POP TIPS: Black Knot on Cherry and Plum Trees

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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.

Additional resources

This POP Blog Post prepared by 2019 POP Intern Piotr Wojcik.  

SUPPORT US!  If you found this entry useful, informative, or inspiring, please consider a donation of any size to help POP in planting and supporting community orchards in Philadelphia: phillyorchards.org/donate.

POP Partner Feature: Monumental Baptist Church

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Volunteers clear the fence adjacent to the new orchard. Many hands make light work!

Over the last five years, Monumental Baptist Church in West Philadelphia has been steadily greening its churchyard. What started out as a large unused garden plot behind the church and two neighboring row homes has now become the Community Garden Project (CGP), a prolific teaching garden and orchard, serving the congregation’s 300 members and its surrounding community by not only providing healthy food but also teaching children and adults the nutritional value of growing fruits and vegetables, preparing healthy meals, recycling and reusing water and natural fertilizers. CGP also uses its program to teach service-learning skills and foster accountability and teamwork. 

The first year, Mecky Pohlschroder, a 19-year member of the church, and a few volunteers began on their own, removing debris, pulling up overgrown weeds, and sowing seeds for what would become plentiful harvests and Sunday morning distribution by the youth tending the space. The second year, the church joined PHS’ City Harvest program – one year, growing over 500 pounds of produce! 

In November 2016, MBC welcomed POP and volunteers for the first planting of fruit trees and shrubs at the site, including blueberries, cherries, peaches, and more.  On July 15th, 2017, POP will help MBC sheetmulch the orchard space in preparation for additional understory plantings.  Hundreds of perennial herbs, flowers, and groundcovers will be planted in the fall to complete the food forest.

Children ready the ground for planting fruit trees in fall 2016.

“It was one of my absolute most cherished days when POP came to start the orchard,” Pohlschroder recalled. “It was just after the election and there was so much sadness, so many emotions, but all these volunteers came out and it was just an awesome day of everyone working so beautifully together. It was filled with love.”

In addition to working with the MBC youth, CGP was extended to the community through after-school programs bringing together immigrant and local students teaching them social and academic skills through gardening. Pohlschroder describes the garden as a place where students’ wonder is cultivated. “The kids really love it,” she said. “They learn it all – from A to Z. They pull out the carrots, wash them, and eat them right there. They’re amazed. Our seniors love it, too. Many of them grew up in the South and had lived on farms, so it’s something that brings back a lot of memories.”

The site houses four garden beds in the church’s front yard, and five in the back, which produce crops such as kale, collard greens, peas, carrots, eggplant, tomatoes, and okra. Since planting the orchard, Pohlschroder says she’s hopeful that it will encourage more church members to spend time in the backyard connecting with community, now that there’s nice trees, with ample shade and room for the kids to play.

As a church, the garden and orchard are also a place where the Bible comes alive — a huge part of it being about working together and sharing the harvest, she says. “Jesus uses many parables in the Bible, from the story of cultivating faith as small as a mustard seed to references of fig trees. It really makes a lot of sense if you can actually see things grow.”

Monica Morgan (pictured here) and church members greet the new trees with joy and anticipation of future harvests!

More than anything, the garden and orchard at MBC is a place of inspiration. It’s a space where children learn to grow, where cookouts unite community members through shared food and cultural connection, and a place that nurtures the visioning of future gardens. One grandmother and granddaughter have begun planting their garden after seeing the church’s – learning about building beds and collecting compost from the Fairmount Park Recycling Center. “It’s amazing to see what sparks are ignited.”

This POP Partner Feature written by Education Director Alyssa Schimmel.

SUPPORT US!  If you found this entry useful, informative, or inspiring, please consider a donation of any size to help POP in planting and supporting community orchards in Philadelphia: phillyorchards.org/donate.