Black Knot Fungus

• Black knot is a serious and common disease in plum that is caused by the fungus Dibotryon morbosum. The visible symptoms, usually presenting 6 to 12 months after infection, look like elongated swellings or soft greenish knots forming on branches and around the trunk. The knots eventually turn black and all growth around the bulges dies. During blossom time spores produced inside the swellings release profusely into the wind, and wet weather allows the infection to spread.
  
  

Plum Pox Virus (PPV)

• Plum Pox, spread by aphids and grafting infected stock, is found in stone fruit trees including plums and peaches. The variety, age and nutritional condition of the host along with the specific strain of PPV determine the severity of symptoms. Indications on leaves range from subtle yellowing veins to yellow or light green rings on the surface of the leaf, often combined with yellowing veins. The infected fruit develop rings or line patterns of a lighter color than healthy fruit and black or brown dead spots appear to ripen.
  
  

Brown Rot Fungus

• A sick tree is visibly distressed.Hemera Technologies/Photos.com/Getty Images
  
  Brown rot is a fungus that develops when the flowers are wet for five or more hours, especially at temperatures between 60 and 70 degrees Fahrenheit. It causes the flowers to brown, wither and die. Any plums that do mature develop progressively larger brown spots covered with tan-colored spores. When the flowers do not fall off, the infection reaches into the limbs, develops cankers and quickly spreads to the rest of the tree.
  
  

Plum Pockets

• Several Taphrina fungi contaminate plum and cherry trees; the most obvious symptom is fruit swelling, ranging from a sickly yellow to a bright red, to as much as 10 times the size of a healthy fruit. Wild and domestic plum cultivars display deformed shoots and curled leaves developing. Infected fruits, shoots and leaves mold and turn black by late summer. Spores in small crevices remain inactive until the following spring when infected parts mature and scatter in the wind.