Stewart's wilt is a bacterial disease of corn caused by the bacterium Pantoea stewartii. The disease is also known as bacterial wilt or bacterial leaf blight and has been shown to be quite problematic in sweet corn. The causal organism is a facultatively anaerobic, gram-negative, rod-shaped bacterium. The disease is endemic in the mid-Atlantic and Ohio River Valley regions and in the southern portion of the Corn Belt. Stewart's Wilt causes minor reductions in field corn yield, despite common occurrence, because most hybrids grown in the Midwest have adequate resistance. However, the disease can be problematic in seed production because many countries have restrictions on maize seed from areas where the Stewart's Wilt occurs.
Stewart's wilt affects plants, particularly types of maize such as sweet, flint, dent, flour, and popcorn. Sweet corn and popcorn cultivars are more susceptible to Stewart's wilt than field (dent) corn, but some dent corn inbreds and hybrids are susceptible. The production of virulence factors can be caused by the communication system between the bacteria known as quorum sensing. Stewart's wilt causes yield reductions by decreasing the size of corn stand or by limiting its production, resulting in fewer and smaller ears of corn.
Disease Cycle and Symptoms
Chaetocnema pulicaria, the primary vector for P. stewartii, overwinters as adults and begins feeding on corn seedlings early in the spring. The bacterium overwinters in the gut of the adult corn flea beetles. Warmer winter temperatures allow for greater beetle survival and in effect, higher populations in the spring. Emerging beetles in the spring transmit the bacteria into corn leaf tissue through feeding wounds. The corn flea beetles wound the leaf and contaminate the wounds with insect frass, which additionally contains the bacteria. When large populations of corn flea beetles are feeding, skeletonization of leaves and death of seedlings can occur.
Stewart's wilt has two phases of symptoms: the wilt phase and the leaf blight phase. The wilt phase occurs when plants are infected during early vegetative growth stages and become systematically infected. The degree of multiplication of the bacteria is highly dependent on susceptibility of the cultivar. In most cases, the wilt phase occurs on seedlings, but for certain corn types (i.e. sweet corn), more mature plants can wilt. The wilt phase is systemic, meaning the majority of the plant is infected via bacterial movement and colonization of the plant's vascular system. When the bacterium spreads within the plant, leaves begin withering and can die. Plants become stunted and at times, the whole plant may wilt and die. Dwarfed, bleached tassels are common. Often, plants that have wilt symptoms also have leaf blight symptoms.
The second, more common phase of the disease is the leaf blight phase, which occurs on the leaves at any vegetative growth stage.
In certain corn varieties, kernels can be infected later in the growing season after flowering occurs. Although corn kernels can be a source of inoculuum, seed transmission is quite rare.
Environment and control
The number of flea beetles emerging in spring from hibernation depends on the severity of winter temperatures. Warm winter temperatures favor the survival of flea beetle vectors and increase the risk of Stewart's disease. The numbers of emerging adults can be estimated by calculating a winter temperature index by averaging the mean temperatures for December, January, and February. If the mean of the mean temperatures is or greater, the beetles will survive in high numbers and the disease risk is high; if the mean is between , the risk is moderate to high; , moderate to low; and a mean less than represents low risk. The toothed flea beetle, adult 12-spotted cucumber beetle, seed corn maggot, wheat wireworm, white grubs, and larvae of corn rootworms can also carry P. stewartii from one plant to another during the summer. These pests cannot overwinter and transmit this disease.
All sweet corn varieties are susceptible to wilt in the first leaf stage. Susceptibility decreases and natural control is obtained as plants grows older. External disease control is conducted by insecticide spraying to stop early feeding of overwintering flea beetles. Common insecticides used for control of Stewart's wilt are clothianidin, imidacloprid and thiamethoxam. These insecticides are most effectively used at rates of 1.25 (mg ai/kernel), with clothianidin being the most effective at that rate. Application rates on the labels may vary a little, so follow the label rates for each insecticide. Better results are obtained when seeds are sprayed prior to germination. Hybrid varieties of sweet corn are also available for control. Dent corn hybrids are more resistant to the disease than sweet corn, hence do not require insecticides.
Importance
The susceptible varieties suffer losses ranging from 40 to 100% when infected prior to the five-leaf stage. The losses are 15–35% and 3–15% for seven-leaf and nine-leaf stages, respectively. Stewart's wilt may add additional costs for phytosanitary regulations from trading partners. Such regulations primarily affect seed commerce by preventing seed from being exported or by creating additional costs for phytosanitary inspections prior to export. During epidemics in the 1990s, Stewart's wilt was a significant economic issue for the corn seed industry because of the logistics of trading and exchange of large volumes of field corn seed throughout the world. Stewart's wilt also creates indirect costs for seed producers - in the same way any important disease does - because resources must be used to screen germplasm and breed corn for Stewart's wilt resistance to develop hybrids that efficiently and effectively control the disease.