Black Knot of Plum and Sour Cherry

Most sour cherry and plum growers are familiar with Black Knot caused by the fungus Apiosporina morbosa. It is also frequently observed on wild plum, cherry and occasionally on other wild Prunus trees, which can be a source of inoculum to near by susceptible commercial orchards. Sweet cherry, occasionally peach and apricot can also be infected by this pathogen but to a much lesser extent than sour cherry or plum. Late winter and early spring is the best time to prune out mature black knots to eliminate the major inoculum source before infection periods occur this spring.

Only woody tissue of susceptible trees are affected by the disease which produces the familiar black round or elongated swellings ranging anywhere from 1.5 to 30 cm in length (figure 1). Fruiting bodies are produced on the outside of the over-wintering mature black knots either during the fall or over the winter. In the spring, the fruiting bodies begin to produce spores (ascospores) that usually require a brief maturation period that is dependent on temperatures. In Ontario, studies have shown mature spores are released from the knots during rainy periods from as early as the end of March through until the middle of July. The peak period for spore release usually occurs from late May (shuck split) through the end of June. At least 2 mm of rain are required for spore release from knots on sour cherry and spores can continue to be released for up to 3 days after the rain has stopped. In one study, a prolong dew period was all that was required for spore release. Infections can occur on developing shoots when temperatures reach around 10-11°C and a wetting period occurs for at least 6 hours although optimum conditions for infection are 20°C with wetness periods of 48 hours following rain.

Figure 1: Mature expanding black knot in large plum branch.

Once the spores land on twigs or branches of a susceptible host, they germinate and infect. A small (1 cm) light brown wart like swelling develops and somtimes noticed on infected plum twigs as early as August or September during the same year as infection. Usually the swelling does not appear until the following spring. Occasionally, a few small knots on plum twigs will develop in to mature black knots during the same season of infection. However, the swelling usually develops into the diagnostic black knot the following spring which then over-winters and produces mature spores 2 years after infection actually occurred.

In Montmorency sour cherry, the knots do not develop much during the year of infection and only appear as a small swelling around the point of infection. The following spring, during white bud stage, the swelling increases in size causing the bark to split. During bloom to shuck split, the swelling increases rapidly creating a black knot with similar appearance as those observed in plum. As the knot matures, fruiting bodies are produced during the fall and winter capable of producing spores the following spring and the cycle repeats.

Black knot management relies primarily on sanitation and properly timed effective fungicide applications. Knots should be pruned at least 8-10 cm below the swelling during late winter or early spring before spores are released and susceptible shoot tissue develops on the hosts. The black knot infected branches should be completely remove from the orchard or burned. Knots left on the orchard floor or in trees will be source of spores that will be released in the spring and cause infection. Wild plum and cherry trees in near by wooded areas should also be removed during late winter or early spring. Applying an effective, registered fungicide at petal fall and again at shuck for sour cherries and at pre-bloom, shuck and again at first cover for plums will protect shoots when a few knots were missed during pruning or inadvertently left on the orchard floor (see OMAFRA Publication 360). Applying fungicides without pruning out knots will not effectively manage this disease, however an integrated approach with both pruning and properly timed fungicide applications can provide up to 90% control.

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