The fungus, Botrytis cinerea, is widespread in nature and overwinters in plant debris which is the main source of primary inoculum in spring. Gray mold (Gray Mould) or Botrytis vine rot is most common in fields that have high humidity and poor air movement. The disease develops very slowly and becomes most visible late in the latter half of the season. Gray mold often becomes established in dying flowers in mid-season which then act as sources of further infection.
Gray mold attacks both plants and tubers and it is usually found on the older, lower leaves that are damaged or weakened by shading and old age. The fungus grows as a gray or brownish, non-glistening mold on infected leaves, petioles and stems under cool, humid conditions. It can be confused with late blight, but late blight fungus produces a white, glistening mold growth. Proper identification of this disease is important. In severe gray mold infections, all the leaves are blighted and a soft gray rot develops in the stems. Gray mold on tubers is rare and not apparent at digging but develops during storage and may be severe. The infected tuber surface becomes wrinkled and the underlying tissue is flabby, temporarily darkened and later becomes watery with brown decay.
Control
Fungicides used for late blight control do not always control gray mold. Use of protectant foliar fungicides (see Publication No. 1300A) will provide control of the disease. Avoid mechanical damage to foliage and tubers. Tuber rot problems can be reduced by eliminating soil adhering to the tubers, encouraging tuber skin maturation and storing potatoes at 4EC after a proper wound healing treatment.