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Jim Chatfield received several calls this past week about whether or not hawthorn (Crataegus) is susceptible to black knot disease. The answer is no - unless this is something totally new under the sun. Black knot, caused by the fungus Apiosporina morbosa, occurs only on plants in the genus Prunus. This includes fruiting and ornamental types of plum, peach, apricot, almond and cherry. The fungus infects Prunus in spring and a knot-like swelling of twigs develops over the summer, hardening and blackening by fall. In some cases, this stem tissue is killed in the first season, but often the twig survives and a longer perennial knot-like canker develops. Prune out cankered areas as soon as they are evident to avoid serial infections over the years and serious health problems for the plant due to black knot. Jim suspects that what callers are seeing on the hawthorn twigs are swellings and cankered areas due to past infections from the cedar-quince rust fungus (Gymnosporangium clavipes). This fungus alternates between certain junipers and rosaceous hosts such as hawthorn, crabapple and quince. The most notable infections on hawthorn are on the "haw" fruits, and in some cases this becomes quite a nuisance with orangish spores produced in such quantities as to become a problem when tracked into the house onto rugs. Twigs also can become infected from this fungus and do develop spindly gall-like swollen areas, with tissue beyond the girdled twig typically dying back. It is generally not a big problem on the twigs and swollen areas can be pruned out.
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Last Updated ( Thursday, 24 April 2008 18:03 )
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BYGL 
