Black Knot of Plums

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Agdex#: 216/634
Publication Date: April 2005
Order#: 05-017
Last Reviewed: April 2005
History: Replaces OMAF Factsheet Black Knot of Plums, Order No. 91-046
Written by: Neil Carter - Tener Fruit and Grape IPM Specialist/OMAF; Michael Celetti - Plant Pathologist, Horticulture Crops Program Lead/OMAF; Dr. Wendy McFadden-Smith - McSmith Agricultural Research Services/Vineland Station
Table of Contents

1. Introduction
2. Symptoms
3. Disease Cycle
4. 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.

FIGURE 1. Black knot on major limb of infected plum tree.

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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 sympton of black knot is corky brown swelling on current season's growth

Figure 2 The first sympton 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 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.

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.

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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.

Figure 5. Black knot developing on fruit spur of three-year-old wood.

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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.

Figure 6. Trichothecium roseum colonizing a mature black 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

* None

Consult a current issue of OMAF 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.

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