Subscribe and stay up-to-date with the latest news and great offers from DEKALB, Asgrow and Deltapine.
Don't miss out on the latest agronomic news.
Local agronomic alerts.Delivered straight to your inbox.
The most distinct symptoms of bacterial stalk rot or whorl stage infections are the slimy masses of degraded corn tissue and the foul odor. Discoloration of leaf sheaths and stalk nodes are initial symptoms. As the disease progresses, a soft rot occurs. Once this decay sets in, a foul odor can be detected and the top of the plant can be easily removed. Split stalks reveal a soft, slimy rot and discoloration at the nodes (Figure 1). The bacterium can infect the plant at any node from the soil surface up to the ear leaves or tassel. At the whorl stage, upper leaves die and are easily pulled from the whorl. Tasseling and pollination may be disrupted from infections high in the plant.
Figure 1. Corn stalk with bacterial stalk rot.
High humidity and warm temperatures during mid-season favor the development of this disease. It can be a problem in areas with heavy rainfall, overhead irrigation, or where water is pumped from a lake, pond, or slow-moving stream. Infection at the soil line occurs if plants have been in standing water for a few days following heavy rain and warm temperatures. Infection is also associated with water remaining in the whorl for extended periods.
Management. The bacterium can survive in corn or sorghum stalks and residue and is a sporadic disease, often affecting individual plants. There is very limited host resistance to soft rot bacteria. Therefore, the best management practices are fall cultivation to incorporate residue, adoption of practices that reduce pathogen inocula, and an avoidance of excessive irrigation.
Goss’s Wilt occurs at one of two phases within the corn plant: a seedling wilt that can result in a systemic infection, and an adult-plant wilt ,which is typically associated with leaf blight. Although the seedling systemic wilt is observed less frequently than the leaf blight, early infection of seedlings can have devastating effects on plant survival.
Figure 2. Goss’s wilt leaf freckles (top). Leaf freckles appear luminous when held up to block the sun (bottom).
The leaf blight phase causes gray to light yellow lesions with wavy margins that roughly follow leaf veins. Two characteristic symptoms of Goss’s Wilt are the presence of dark green to black water-soaked spots near the edges of expanding lesions or the appearance of “freckles” within infected areas of leaves. Goss’s Wilt “freckles” are luminous when leaves are held to block the sun (Figure 2). Bacterial exudates (ooze) on leaf surfaces can also be used to differentiate Goss’s Wilt as they have a shiny, shellac-like appearance when dried. Goss’s Wilt can be mistaken with symptoms of northern corn leaf blight, Stewart’s wilt, or necrotic areas of leaves resulting from drought or nutrient deficiencies.
Goss’s Wilt follows weather events in which rain and wind disseminate overwintering bacteria from infested plant residues. Wind or hail damage to leaves and other plant parts create wounds for bacteria to enter the plant. Hot, dry weather can inhibit disease development, except in fields with overhead irrigation.
Management. Goss’s Wilt overwinters in infested corn residue on the soil surface. This infested debris is the primary source of inoculum for the following corn crops. Continuous corn rotations enhance damage from Goss’s Wilt due to an abundance of overwintering inocula. Additional hosts for this pathogen include green foxtail, shattercane, and barnyardgrass. Infection requires leaf injury (hail, sand-blasting, wind, equipment). Insects are not known to spread the bacterium.
Planting corn products with genetic resistance to Goss’s Wilt is the best method to manage this disease. Tillage that buries corn residue reduces levels of overwintering inocula. Rotating away from corn for two or more years with soybean, dry bean, small grains, or alfalfa allows time for infected residue to degrade, reducing inocula levels.
Lesions initially appear as dark green, round to oval shaped, about 1/4 inch diameter, water-soaked spots, on the tips of lower leaves. As the lesions mature, they become light tan to nearly white in color and eventually dry and turn brown, often with reddish to brown margins. Larger lesions may be surrounded by yellow halos (Figure 3). Lesions can grow together forming irregular spots and streaks of dead tissue.
The bacteria overwinter in crop debris and enter leaves through the stomata (transpiration openings). Scouting should be done prior to the silking growth stage, although there is no in-season management options for Holcus leaf spot. The disease can be confused with eyespot, which has similar lesions, or with injury from paraquat herbicide drift. Generally, the potential for yield loss is small.1
Favorable conditions and elements for Holcus leaf spot development include: rainy, windy, hail, blowing soil, and warm temperatures (77° to 86° F), especially early in the season.
Figure 3. Holcus leaf spot with distinctive halo. Photo courtesy of University of Illinois.
Management. Planting resistant corn products, crop rotation, managing weedy hosts such as foxtails and johnsongrass, and tillage can help reduce the potential for infection.
Figure 4. Stewart’s wilt symptoms showing flea beetle feeding wounds.
Similar to Goss’s wilt, Stewart’s wilt has two phases. The seedling wilt phase occurs when the overwintering generation of flea beetles infect plants with the bacterium soon after early-planted corn emerges. Long, chlorotic lesions with wavy margins follow leaf veins as a result of bacteria in the xylem. The leaf blight phase occurs near or after tasseling. Water-soaked lesions extend the length of the leaf and become necrotic (Figure 4). Vascular tissues can be discolored and decay or death occurs at the base of the stalk on susceptible products. Lesions are similar to Goss’s Wilt lesions late in the leaf blight phase. The two bacteria can be differentiated in a lab.
Stewart’s wilt infection is solely dependent on the survival of corn flea beetles. If the sum of the average monthly winter temperature for the months of December, January, and February is greater than 90 °F, corn flea beetle survival and disease risk is considered to be high. However, if the sum of the average temperature for those months is less than 80 °F, corn flea beetle survival and disease risk is low.2
Management. Many corn products are available with Stewart’s wilt resistance. Neonicotinoid seed-applied insecticides can help manage corn flea beetles and have been associated with lower than expected levels of Stewart’s wilt in recent years.