Black knot in nurseries: Check for galls in March

Nursery growers who grow ornamental trees in the Prunus genus need to scout for galls in their nurseries and take appropriate management steps beginning in March.

March is the time for nursery growers to scout their field and container-grown Prunus species for the presence of black knot galls caused by the fungus Apiosporina morbosa, according to Michigan State University Extension. This fungi causes “knot-like” growths (Photo 1) on the branches and limbs of many ornamental trees such as flowering plum, sand cherry, flowering almond, black cherry, pin cherry and fruit-producing cherry, plum, apricot and peach trees.

Black knot galls
Photo 1. Black knot galls on Prunus Spp. Photo credit: Bill Shane, MSUE

The disease can be confused with crown gall, caused by the bacterium Rhizobium radiobacter (formerly Agrobacterium tumifaciens), which can occasionally cause tumor-like swellings on above ground plant parts. Black knot galls are generally darker and more abundant than those of crown gall.

The swollen galls (Photo 2) contain the spores that will be ejected in the spring when temperatures reach a minimum of 43 degrees Fahrenheit and are spread by moisture – either rainfall or irrigation. Thus, the galls should be pruned out of trees that may show these symptoms before the spread occurs. Scout your trees in the nursery and also check any wild Prunus species that may be on the perimeter of your tree blocks for the galls as they could serve as a source of inoculum.

Black knot gall close-up
Photo 2. Close-up of black knot galls. Photo credit: Joseph O’Brien, USDA
Forest Service, Bugwood.org

Pruning cuts should be made at least 4 to 6 inches below the visible swelling to insure the entire canker is removed. Disinfect pruning tools with either a 10 percent bleach solution or isopropyl alcohol between making cuts. Destroy pruned material so it does not serve as a source of reinfection in the nursery.

Trees being sold for fruit production can be treated with either chlorothalonil or thiophanate-methyl fungicides during active shoot extension in the spring, i.e., bud break until one to two weeks after pink bud to reduce the incidence of black knot, if it was seen in your nursery.

This fungus has a two-year lifecycle in which an infected branch may not show any swelling during the first year. The swelling becomes very evident during the second year.

For more information on commercial ornamental nursery production, contact Tom Dudek at 616-994-4542.

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