My understanding of black knot is that the black turds spread spores during wet periods, and establish themselves either on young shoots, or on injuries. In most of the articles on control, they suggest pruning in fall, or at least before spring, and to sterilize the pruners between each cut. The nominal notion behind this is that if you cut too close to the knot, then your snips are potentially contaminated by mycelium in the wood, and that this mycelium is transmitted to the cut on the next branch. I can see this as a scenario if doing summer pruning. Is this likely during the leafless season? There is little sap in the wood. The mycelium has to stay viable at cold temperatures, subject to drying and sunlight. Possible experiment: * Find a wild choke cherry infected with black knot. * Make one snip through the black knot. Surely this will infect the snips. * Now proceed to give that chokecherry a hard pruning giving it 20 snips using the usual 6" below any existing knots, and filling out the numbers with general hard pruning. * Flag the bush so you can find it the following year, and count the number of knots. Note that young infections aren't very obvious. It doesn't develop the turd look until the after the first winter. * Check again in the second summer. Can someone point to research that shows that pruning tools are effective infective instruments during winter dormancy?
Hi @sgbotsford - Although Black Knot / Apiosporina is an ascomycete fungi, I think your post looking for prevention/treatment strategies might get more relevant responses in the Cherry Forum? @wcutler - What would you advise?
I posted here, because I want to verify how the transmission works. There is a lot of horticulture practice that is truely lore: It's done because it's always been done that way. I also suspect that practices that are necessary for summer pruning may not be needed in dormant season pruning, but since the more stringent practices work in the off season, that's what everyone recommends. Moderator: I would suggest leaving it here for a few days to see if it gets any attention, and let me know if I should repost to the prunus forum.
Here is fine for now -- we can perhaps page people in who don't visit this forum often but do a lot of fruit tree care (maybe @vitog ?) I wonder about the lore aspect as well -- particularly as you mentioned with respect to dormant season pruning. I think one might need to know the lifetime of the black knot fungi spores.
I have no idea how transmissible Black Knot is during the dormant season; I always remove diseased portions of the tree as soon as I notice them. I just cut off a plum branch with Black Knot a few days ago. I've been doing this for over 40 years with no signs of the disease's spreading and have never sterilized the pruning tool. I think that the probability of spreading is overblown, but perhaps I've just been lucky. Black Knot only appears on the Italian Prune portion of my grafted plum tree about once a year, and I never see it on my other 8 fruit trees.
