Black Knot

• Black knot disease is a serious infection caused by the fungus Apiosporina morbosa. It is characterized by black knots that start to form as green swellings on small twigs and branches and eventually form elongated growths that encircle and kill branches.
  
  While Santa Rosa plums are less susceptible to black knot than other varieties, they are not immune.
  
  The best way to control black knot is to plant trees away or upwind from old or infected orchards and to prune and destroy or bury infected branches as soon as signs of infection appear. Once black knot disease becomes well-established in a plum orchard, it is often impossible to eradicate.
  
  

Brown Rot

• Brown rot is another fungal infection affecting Santa Rosa plums. It appears mostly during periods of wet weather and is evidenced by clusters of brown-gray spores. Pruning your plum trees to open up the canopy and encourage air flow helps to prevent infection of this and other fungal diseases. Brown rot can survive through winter in mummified fruit, so remove any unharvested fruit from the orchard before or during the dormancy period.
  
  

Oak Root Rot

• Plum tree in blossom.plum-tree blossom image by Roman Ivaschenko from Fotolia.com
  
  Oak root rot is another fungal disease that can affect plum trees including Santa Rosa plums. The fungus survives underground for many years, so horticulturalists at the University of Florida extension service suggest that the best way to minimize the risk of this root disease is to plant your plum trees in an area that has been cleared of oak, hickory or other forest trees for at least 20 years.
  
  

Plum Leaf Scald

• The first signs of plum leaf scald are yellowing at the edge of leaves. Next, the leaf tips brown and fall away. The disease is carried by an insect known as the glassy-winged sharpshooter. Signs of the disease generally do not appear until the whole tree is infected. In fact, some trees at first appear greener and fuller than their healthy neighbors because the disease has the effect of shortening leaf nodes. Once infected the plum tree will eventually decline and die. Because few insects are needed to infect a tree, insecticidal control is not practical. Some growers introduce small wasps that feed on the glassy-winged sharpshooter's eggs in order to avoid infection.
  
  

Cultural Control of Plum Diseases

• Red plums.Violet plums. It is isolated on a white background. A close up. image by Andrey Khritin from Fotolia.com
  
  Santa Rosa plums, like all trees, resist disease best when growing conditions are otherwise ideal. Trees should be planted in well-drained soil and kept moist during periods of drought. Santa Rosa plums are a Japanese cultivar and are less cold-tolerant than their European cousins so should be avoided in colder climates.
  Inspect your plum trees often for signs of infection and remove infected growth promptly.