At this time of year with leaves off the trees, we often see black masses or galls attached to outer twigs of plum and cherry trees in the genus Prunus. Both ornamental and edible varieties are susceptible to this fungus, along with the native chokecherry. Sometimes apricot and peach trees are impacted. This disease produces rough, irregular, black growths that encircle and eventually kill the twig or branch. Insects take advantage of the growth, drill into it to seek shelter and may cause other problems.
The disease cycle starts in spring when temperatures are above 60 degrees. At that time, disease spores emerge from existing black knots and travel to other parts on the tree or to nearby hosts. When it is wet, rainy and cool, the probability of these conditions are even greater. Spores can germinate on a thin film of moisture. They may not show up as visible black knots until the following season.
What to do?
- Wait for a dry day to prune out infested parts. Bag them and put in the trash. Clean pruning tools with a spray of bleach (1/2 cup to 1 gallon of water) between cuts. We have even cut a prune plum down to a framework of low branches, and it is resprouting with healthy new growth. In some cases, however, removal of an entire infested tree and its stump may be the best solution.
- Be vigilant – check trees frequently and prune out the black knot as soon as it is spotted, using the procedures above. Do not let it get a toehold!
- Choose disease-resistant varieties of apricot, peach, plum and cherry to head this fungus off at the pass. Some recommended varieties are not cold hardy in our region (see link below).
- Work to build soil biology and balance nutrients in the soil around the trees via soil testing, foliar and root feeding with compost tea, and granular soil amending.
- Spraying is best done in the dormant season before blossom and leaf emergence. Unfortunately, spraying cannot rescue a tree that already has black knot, but can help stem the development of new spores. In an organic program, Neem Oil, Liquid Copper, and Lime Sulfur are the materials of choice and are sprayed in late winter as a preventative.
- Consider eliminating these trees from your property once they are infested, in favor of the native chokecherry at the perimeter, a host plant for numerous native caterpillars and the birds who eat them! It may be disappointing to forego harvesting your own cherries and plums, but these items are readily available at the grocery store or farm stand in season.
For more information, here is an excellent fact sheet from Ohio State University about black knot: ohioline.osu.edu