Ah, bloom. The time when a pathologist’s thoughts turn to …. disease management!
Black knot is a perennial problem in tart cherries and both yellow and European plums. The fungus, Apiosporina morbosa, survives in swollen black knots in which the ascospores develop in the spring.
Ascospores can be released when at least 0.25 mm of rain occurs at temperatures over 10°C. More ascospores are released with increasing amounts of rain and optimal temperatures of 16 to 27°C. Infections typically start where water collects in the “V” where leaves attach onto shoots.
Ascospore release from knots occurs after bloom so it is critical to maintain protection with effective black knot products. Since ascospore release requires rainfall, spore release can be delayed until rainfall is sufficient and in some years, this can be well into terminal development.
The graph below shows the proportion of ascospores released from knots 2013-2015 and growth stage of a neighbouring plum block.
Pruning and destroying knots should be your first line of defence. All knot removal should be completed before the new shoots that develop in late bloom are exposed to infection. The black knot fungus is present in limbs well beyond where knots are visible so prune a minimum of 15 cm below the knot. Knots can release ascospores for up to 4 months after they are removed from the tree. Brush piles should be burned to completely eradicate knots. Flail mowing knots into small pieces will also reduce knot viability and ascospore release.
Fungicides effective against black knot should be applied post bloom and every week to two weeks, especially before rain, until terminal growth stops. Make sure that sprays get to the tops of pollinizer trees as these often harbour many knots. Unfortunately, the fungicide options for managing black knot are limited and are about to become even more limited. Chlorothalonil, the active ingredient in Bravo and Echo, is very effective but is not labeled on plum and has been under review by PMRA with the proposed cancellation of all outdoor uses. Corteva will be phasing out Indar, with the last use December, 2025. Propiconazole, the active ingredient in Bumper, Fitness and Princeton, has weak activity against black knot. Captan is limited to one application per season, it isn’t labeled for black knot on tart cherries and the recommendation is to not use it on plums until later in the season due to potential shot-holing of foliage and generally after ascospore release has completed. On a positive note, the label for Senator 50 SC has been expanded to include control of black knot in plums and tart cherries. Refer to the label for more information: https://pr-rp.hc-sc.gc.ca/1_1/view_label?p_ukid=313981989. As shown in the graph below, in spray trials conducted several years ago in Niagara, Senator performed well against black knot.
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