Bacterial wilt occurs primarily on cucumbers and melons, but also may be a problem on squash and pumpkins. The disease is caused by the bacterium Erwinia tracheiphila. Symptoms vary on the different host species, but typically the leaves turn a dull green color and a progressive wilting of lateral leaves occurs. The pathogen moves through the main stem, plugging the vascular tissue, and eventually causes wilting and death of entire plants.
The "string test" can be useful in identifying the disease. Start by cutting a wilting stem. Push the two cut ends together and slowly pull apart. If bacterial wilt is present, a string of bacterial ooze should appear, connecting the two cut ends as you slowly pull them apart. Another diagnostic technique is to place the cut end in a glass of water. If bacteria are present, they will ooze out into the water, causing a cloudy appearance near the cut end. Bacteria are microscopic so you must look closely. If the stem pieces are already dead, these techniques are not useful in diagnosing the disease.
The disease is closely associated with cucumber beetles. The bacteria overwinter in cucumber beetles. The beetles transmit the disease when they feed on young leaves in the spring. Once inside the plant, the bacteria multiply and spread rapidly. The disease is moved from plant to plant by beetles. The disease is usually first seen on the edges of plantings, where the beetles first land. Control of the disease is dependent upon control of the cucumber beetle. There are also varieties available with bacterial wilt resistance.
Keep in mind that other disease, insect, pesticide, or environmental factors can cause wilting of leaves and death of vines. An accurate diagnosis is important in order to select the appropriate control measures.
This article originally appeared in the August 22, 1997 issue, p. 129.