Cause Erwinia tracheiphila, a bacterium that overwinters in the bodies of the striped and 12-spotted cucumber beetles. In Oregon, the disease is on cucumbers occasionally. The bacterium affects plants only in the cucumber family such as cucumber, muskmelon, pumpkin, squash, white gourd, and a number of wild plants.
The relationship between the bacterium and the beetles is very close: if the beetles are eliminated, so is the bacterium. The bacterium hibernates in the digestive tract of a few beetles and in spring finds its way through the feces of the carriers to the young plant. It can enter plant tissue only through deep wounds such as those produced by feeding beetles or, later in the season, grasshoppers. Bacteria are not carried in or on seed.
When beetles chew on affected leaves, which they seem to prefer, their mouth parts are contaminated with bacteria, which they carry to the next few plants on which they feed. Infection occurs only if there is a film of water sufficient for the pathogen to reach the wound in the inner leaf tissue. The beetles do not migrate in rainy periods, and the disease rarely is found where annual rainfall is more than 45 or less than 18 inches. Optimum average daily temperatures for disease development are 52°F to 60°F.
Symptoms General wilting of the plant. Infection begins on one or a few leaves, which droop suddenly. This usually is associated with insect damage. The disease spreads from foliage down to the petioles and stems until the plant has wilted and died. Fruit also wilts and shrivels.
Cultural control Controlling carrier beetles is effective. See PNW Insect Management Handbook for details.