ATTRA Question of the Week

R.H.
Ohio

Answer: Thank you for requesting information from ATTRA on controlling black knot in your 30 acre organic plum and peach orchard.

Black knot is caused by a fungus called Apiosporina morbosa. It can infect wild cherry and plum trees, which can be sources by which it may come into your orchard. You may want to check around the border of your farm if you have any wild Prunus tree species, to see if they are infected. You will need to make sure all sources of inoculum are removed as soon as possible, or nothing you do in your orchard will help stop it.

To control black knot organically, you must 1) identify and remove sources of inoculum, 2) prune out the infected twigs and branches in your plum and peach trees 3) burn, bury or use a flail mower to shred the infected prunings and disk them in lightly.

When pruning the knots out, make your cuts 2-4 inches below the beginning of the knot – the fungus infects the wood outside the knot region throughout the growing season. If the knot is on non-prunable wood like a scaffold limb or the trunk, you can also cut the knot out down to the wood, and dig out about a ½ inch outside the knot edges as well.

If you can't find the source of inoculum for your orchard, or you think the problem is severe enough that it will reoccur, then we recommend spraying with an organically approved fungicide, like lime-sulfur, starting at budbreak. If you are going to use a fungicide, it is CRITICAL that you also prune the knots out and use proper sanitation. Otherwise you will be poisoning your soil for no reason. The most critical time to apply the fungicide is right before a rain event, and particularly when the temperatures are going to be above 55°F. The fungus opens it's asci during the first spring rain and release the ascospores into the air, which get carried by water and wind currents, to land on fresh, green wood. If it stays wet, this is the perfect environment for the ascospore to "germinate" and begin an infection into the wood.

Make sure you scout your orchard at least once a year for these knots; they will not be very noticeable until at least one to two years after the infection.

Please see the Fact Sheet from Cornell University extension enclosed for further details.

References:
Wilcox, Wayne. 1992. Black Knot of Plums. Tree Fruit Crops Disease Information Sheet No. 6. Cornell University Cooperative Extension. http://www.nysipm.cornell.edu/factsheets/treefruit/diseases/bk/bk.asp