
can result in the regrowth of the knot. No amount of pruning can restore
a severely infected tree to productivity.
Site Selection
Avoid planting near abandoned orchards that may have black knots. Do
not plant new plum orchards next to trees or orchards bearing black knots.
If possible, remove wild plums from fencerows and wooded areas within
150 m around orchards to reduce the potential of spores from knots on
wild hosts blowing into plum orchards.
Resistant Varieties and Healthy Stock
Cultivated plums vary greatly in their susceptibility to black knot.
Table 1 shows the susceptibility of some of the
more common cultivars. None of the plum cultivars recommended for commercial
production in Ontario are resistant to black knot; however, some cultivars
are less susceptible to the disease.
Purchase and plant disease-free trees from reputable nurseries. Never
buy or plant any nursery stock that has visible knots or abnormal swellings
on the twigs and branches.
Biological Control
Mature black knots may be naturally colonized by a parasitic fungus (Trichothecium
roseum) making them appear creamy-yellow to pink in colour (Figure
6). Black knots colonized by this beneficial fungus do not produce
spores. However, T. roseum does not provide the level of suppression or
control of black knot that is required in commercial production.
Figure
6. Trichothecium roseum colonizing a mature black knot. This
knot has also been infested by borers - note frass (insect excrement)
near bottom of knot.
Fungicides
Fungicides will not provide adequate control of black knot without proper
orchard sanitation (pruning, removal, and burning of black knots). Many
fungicides registered for brown rot control can help with black knot management,
but only by suppressing the black knot disease. Most fungicides targeted
for brown rot control will prevent 40%-50% of the potential black knot
infections under high disease pressure. Where cultural controls such as
pruning knots are combined with the use of fungicides, up to 90% prevention
of black knots is possible.
Table 1. Relative
Susceptibility of Plum Cultivars to Black Knot
Very Susceptible
- Bluefre
- Damson
- Myrobalan B
- Ozark Premier
- Stanley
- Veeblue
- Vision
Moderately Susceptible
- Early Italian
- Italian
- Valor
- Vanier
- Verity
- Voyageur
- Valerie
- Vanette
- V70034
Slightly Susceptible
- Burbank
- Early Golden
- German Prune
- Shiro
- V72511
Not Susceptible
Consult a current issue of OMAFRA Publication 360, Fruit Production
Recommendations for a list of fungicides currently registered for
black knot control. Some products are registered for control of both brown
rot and black knot. It is important not to overuse these products as reliance
on single products or single chemical families selects for and promotes
the establishment of resistant fungal populations.
It is important to protect the tender developing shoots before bloom and
until they are about 15 cm long. Choose an appropriate fungicide and application
time carefully to obtain optimum control of both brown rot and black knot.
For best results apply 3-4 applications of a protective fungicide at 7-10
day intervals. Intervals between sprays may need to be shortened to 3-5
days if 2-5 cm (1-2 in.) of rainfall occurs. In years with dry spring
weather, delay fungicide applications until after bloom, but prior to
rain, in order to protect new growing tissue before spore release and
infection occurs.
