Overview of bacterial wilt
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.
Signs and symptoms of bacterial wilt
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.
Disease cycle of bacterial wilt
The disease is caused by the bacterium Erwinia tracheiphila and is closely associated with cucumber beetles. The bacteria overwinter in cucumber beetles. Two species, striped, and spotted cucumber beetles, carry the bacterium from plant to plant, and infection often happens through beetle feeding wounds. 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.
Type of Sample Needed for Diagnosis and Confirmation
The Iowa State University Plant & Insect Diagnostic Clinic can help you to investigate and confirm if your plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each states diagnostic laboratory for U.S. residents can be located at the NPDN website. If you have a sample from outside of Iowa, please DO NOT submit it to the Plant & Insect Diagnostic Clinic without contacting us.
Management of bacterial wilt
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. Most growers rely on insecticides, but chemical warfare can require many applications per year, which is expensive and may also damage non-target insects, including the bees that pollinate cucurbits. For more information on insecticide use for commercial production see the Midwest Vegetable Production Guide for Commercial Growers guide available at https://store.extension.iastate.edu/Product/1774
The use of row covers can help suppress bacterial wilt transmission by excluding the insect vector at a critical time (10 days after anthesis). The efficacy depends on the beetle's population numbers.