Black knot fungus
Focus on sanitation. You need to trim out the fungus.
Spraying fungicides is of secondary importance and usually only done under severe situations. Lime sulfur is usually recommended. Another option is Bordeaux mix (copper sulfate).
Spray now, before growth begins.
Fighting black knot is usually a losing battle, especially in areas where wild plums may be around. Focus on pruning. Pruning. Pruning.
- Tom Kalb