May 12, 2011 at 8:45 a.m.

'Black Knot' on fruit trees and shrubs

'Black Knot' on fruit trees and shrubs
'Black Knot' on fruit trees and shrubs

Some of the Chisago County Master Gardeners have noticed black knot on fruit trees and shrubs. Michelle Grabowski, U of MN Extension Educator writes that there have been some outbreaks of black knot on plum, cherry, and other fruit trees and ornamental shrubs.

Black knot is caused by a fungus that can be easily identified by black knot galls that stand out while the plant is dormant. It causes elongated, rough black swellings on twigs, branches and twigs. The growths begin velvety olive green then the following year turn hard, brittle, and coal black. Stems girdled by black knot will die outward from the growth. In spring, fungal spores are released from existing knots and blown in the wind to other trees. Trees will gradually weaken and die if effective control measures are not taken.

The damage done by black knot can vary greatly depending on the species, variety and intended use of the tree. Some ornamental trees can have a canopy full of galls and still produce a beautiful flower show in the spring and a healthy flush of foliage in the summer. In other trees, galls can distort twig growth and eventually girdle the twig, killing leaves and branches beyond the gall.

Multiple infections of this type eventually reduce the vigor of the tree and send the tree into decline. Occasionally black knot infestations can be found on the main trunk of the tree. These infections are swollen and covered with lumpy black gall-like growth and the often crack and ooze sap. Although the black knot fungus will not cause trunk decay itself, the crack formed by a trunk infection can provide an entry point for other wood rotting fungi.

Most infections occur between budbreak and two weeks after bloom if the weather is wet with 55 to 75 degree temperatures. A copper fungicide should be applied every two weeks from the time the leaf buds open until three weeks after flower petals fall. Fungicides prevent new infections but do not cure diseases already present. There are many formulations of copper fungicides. The one you use must be labeled for black knot fungus on your type of plant.

There are a few disease resistant fruit tree varieties, but unfortunately most are not hardy in Minnesota.

The last class in our Spring Series will be on May 3 at the North Branch Senior Center at 6:30 p.m. It will be on raising mushrooms, which is part two by Leslie Jo Meyerhoff. She did the first part at our Bonanza. We still have some bare root plants for sale. If you need any information about the classes or ordering plants, call our office at 277-0151, or you can call me at 651-257-4496 and leave a message.


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