Stewart’s disease of corn, or Stewart’s wilt, is caused by the Erwinia stewartii. Stewart’s disease is generally more destructive on sweet corn than popcorn or dent corn, but some hybrids and inbred lines of popcorn and field corn are very susceptible.
Signs and Symptoms
Symptoms of Stewart’s disease most commonly and initially appear as leaf lesions originating from flea beetle feeding scars. Lesions begin as pale-green to yellow streaks extending along leaf veins. Margins of the lesions become water-soaked and irregular or wavy. Lesions may extend the entire length of the leaf in susceptible lines or only 1-2 inches in resistant hybrids. Lesions range in width from 1/16 to ½ inch. As the disease progresses, the lesions turn necrotic and become dry and brown. Entire leaves may be blighted as lesions coalesce. In severe cases, yield can be significantly reduced and the diseased plants may become more susceptible to stalk and root rots.
Numerous insects can acquire, carry, and transmit Erwinia stewartti to corn plants. The beetle stages of all the root worm species and several flea beetle species have been shown to be vectors of the pathogen, however, the primary vector of the Stewart’s disease bacterium is the corn flea beetle, Chaetocnema pulicaria.
The corn flea beetle is the only vector in which the bacteria are able to overwinter. The bacteria survive low winter temperatures in the bodies of dormant, adult corn flea beetles. As the adult beetles emerge and feed in late spring and early summer, bacteria are deposited in feeding wounds and enter the vascular system of corn leaves. Newly hatched flea beetles feed on infected tissue and further spread the disease throughout the season.
Flea beetles are also blown into Iowa by southerly winds and are attracted to grassy weeds. The beetles move from grassy areas into adjacent corn plants, therefore, the incidence of Stewart’s disease is usually higher along grassy areas of fields.
Temperature and Flea Beetle Survival
Warm winter temperatures favor the survival of the beetle vectors and increase the risk of outbreaks of Stewart’s disease. A generally accepted method to determine the survival of overwintering adult flea beetles is to add the mean monthly temperatures for December, January, and February. If the sum of the average temperatures is 90°F or greater, survival of the beetles is favored and the potential for disease outbreaks is increased. If the sum of the mean temperatures is less than 80°F, most of the beetle vectors are killed and little disease can be expected. Heavy snow cover acts as insulation and may also favor the survival of the beetles.
The bacteria have been detected in the endosperm of seeds from severely diseased, systemically infected plants. Transmission of the bacteria from infested seed to young seedlings has been demonstrated in both field and greenhouse conditions, but this is extremely rare.
- Planting resistant varieties and hybrids is the most effective means of disease control.
- Winter temperatures can be used to forecast the survival of flea beetles and predict the risk of outbreaks of Stewart’s disease the following season.
- Insecticides can be used to control corn flea beetle populations but has not been shown to reduce the incidence and severity of the disease in susceptible lines.