Are you taking more time getting from one place to the next rather than making a beeline to the warmth of shelter? There is a lot to see outside this time of year. Black knot is one of the curiosities that some people are taking notice of lately. Several examples of this disease were submitted to the ISU Plant Disease Clinic over the past few weeks.
Black knot is a widespread fungal disease caused by Apiosporina morbosa (another name for the fungus is Dibotryon morbosum) that occurs on wild and cultivated plums and cherries. It is one of the few diseases that can be diagnosed over the phone because the symptoms are easily recognized. Swollen black knots (galls), longer than wide, elongate up to a foot along branches, twigs, and/or the main trunk. Sometimes the galls will have a whitish powdery coating. Fungal spores are produced on galls that are at least one year old. Young succulent twigs are the most likely candidates for new infections. Newly formed knots are greenish and soft, but blacken with age
Good sanitation through gall removal is the primary control measure. Pruning, preferably during dormancy, should be done at least 2-3 inches below the swellings, since the fungus may extend beyond the visible portion of the black knot. Sanitation usually keeps the disease in check. Fungicides may aid in disease control, but are not typically necessary. Fungicides used without good sanitation will not control the disease. Diseased wild plums or cherries nearby, such as in wooded areas or fence rows can be a spore source, so keep an eye on them, too.
If you have no cherries or plums, take heart, this article still may be of use to you! Chances are that one-day you will unexpectedly happen upon this peculiar formation and you will know exactly what it is. Black knot.
This article originally appeared in the April 20, 2001 issue, p. 44.