POP TIPS: Black Knot on Cherry and Plum Trees
Late-stage black knot infections on cherry (image from Cornell University)
Close up of late stage black knot infection on plum (image from Michigan State University)
Black Knot identification, symptoms, and life cycle
Sometimes, in the springtime, cherry and plum trees may develop subtle, velvety olive-green swellings on their branches or twigs. If left unattended, these swellings turn into large, brittle, unsightly black galls that can kill the whole limb or even stunt the growth of the entire tree. These galls are caused by an infection by the fungus Dibotryon morbosum or Apiosporina morbosa and are commonly called black knot disease.
The fungus overwinters in the knot and may take a few seasons to display visible symptoms, but it generally worsens from year to year. Galls vary in length between just an inch to nearly a foot and many times do not completely encircle the branch. Those a year old or older may become covered with the pinkish white mold of another fungus and may become riddled with insects, especially lesser peach borers. Trees with multiple infections become dwarfed and misshapen, markedly reducing their productivity and attractiveness. It’s extremely important to identify and remove any knots in the orchard to keep the disease from infecting other trees, since spores of the fungus are discharged from tiny sacs in the surface of the knots. Spores are spread by rain and wind to new growth, so discharge and infection are greatest during wet periods, at temperatures ranging from 55 to 75°F. A few greenish, corky swellings may become visible the fall after infection occurs, but most will not be noticed until the following spring. Often times, winter is the easiest time to inspect trees for infection since there are no leaves to hide any potential knots.
(Diagram from Dr. Wayne Wilcox, Cornell University)
Black Knot management
When getting rid of knots, prune off infected limbs 6 to 12 inches below the knot. Disinfect pruners between cuts with rubbing alcohol or a 10% bleach solution (1 part bleach to 9 parts water) and remember to deeply bury or remove the prunings from the site! If possible, remove any wild plum or cherry trees nearby. For persistent infections, apply two sprays of lime-sulfur, 7 days apart, before the buds begin to grow in spring. Spraying can help to limit the spread of this disease, but this must be combined with conscientious removal of galls as soon as they are identified.
Infection rates also depend on the particular cultivar of fruit tree. Orchardists looking to start growing plums and cherries should consider avoiding highly susceptible cultivars such as Shropshire and Stanley. Some recommended plum varieties include AU-Cherry, AU-Producer, AU-Rosa, AU-Rubrum, President, and Crimson. Meanwhile, tart cherry varieties such as Evans Cherry are reported to be less susceptible than other cherries.
Early stages of black knot infections result only in small galls, but remove them as soon as possible to protect your trees from further damage! (Cornell)
Make sure to check out POP’s plum and cherry scouting guides to be prepared for any other potential issues.
This POP Blog Post prepared by 2019 POP Intern Piotr Wojcik.
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