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?