What is the best time to prune a sweet cherry tree, keeping in mind that it has a few spots of bacterial canker I would like to avoid spreading?
Warm dry weather is the best. I have sometimes painted over recent cuts when canker may be a problem, so you could always add that as insurance. While the opinion today is generally there is no need to paint over pruning wounds, I think there are a few exceptions, both for canker and fireblight susceptible plantings. I have seen too much canker/blight at pruning wounds.
Would a bordeaux mix painted lightly on the wounded surface of the cut pose any toxicity problems?
Scott, just because you get cankers on wounds is no proof at all that painting is helpful. It may just as well aggravate the problem, depending on the compound. If peach trees get less canker when in active growth it is probably because they have the most energy to wall off infection at that time. My guess is same would apply to cherries, but it is just a hunch.
I have very little canker issue with cherries here, and I don't know why. I do aggressive structural pruning when they are dormant just because that is the convenient time and come back during the spring and summer to head back new growth and encourage secondary branching and more small, fruit bearing wood. That is the trick with cherries- getting smaller wood quickly.
Hman, I don't have much data to go on other than cankers appearing at pruning wounds. When I take out a canker I will often paint it over in case I missed any. I generally do not put paint on other pruning cuts but maybe it would be a good idea if a disease is very active. I had many apple (un-painted) pruning cuts infected with fireblight late last fall, it caused a huge amount of damage in my orchard.
Noogy, I doubt the copper would cause any problems if not too strong but thats just a guess.
Scott, I believe if the paint was affective it would be well documented. There would be a very big market if it reduced FB damage and would be relatively easy to research. FB affects living tissue and I'm not sure the paint or compounds might not make tissue more vulnerable on the interface between compound and natural bark because it stops wounded tissue from being able to dry out as quickly. Fungus is the primary enemy of plant tissue and painted wounds may allow fungus to get a foothold, spreading beyond the wounds original edges and making these edges vulnerable to FB. It could work either way from my thinking.
It is interesting you have witnessed a lot of FB starting at wounds and all mine starts on small annual wood. The literature mostly talks about it originating on flowers.
You may have discovered another problem with close planting in the east coast. I summer prune during all weather conditions- but don't start until July for apples.