To be or knot to be: Battling black knot in plums

Black knot has become a severe problem in many plum blocks in recent years. Most newly formed knots originate on current season twigs, with a small proportion originating on branches more than 1 year old or on relatively fresh pruning stubs. While numerous knots can be found on large branches, these trace back to infection through a small lateral. New knots develop the year after infection occurs and they do not release spores until 2 years after infection.

The development of spores in the knots is temperature-dependent. Spore release is moisture-dependent: spores are shot out from the knots during rain of at least 2 mm and can continue to be released for up to 3 days after the rain has stopped.

The peak period for spore release usually occurs from late May (shuck split) through the end of June. In a research trial conducted at Jordan Station in 1992, the majority of infections occurred between shuck split and shuck fall.

Black knot spore are spread by wind and rain to twigs where infection takes place through unwounded tissue. Unlike fire blight, infection does not occur through the blossoms. Infections can occur on developing shoots when temperatures reach 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.

The Niagara Peninsula Fruit and Vegetable Growers Association has funded a 3-year collaborative project including Dr. Deena Errampalli (AAFC) and Dr. Jay Subramanian (U of Guelph) and myself to look at various aspects of plum black knot biology and management.

In the meantime, here are some Q&A's to help with black knot management:

How do I get black knot out of my orchard?

  • For knots on smaller shoots, make the cut at least 15-20 cm (7-10 inches) below the knot. If possible, prune infected branches further back to an appropriate location, such as a healthy collar, rather than leaving a stub.
  • Remove knots on scaffolds surgically using a knife, chisel, two-handed drawing knife (see image) or saw. Remove tissue 10 cm (4 inches) above and below the knot and deep enough so that no black discoloration is evident. The fungus colonizes the inner bark beyond the visible swelling. Failure to remove enough tissue can result in the regrowth at the edges of the wound.
  • To ensure that you aren't spreading black knot during surgery with tools, sterilize them with 90% ethanol or bleach between each knot.
  • Some older sources I've read suggest wiping the wounds with lime sulphur, Bordeaux mix or turpentine (reference from the 1880's). I don't know for sure whether this works or not, but it can't hurt other than the time it takes. We'll look at this in the research project.
  • Make sure to remove knots from the tops of pollenizers as they are an excellent source of spores for neighbouring trees. Wild plums can also act as a source of spores for orchards.
  • All knots should be removed from the orchard and burned before bloom. There has been some success with flail-mowing knots dropped into the row middles, but this does not ensure complete removal of spore sources. Some sources have suggest that removing knots in late winter, when trees are fully dormant, as opposed to early spring, may be beneficial. We'll look at this in the research project.

Can black knot from sour cherry infect plum?

This question is up for debate. In my (and other) research trials in which spores from sour cherry black knots were sprayed on plum and vice versa, no infections occurred while when plums were inoculated with plum black knot spores infections did occur. A molecular analysis of isolates of the black knot fungus from chokecherry and plum showed differences between the two so they are genetically distinct. The relationship between sour cherry and plum strains has not been examined. We hope to address this in the research project.

When should I spray?

  • In a high-inoculum orchard, start spraying at 10% bloom
  • Make sure that you include a black knot fungicide in your spray program between shuck split through the end of June.
  • In years with a cold spring, spore development may be delayed and in a dry spring, spores develop and mature but are not released until rainfall occurs. There is no definitive answer as of yet as to when you can stop spraying for black knot. An aspect of the research project to determine when spore are released and also when they stop being released.

What should I spray?

While fungicides will not provide adequate control of black knot without proper orchard sanitation (pruning, removal, and burning of black knots), they are good additional insurance.

Several fungicides registered for brown rot control can help with black knot management, but only by suppressing the black knot disease. Those listed in publication 360 on plum include: Supra Captan, Maestro, Indar, Topas, Mission and Jade.

Chlorothalonil (Bravo/Echo) and captan products consistently provided the best control of black knot in trials in Ontario and New York in the 1980's and 90's. Chlorothalonil is not registered for use in plums and both captan and chlorothalonil caused injury on fruit if applied after shuck split. A trial in the 1970's in Michigan reported that thiophanate methyl (Senator contains this active ingredient and is registered for brown rot control on plum) provided good control when applied at 5 times starting 75% bloom and every 10 days after that.

Some old reports (1880's) suggest a delayed dormant application of "1:8 lime sulphur applied about the time buds open followed by a second application of 1:40 lime sulphur when the shucks were falling, and a third of the same strength some weeks later". Lime sulphur plus dormant spray oil is labeled for use on plums for management of "San Jose scale, European scale, Mites, Aphids eggs, and General clean-up". The oil should not be included in the tank mix once there is green tissue showing.

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