Prune out infected twigs to keep plum trees disease-free
Helen Chesnut
Q As I began picking my prune plums, recently I noticed a few twigs encircled by corky swellings. What are they, and what should I do about them?
A: Rough swellings that develop on (mainly) plum and cherry trees signify the presence of black knot, a fungus disease. In spring, the knots form spores that spread to infect young growth in wet weather. Newly infected shoots form swellings that harden and darken through the summer.
Unfortunately, Italian prune plums are highly susceptible to the disease. It is recommended to inspect the trees in winter and prune out infected twigs and branches. I prefer not to prune in winter because cuts heal slowly at that time and exposing cuts on disease-prone trees to cold, wet weather seems unwise; however, if I did spot black knot in my tree in winter I would cut it out by late February, before the spring spore formation period.
So far, I’ve avoided having to do any emergency winter pruning on my prune plum tree because I check the tree thoroughly at this time of the year, as I work my way through the tree to pick the fruit. At the same time, I prune out any diseased growth I see. I always have hand pruners with me as a pick.
I watch for black knot, and make cuts around 10 cm (four inches) below any knots I find. I also look for shrivelled fruit covered in a greyish fuzz, indicating another common fungus disease of stone fruits — brown rot — and remove these infections by pruning off entire twigs bearing diseased fruit.
Twigs and fruit infected with black knot or brown rot should never be composted. Keeping a tree cleaned of infected fruit and twigs is the best prevention measure possible. It also helps to keep the tree pruned to be open shaped and uncrowded.