Dealing with severe black knot

I have had mild black knot for years. Control was no big deal: find a knot, prune it out. But late last fall I noticed a ton of black knot in my plums. One gage plum was completely covered and I had to take it out. Many other plums had numerous strikes. The particular thing I am noticing in this attack is lots of the smallest limbs have it, usually the strikes are on larger limbs. I didn’t prune out all of those last year because I was not sure they were full infections as all I noticed was slightly swelled limbs, but its pretty clear now that they need to be removed. So, I am hacking away at them.

Has anyone dealt with such a bad infection before? Any ideas on where it came from? I usually am removing all the knots so either I missed some or there is a wild tree nearby causing all my problems. I have two plum stands that are 100’ apart and both had big problems, so it seems it could also be related to some perfect weather for the spread. I have not done any spraying beyond my usual dormant copper a few weeks ago, but I am wondering if I should spray some copper or something else on it. The cycle of the disease is it takes two years to make the spores for more infection, so if you get it all 100% pruned out you should be able to break the cycle.

Scott,

I’ve used a little bit of chlorothalonil on plums for black knot. Bravo Weather Stik is labeled for it.

I do think I will spray something now, spring is the period for infection. It could be there is some knotted prunus in the neighborhood oozing spores. Maybe I will also go on a scouting mission. I have gone through my own orchard very thoroughly and I have found no 2nd year infections, I don’t know where the original source is coming from.

The saddest part is I have half a dozen different Gage plums fruiting a lot for the first time. I finally figured out how to prune them for the low light / tight spacing they are in. But I am now removing most of the fruit because there is a knot between the trunk and the fruit.

I don’t know what your neighborhood is like, but you probably know there are a number of species that can host black knot. Around us choke cherry and wild black cherry are the biggest feral sources. The black cherry seem less affected, but they can be so tall that. I suspect they cast a much wider “net”.

I think like many diseases, sometimes everything lines up just right with weather, spores, and minor injury to trees.
My understanding is that it can only infect young twigs, and sometimes injury sites. I believe if your seeing it on older wood you have had infection running since it was young.
Good luck with it

I decided to put down some chlorothalonil. I haven’t used that stuff in years, I hope my bottle is still good. I am a few days late on the application limit of shuck split on some trees, but close enough I figure.

All of the knots I found on my own trees are fresh infections from last year, I did a very thorough inspection. So even if I missed a few I expect they are not infecting this year; either I pruned out the bad guys last winter (I did prune out a number of knots then) or it came in from a neighboring tree. I have a few non-fruiting prunus trees and they are all clean from what I can tell. Rob, the infections are not only on young wood, the bases of small shoots on larger wood were often infected with fresh knot. If there is no shoot at all (largest wood) then there is no new infection generally.

For people growing unusual plums I thought I would summarize the sensitivity I saw. There were enough spores and enough strikes that I got some really good data.

Scary susceptible:

  • Sweet Treat Pluerry - tons of strikes and they spread up and down the shoots very fast.

Highly susceptible:

  • All gage plums. The worst was the new gage plum, Rosy Gage, which I had to remove.

Susceptible:

  • Santa Rosa / Weeping Santa Rosa
  • Flavor Supreme
  • Mariposa
  • Laroda
  • Elephant Heart
  • Earli Magic
  • Valor
  • Mirabelle de Metz
  • Emerald Beaut

Mildy susceptible:

  • Purple Heart
  • AU Roadside
  • Coe’s Golden Drop
  • Jefferson
  • Middleburg

Resistant (few or no strikes)

  • Satsuma (yet another reason why this is the best all-around Japanese plum for eastern growers)
  • French Prune (ditto for probably the best Euro plum)
  • AU Producer
  • Beauty
  • Spring Satin
  • Lavinia
  • Shiro
  • Reine des Mirabelles
  • all other stone fruits (sweet and sour cherry, peach, apricot - no strikes on any of these)
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Shiro and Satsuma have exact same susceptibility in my nursery and elsewhere as most other plums I grow. I get a pretty good chance to evaluate the plums I grow because I spread my nursery trees in various spots as far as eastern exposure and I have about 30 trees each of those two varietes. I also manage about a score of Shiros at that many different sites. Satsumas are newer for me so I’m managing half as many on smaller trees in various orchards.

As I have stated often, the only extreme standout in terms of vulnerability here is Methely.

ACN claims Bluebird is resistant and I know it to be a very good E. plum of decent precocity. I’m thinking of using it as a base plum and grafting other varieties on it as a nursery staple if the one in my orchard continues to be BK free.

I have rescued trees in which every leafing branch is consumed by blacknot by butchering the existing smallish wood and regrowing the tree. For trees to get as bad as you mention has always required a certain level of negligence from my experience, so if you were keeping your eyes on things I don’t know how things got so far.

I have my helper carefully go through my entire nursery and orchard at least 3 times a year in addition to my removing it whenever I see it. There was more on the trees when I was pruning this spring than should have been but I believe they were galls he missed. It wasn’t spread all over the tiny wood but on trunks.

As the literature led me to expect, chlora hasn’t been very useful as a tool for suppressing BK here.

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Thats good to keep in mind. I suppose I could have done that with my Rosy Gage. It is now starting all over from the trunk.

