Hi All, I know Black Knot is a common problem with Plum trees but I'm wondering how everybody else manages this fungus. From what I've read online, the only way to deal with Black Knot is to prune it off during the latter part of winter & to dispose of the infected branches in garbage bags & throw them into the trash bin. During the last few years I've done just that but what I've noticed is that the Black Knot comes back in full force the next spring/summer. In essence, I'm pruning off much more than the tree can grow in a season & to me, this is a fight that the tree cannot win. Any insight/tips would be greatly appreciated. Thanks, Paul
I find Black Knot growths on my Italian Prune plum fairly regularly, but not in large amounts. I prune them off as soon as possible and have not had any difficulty keeping them under control. If you leave the fungus on the tree until it releases spores, that will allow it to spread to new locations. In principle, removing all of the Black Knot growths during the dormant season should prevent them from spreading, but perhaps you are missing some of them. The other possibility is that they are spreading to your tree from another location. Are there any other plum trees nearby, such as common flowering plums?
Unfortunately, there are other plum trees in the neighborhood which are likely affected by Black Knot as well. I suspect that this is the source of my continued infections. If this is the case, then should I still continue to prune all of the Black Knot that I see on my tree? If so, is there any risk of over pruning & killing the tree? Thanks, Paul
You should continue to remove all infected branches; otherwise, the number of infections will get worse. Over-pruning is not likely to kill the tree but could result in a tree with a rather ugly shape. You could try spraying with a fungicide during the late dormant season and at flowering, which is when spores are released. An old pest control booklet from the BC Department of Agriculture suggests spraying with lime sulphur at 100 mL per L of water at the dormant stage and additional sprays of lime sulphur at 20 mL per L of water at full bloom and at petal fall.
