Whether to leave apple tree pruning cuts open or coat with something?
Hi- I live in heavy fireblight country. I need to prune off a one inch diameter limb at the trunk due to a blight canker two feet out on the limb. Should I leave the pruning cut open to the air (and blight) or paint it with something like Doc Farwell's Pruning Compound? I know for pruning in general, the practice changed from painting (long ago) to leaving open (more recent.) Thanks. Steve
You are correct that painting pruning cuts is no longer recommended.
We do suggest that you prune now, but given our weather there is no guarantee that the pathogen hasn't already moved towards the trunk.
The 'ugly stub' method of pruning is helpful for difficulties with fireblight.
More on that here:
Does fireblight always discolor cambium to orange or brown?The cambium in all my "blight?" cankers is beautiful bright green. Various friends tell me that can't be fireblight, maybe an apple canker like Nectaria or Botryosphaeria. It sure looks like fireblight. Can you help clarify? Thanks. Steve
It is difficult to tell from the images which infectious agent caused these cankers and dieback. For the future, it is best not to do a cambium inspection on the trunk (as this appears to be) as this type of wounding on such a vulnerable site could open up the possibility for further infection. (On a smaller branch, at least, if this were to happen, the branch could be pruned off whereas the same can't be said for a trunk.) Green cambium is generally healthy cambium, yes, but this "scratch test" could be just outside of a diseased area. Plants will try to compartmentalize diseased wood, where healthy tissue dies immediately outside the infection zone in order to stop the spread of the pathogen (which usually requires living wood to spread). The cut end of the wood appears to have borer damage visible - it isn't perfectly clear and we can't tell how old it may be - in the heartwood. Although heartwood damage is less critical due to its natural state of being dead wood, to get to that location, the insect would have had to have bored through healthy cambium to reach that spot. Outside of the immediate damage from this, the wounding also exposes the tissues to possible infection. It is possible your tree is suffering from fire blight in addition to another disease either simultaneously or in succession. Typically, stressed or wounded plants are much more likely to succumb to infection or infestation over healthy, vigorous plants, and it is not uncommon for more than one ailment to strike at once.
We would still recommend pruning out all damaged and diseased wood, though if the cankers pictured are on the trunk, there isn't much that can be done except to wait and see if the tree recuperates. Damaged tissues don't heal in the same way that animal tissues heal, but they can compartmentalize damage and cover cuts with callous tissue if the callous-forming tissues aren't damaged.
The Univ. of California has a page on Nectria diseases (http://ipm.ucanr.edu/PMG/GARDEN/PLANTS/DISEASES/nectriacanker.html) as well as Penn State (https://extension.psu.edu/nectria-canker). If you have not seen the characteristic spores, it's less likely this is the causal agent. Ohio State's page on Nectria (https://ohioline.osu.edu/factsheet/plpath-fru-42) does suggest that such infections commonly follow fire blight damage, but the description of the cankers sounds different enough that this too may not be the culprit. Unfortunately, canker diseases tend to have similar surface wounds due to the nature of canker-forming diseases. Nonetheless, at this stage, maintaining good tree health is the best you can do in the hopes that the tree is able to recover and halt the spread of infection.
I would still like an answer to my main question:
Does fireblight always- usually- never- discolor the cambium?
My cambium is bright green everywhere I checked.
Fire blight - as with essentially any canker - will discolor cambium, yes, as any unhealthy cambium is discolored. Discoloration in diseased tissue tends to occur mainly on the leading edge of the infection only, as behind this the wood has already died or succumbed to a secondary infection, which can muddle attempts at nailing-down a diagnosis of the primary agent. If your cambium is green, you may be sampling spots that are too far ahead of the progression of the infection. There is clearly a cankering agent at work in the tissues; the difficulty without spores visible is discerning which agent caused them first.
As they point out in Penn State's fire blight posting (https://extension.psu.edu/apple-and-pear-disease-fire-blight-dormant-removal-of-cankers), bacterial diseases are inactive in winter, so changes to the cambium may not be apparent until spring. You can re-check the edges of the cankers in spring if any haven't yet been removed (or new ones appear) to see if there is a progression towards browning, blackening, or a rosy color. Any color other than green, however, tends to be a sign of a serious problem that may lead to the death of the branch (or, if on the trunk, entire tree).
Thanks- that was very helpful. I found bright green cambium inside the cankers. No discolored cambium anywhere.
Have to decide whether to saw the trunk off below all the cankers as they cover at least 50% of the circumference of the trunk and some of them are not sharply defined. If I don't cut and just wait to see if the tree heals, the cankers can ooze this spring and infect all the other orchard trees.
What a dilemna.