I know that at dormant pruning time the previous winter I was more or less knot-free. At pruning time I remove every knot, and the trees are all pedestrian height so its not hard to catch them. In this latest round I got most of the infections at the base of small shoots - it is a different profile than the usual knots I was getting. So I am inclined to believe I had some perfect storm of black knot circumstances.

Sounds like it.

Alan,

Do you just rely on pruning to control black knot?

I spray a little chlorothalonil on the plums and they also get some Captan early season to keep rot down (which is also supposed to help control black knot).

I’m inclined to attribute this to the fungicide because we get very wet springs here and I’ve not seen any black knot on cherries or plums, including Rosy Gage which is a full sized tree.

Do you have native cherries nearby, full of galls. If not you probably don’t have a source of inoculum. I used to spray chlor at one site with a particularly bad problem with it and it made no appreciable difference. Not very scientific, I know, but Cornell says it only provides a bit of control and pruning them out is what must be done and is what I do- again and again.

Choke cherries are our #1 pioneer tree species. Land that was cleared and let go tends to sprout them like crazy- they also dominate hedge rows.

Choke cherries are native to this area, but I haven’t paid enough attention to wild tree/shrub flora to say how prevalent they are. My wife is better at wild plant identification and says she’s seen them around here, but probably not a lot close to the backyard orchard because there’s not much wild vegetation close to the house (pretty much lawns and fields).

The farm has a lot more wild vegetation close by. Perhaps in time I will see infections there due to the heavier pressure you mention.

I have been looking around my house for wild sources and have come up empty so far. The neighbor used to have a plum tree and I thought that was the source perhaps, but its now just a stump. I don’t see chokecherry around here and we are out of the normal range of it. Its a mystery…

It is interesting how I can still find an occasional gall. Now I am finding a few in the plums that are the most resistant. Since they looked so much better than their neighbors I was not scanning them as thoroughly in the original scans.

Yes, it is. But I think it can be self sustaining in an orchard once it has a foothold. You might have received wood or a tree with inoculum. I know I have. Takes a while for the galls to develop, as you know.

It is still surprising that it became such a problem in your orchard when you are so observant and attentive.

Most of my problem sites do have a lot of choke cherry on the edges.

When I first started my business I remember a commercial grower telling me that a lot of them that happen to grow plums consider them something you replant frequently because controlling BK by pruning out the galls is so labor intensive. You just cut them down when they become too diseased to be useful and replant somewhere else. At least it doesn’t infect the fruit.

Like Scott, I thought I had black know controlled at a low level. I’ve pruned out a handful of strikes each winter for the last couple years. Then, today, when doing a few last grafts and thinning fruit, I found 15-20 strikes. I think most were new, 1 yr olds (not the 2 yr old black ones).

The trees with the most were Valor, Flavor Supreme (both on Scott’s list of susceptibles), Vision, and Satsuma.

I also had a particularly vexing one on Leonie, a Euro I planted last spring. It must have been infected within moments of planting. The knot was at the first large branch, right up against the trunk. I used a saw to cut it off, without topping the tree. But I left it a bit of a mess and it remains to be seen how it heals.

Most of my trees are also multi-grafts. And a downside of multi-grafting plums is what happens with the knot is between the graft and the trunk. Here’s one on the Valor where I used pruners to excise the know. But, I definitely didn’t cut off the recommended buffer.

You can just make out the first cut I made with the pruners on the right, before deciding to take a pic:

After removal:

Later in the day, I was spraying anyway, so I made sure to hit the wound. But, I’m not sure than Immunox does anything for black knot. I checked and it isn’t listed in the specs.

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I used bleach on the big knot I cut off the trunk last year, and I haven’t seen it return

Yet

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We had good knot conditions here as well, I also had more knots than usual this spring. Spraying copper or cholorothalonil on them after pruning is a vey good idea, but I confess I almost never do it - I prune knots right when I see them and I don’t have spray in hand. I have a miniature blowtorch I could keep on my belt, that might be a good thing to have handy.

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Hello, we had a black knot outbreak 10 or more years ago, thousands of strikes, many hours spent cutting them out, and a couple of years before we got back to the background level.

I blamed it on Hurricane Floyd which blew thru here the summer before damaging the branches allowing massive infection. We have tons of choke cherries in the woods and tree lines which have knots on them so we have a constant source of infection. I used to try and cut them back but there are just to many of them.

I read an article a while back about apple orchards in Nova Scotia having severe fire blight problems after a hurricane damaged the branches and blew spores everywhere.

I use chorothalonil for the spring fungus sprays but do not think it is very effective for black knot.

We just prune it out, and if it is on a major branch or trunk, use a hatchet to cut it off, leaves a big scar, but the trees handle it.

Eric

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If a good pruning saw can’t handle the task, I use my small chain saw- but this is very rare and only on big, old trees.

Some sites I manage have such bad black knot that even apricots get it.

Found two black knot areas on my Shiro, one of them was right at the graft union of Lavina which is fruiting for the first time.

I did like you did, cut out a chunk of affected wood, probably not enough. It was an attempt to keep that fruiting Lavina alive so I could try the fruit.

I planted one Methley plum to pollinate my other plum trees but apparently they are black knot magnets. As I make my rounds through my orchard I scan the limbs for problem areas. My last trip I spotted two knots about 1/2" in size. These removed fairly easy but I’m wondering how big of a problem this tree will be going forward. I suspect that the key for controlling BK is early detection.