| Black knot is a name used to describe a disease of
woody stems with black, knotty, outgrowths. It is
most widespread on plum and cherry trees, but also
infects apricots and peaches. Infection occurs in the
spring, but symptoms of the disease may not be
obvious until the following spring or even in
later seasons.
Symptoms and Diagnosis
Initially the bark of infected twigs and medium-sized
branches will begin to crack in the springtime. A
light yellow discoloration and cork-like swelling
occurs on infected areas. Knotted areas later turn an
olive-green color and finally turn black and harden
during the summer. The swelling weakens branches
and may stunt or kill smaller branches due to
decreased water and nutrient flow.
Life Cycle
Dibotryon morbosum is the fungus responsible for
causing black knot disease on cherries and plums.
Spores overwinter in the blackened "knots" of
infected branches. Disease spores are spread to new
hosts by wind during warm spring weather when
temperatures are between 55–75 degrees F and
there is rain or high humidity. Trees are weakened
because the knots disrupt the normal flow of water
and nutrients throughout the branches. Twig dieback
is common.
Integrated Pest Management Strategies
1. Plant resistant varieties. Plums such as
‘President’, ‘Santa Rosa’, or ‘Fellinberg’.
2. Prune and destroy all knotted branches in the
winter or before March 1. Prune 3–4 inches
below the knot to remove all fungal material. Dip
tools in a solution of 1–part bleach and 9–parts
water between cuts.
3. Remove any wild cherry and plum trees
surrounding your property. These trees may be
harboring black knot and it spreads easily to
cultivated varieties. Pruned material should be
disposed of or burned.
4. Apply a fungicide cover spray at bud break.
Pesticides registered for use include copper,
chlorothalonil (Daconil), mancozeb, and
thiophanate methyl (Cleary 3336). | 
