Plums With Black Knot: How To Treat Plum Black Knot Disease

By: Teo Spengler

Plum black knot disease is named for the warty black growths that appear on the branches and shoots of the fruit trees. Black knot on plum trees is quite common in this country and can affect both wild and cultivated trees. If you have plums or cherries in your home orchard, you need to know how to identify this disease and how to treat plum black knot. Read on to learn more about plum black knot control.

About Plum Black Knot Disease

Plum black knot disease is a nightmare for gardeners, since it can easily result in the death of plum and cherry trees. It is caused by a fungus called Apiosporina morbosa or Dibotryon morbosum.

Most cultivated plum trees are susceptible to black knot, including American, Japanese and European plum tree species. The popular cultivars Stanley and Damson are very susceptible. You also see ornamental cherries and plums with black knot.

Symptoms of Plums with Black Knot

So how can you tell if your plum has black knot? The principal symptoms are rough black swellings or knots that appear on the woody portions of the tree, usually small twigs and branches.

The knots grow longer and wider until they encircle the branch. Initially soft, the knots harden over time and turn from green to brown to black. Plums with black rot lose branches as the knots cut off the water and food supply, and eventually the disease can kill the entire tree.

Plum Black Knot Control

If you are wondering how to treat plum black knot, the first step is to catch it early. If you become aware of black knot disease when it first develops, you may be able to save the tree. The spores that spread the fungus are released from mature knots in spring when it rains, so removing the knots in winter prevents further infestation.

The knots may be difficult to see while a tree is covered with leaves, but in winter, they are obvious. Plum black knot control starts in the winter when the trees are bare. Search each tree for knots. If you find any, prune the branches out, making the cut 6 inches (15 cm.) into healthy wood. If you find black knot on plum branches you cannot remove, scrape off the knots and the wood under it. Cut it away ½ inch into healthy wood.

Fungicides can help protect your plum trees, although they cannot cure a severe infection of black knot on plums. Use a protectant fungicide if your plum is among the more susceptible varieties like Stanley, Damson, Shropshire and Bluefre.

Spray the fungicide in spring when the buds begin to swell. Wait for a warm, rainy days when the tree leaves are wet for at least six hours. Reapply the fungicide every week during periods of heavy rain.

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Black knot

Black knot is a common fungal disease of Prunus trees including ornamental, edible, and native plum and cherry trees.

Hard swollen black galls (tumor like growths) form on branches and occasionally on trunks.

Many Prunus trees tolerate black knot. Tolerant trees have many galls throughout the tree with few negative effects on the health of the tree.

Some Prunus trees are more severely affected by black knot. In these trees, leaves and shoots wilt and die on branches with galls.

Management will vary depending on how severely the tree is affected by black knot.

Ministry of Agriculture, Food and Rural Affairs

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.

What is Black Knot Disease?

Black Knot Disease is the infestation of a fungus technically* known as Apiosporina morbosa that is common to plum and cherry trees as well as other fruit trees like apricot and peach trees. The fungus takes form in small green or brown swellings that, without treatment, turn into large, black “knots” that drain the life from infected limbs and twigs. Not only can this disease disrupt the growth of your trees, but it can also kill them completely.

Black Knot Disease is a chronic issue. It will continue to spread throughout your tree no matter what the season. The fungus lays dormant in the knots during the winter and spreads its pores during the spring months.

How to Doctor Black Knot

Like humans, trees need check ups to make sure they are healthy. If you or your tree-care professional notices Black Knot on one or more of your trees, it’s time to play doctor and remedy the issue. Here’s how in 3 simple steps:

  1. Cut away branches and stems that have any signs of the disease
    If you notice black growths on your plum or cherry trees the first thing you have to do is amputate the infected areas. This may seem like an extreme first step, but unlike humans trees have the ability to regenerate their limbs. Make sure you cut 2 to 4 inches below the growth so you are removing all of the infected wood.
  2. Burn or bury the cut-off branches/stems
    It’s important to make sure the infected limbs are properly laid to rest by either burying or cremating. These are the only two ways to insure the fungus cannot spread to other trees in the area.
  3. Use an appropriate fungicide
    While your tree is in remission, it’s important to use the proper fungicide to keep your tree Black Knot free in the future. Fungicides vary in their effectiveness depending on what region you’re in so it’s important to check with your tree care professional to find out which fungicide is right for your area. Also note that it’s imperative that you follow the label on your chosen fungicide and re-spray at the appropriate time intervals to keep your tree from relapsing.

If you suspect you have a tree with Block Knot disease, give us a call at (201) 444-0315, so we can work with you to start a treatment plan.

*If you want to check out more technical information on black knot fungus, its cycle, and its biology, you can here.

Watch the video: How to Prune Young Fruit Trees

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