Question
I have an ornamental flowering plum tree in my front yard. Last year it contracted a disease called BLACK KNOT. I removed all the limbs that had the black galls on it. Is there any way I can avoid this from happening again. Somebody told me to put garlic cloves in the soil around the base of the tree. Will this hurt the tree or help it ?

Answer
It would do no good for the black knot. Black knot is spread from infected nearby tree by spore carried with the wind in the spring. Here are the normal control and prevention methods for this disease.

Control

A. Destroy Sources of Infection

  1. During fall and winter, or before new growth starts in the
      spring, prune out and destroy all visible knots.  On large, main
      branches and trunks, knots should be cut out with a knife or
      chisel. One inch of healthy bark around the knot should also
      be removed.

B. Obtain Disease-Free Stock

   1. Never purchase nursery stock showing visible knots or
       abnormal swellings on the twigs and branches.

   2. When pruning branches with knots, the cut should be made
       at least 2-4 inches below the lowest part of the knot.

   3. Because spores can develop and spread from knots left on
       the ground or in brush piles for several weeks after removal
       from the tree, they should be burned or buried.

C. Chemical.  Fungicide sprays applied in a timely, regular manner can prevent most new infections. Use Benomyl (Benlate 50% wettable powder) at the rate of 1/2 tablespoon/gallon of water. Spray schedule:

   1. Apply first spray in the spring just as green tissue begins to
       appear.

   2. Again just before and after bloom.

   3. Spray at 2-week intervals until terminal growth stops, usually
       early to mid-June.