How to spot it

Hard, black, rough swellings on twigs and branches, like charred sausages. New knots are olive-green and soft in spring.

What it does

Branches beyond the knots weaken and die, and the tree declines over the years.

What to do now

• In winter, prune out knots 4 inches below the swelling, and bag or burn them.

Keep it away next time

• Remove wild cherries and plums with knots nearby.
• Check trees every winter and prune early.

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Plants it affects