I have a plum tree in my backyard which I don’t think has been pruned for years (I purchased the home 2 years ago). I’ve noticed black knots in the tree and this spring a brownish knot on the branches, quite a lot. I know it’s too late to cut them out, but is there a fungicide that would help the tree through this year and I will prune next winter/spring? The plums have already started to grow. Many thanks,
Plums and cherries get black knot. Early in the season when leaf buds start to break spray with lime sulfur. Later on spray with a sulfur spray and repeat at approximately 2 week intervals until late July. Observe the no spray interval before harvest as stated on the product label.
Cultural control: If you can remove and burn or give to the garbage, not recycling, all knots as soon as noticed as early pruning minimizes the spread. Make the cut at least 10cm (4 in) below the knot. This article suggests destroying all wild or neglected plum and cherry trees.