9/12/11 BLACK KNOT OF PLUM

ORNAMENTAL PLUM WITH BLACK KNOT DISEASE

EPICORMIC GROWTH

YOUNG INFECTED BRANCHES

THE "BLACK KNOT"

WHITEWASHED CHERRY TRUNK

Monday, 8:00 AM.  63 degrees, wind SW, calm.  The barometer still predicts rain but it is evidently a very weak low and the humidity is only 47%.  We sure could use rain, though.
    Black knot of plum and cherries is a very serious fungal disease of species in the  genus Prunus, which infects both wild and domestic cherries and plums.  the first symptom is a swelling at the base of the petioles of infected trees and shrubs, and over a period of years it usually infects the entire plant, eventually exhibiting the “black knots” on stems, branches and trunks that gives the disease its name.  From a distance, leaves will appear wilted and the plant un-thrifty, and a severely infected tree will have a lot of “epicormic growth,” heavy suckering from the trunk at ground level.
    Plums and cherries are relatively short-lived, and if planted for fruit or ornament one must not expect them to be long in the landscape. They are very thin barked, and sun and frost damage often occurs.  Orchardists often whitewash cherry trunks to reflect the late winter sun and prevent sun scald and frost splitting of the trunks. I never use cherries or plums, ornamental or fruiting, as a focal point or anchor plant for these reasons.
    Once infected with black knot, constant pruning out of infected material (along with sterilization of the pruning tools after each cut) is needed to slow the progress of the disease.  Spraying with an appropriate fungicide may help slow progress of the disease, but I think it is essentially a death sentence.  Diseased plant material should be burned or at least properly composted or the disease will spread. Never plant another cherry or plum in the same location as an infected plant.
    Flowering cherry trees, purple-leafed plums, sand cherries and edible cherries and plums are certainly worth growing for aesthetics  or edible fruit, but be aware of their problems and limitations before you invest a lot of time, money and energy in them.  As with all living things, overcrowding and proximity to sources of infection will lead to greater incidence and severity of the disease.
    And, thanks to all who have wished me well after my recent medical emergency!