Recent phone and e-mail contacts tell me that Black Knot disease is showing up on flowering plums and prunes along the east coast. Got a call from a gentleman in Maryland today who had discoverd the disease on a minor branch of a Mt. St. Helens Plum in his front yard. He successfully removed the branch but wanted to know if he should write off the tree and replace it with something not as susceptible.
In plain terms, the answer is no. What this gardener experienced was a minor infestation (at this point). He got good advice from his local nursery, and was seeking a good fungicide (captan) as a follow up treatment.
Black knot is a fungal disease of plums and prunes in both agricultural and ornamental plantings. The disease causes black leisions on branches and contributes to an overall decline in vigor of the plant. The disease is spread by spores which attach to current year wood and which develop into the signature "Black Knot" leision in the same season.
The best way to stay ahead of this disease is to prune out affected branches in the fall when you can see the leisions clearly. Remove and destroy any pruned branches since the spore can arise from prunings, and will simply reinfest a tree if they are left around.
There are some resistant varieties of plum to consider, but for our gentleman in Maryland who would have to replace a 15 foot specimin tree in his front yard, the cost of replacement would be far more than the cost and effort of maintaining the existing tree.
Kudos to the local nursery who gave the right advice as far as pruning is concerned, and who also explained the proper disposal of pruned branches to avoid re-contaimination.
Black Knot Image courtesy of University of Illinois at Urbana Champaign - Thanks!
SG
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