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. Once the bacteria are inside the plant, they colonize the xylem and intercellular spaces of the leaves. When the bacteria reach the corn stalks, the vascular bundles become brown and necrotic. The kernels may also have grayish lesions with dark margins or they may be irregular in shape and dwarfed. Another common symptom of the bacteria is formation of open cavities within the stalk tissue. While the plants are weak and vulnerable, stalk rot fungi can further invade the corn plant. 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. At first, the leaf lesions appear long and irregularly shaped and are light green to yellow and later on, straw colored. On mature plants, yellowish streaks with wavy margins extend along the leaf veins. This leaf blight phase is often prevalent after tasseling and the symptoms look similar to frost damage, drought, nutrient disorders, northern corn leaf blight (caused by Exserohilum turcicum), and particularly Goss's wilt (caused by Clavibacter michiganensis ssp. nebraskensis).
A good way to determine corn has Stewart's wilt symptoms is to look at the leaf tissue under microscope. A good indicator of whether or not the bacteria have infected the stalks is if yellow masses of bacteria are oozing from the vascular bundles. With certain sweet-corn hybrids, yellow, slimy ooze collects on the inner ear husks and/or covers the corn kernels. If the bacterial ooze exhibits non-flagellated, non-spore-forming, rod-shaped bacteria, the likelihood of Stewart's wilt is great.