After several years of speculation about the occurrence of Goss’s bacterial wilt and leaf blight of corn in Missouri, the disease has been officially confirmed from a corn leaf sample from Holt County, Mo. Corn plants in the field showed symptoms suggestive of Goss’s bacterial wilt. When the leaf sample was submitted to the University of Missouri Plant Diagnostic Clinic, there was very pronounced bacterial streaming from affected tissues. The sample tested positive on the immunostrip test for Clavibacter michiganensis subsp. michiganensis, the causal agent of bacteria canker in tomato. This test will also react with other species of Clavibacter including the one which causes Goss’s wilt. The sample was negative on the ELISA test for the Stewart’s wilt bacterium. Using a selective media, C. michiganensis nebraskensis was isolated from the infected tissue. Bacterial cultures were submitted to the University of Minnesota Plant Disease Clinic and personnel there used PCR and sequencing to confirm the identification of the bacterium.
Goss’s bacterial wilt and leaf blight of corn is caused by the bacterium Clavibacter michiganensis subsp. nebraskensis. The disease was first reported from Dawson County in Nebraska in 1969. Over the next ten or so years it was identified in adjacent states. Since about 2006 there has a resurgence of the disease in Nebraska and it has been reported in many of the Corn Belt states.
The bacterial pathogen can cause two main types of symptoms, a leaf blight and a systemic wilt. The leaf blight phase of the disease is the most common. Initially water-soaked streaks may appear in leaf tissue. These water-soaked areas develop into lesions that are gray to light green to light yellow in color, have wavy, irregular edges and run parallel to leaf veins. Small, irregular, dark green to black water-soaked spots (often called freckles) occur within the lesions. Bacterial exudate or ooze may dry on the diseased leaf tissue. This dried exudate will appear shiny or glistening in sunlight. The presence of “freckles” within the lesions and dried bacterial exudate within the lesions are diagnostic features of Goss’s leaf blight.
The leaf blight phase of the disease might be mistaken for Stewart’s bacterial wilt or northern corn leaf blight. The lesions caused by Stewart’s bacterial wilt are very similar to those caused by Goss’s wilt and leaf blight. However, the appearance of freckles or dried bacterial exudate on the lesion surface are good indications that it is Goss’s wilt rather than Stewart’s wilt. Also Stewart’s wilt is spread by the corn flea beetle so the presence of flea beetle feeding scars on leaves would suggest Stewart’s wilt. Stewart’s wilt has not been very prevalent in Missouri the last few years. Northern corn leaf blight lesions may have a water-soaked appearance initially. Lesions tend to be tan to brown to almost bleached in color and have slightly more defined or regular lesion margins or edges than the lesions of Goss’s leaf blight. Northern corn leaf blight lesions may have distinct clumps of mold growth on the dead tissue which could be mistaken for the freckles of Goss’s wilt. Older lesions of both Goss’s wilt and Stewart’s wilt may be colonized by secondary fungi so fungal growth within or over lesions in not a reliable diagnostic feature.
Leaf lesions may expand and kill large portions of the canopy. Severe browning and death of leaves may be confused with leaf scorch associated with drought or hot, drying winds. Goss’s leaf blight lesions develop along the margins of the leaves of within the leaves while leaf scorch tends to begin at the leaf tip and move towards the base of the leaf.
Yield losses will vary depending on susceptibility of the hybrid, timing of infection (infection early in the season may result in higher yield losses) and environmental conditions during the remainder of the growing season.
The wilt phase of the disease is less common and occurs when the bacteria infects the vascular system of the plant. When the bacteria move into the water-conducting tissues, the xylem tissues will be discolored and then plants will wilt and die. Stalks will exhibit a slimy, stalk rot.
The bacteria which causes Goss’s wilt and leaf blight survives in infested corn debris on or near the soil surface. It may be spread by wind-driven, rain, splashing rain and heavy rains associated with hail storms. Infection may be primarily through wounds caused by hail, wind, rain, wind-blown sand or soil, or possibly machinery. Development of the disease is favored by warm, wet and humid conditions. Outbreaks may be found after a hail storm or rain storm with strong winds which would wound plants and spread the bacteria.
There are no rescue treatments for Goss’s wilt and leaf blight. Management strategies focus on preventative measures and include planting resistant hybrids, rotating crops, controlling weeds and managing residues. Hybrids range in sensitivity from very resistant to very susceptible. If Goss’s wilt and leaf blight have been a problem, check ratings of hybrids and try to find hybrids with some level of resistance. Because the bacterium that causes the disease survives on corn residue and debris, severe infestations are more likely to occur when corn follows corn. Rotation to soybean or other broadleaf crops will help reduce the problem. Fields with a reduced-tillage system may also be at greater risk and in those fields hybrid selection, crop rotation and weed control become even more important. Finally, weedy grasses such as barnyard grass, green foxtail, and shattercane can be hosts for the bacterial pathogen. Good weed control of grassy weeds can help reduce inoculum and lower the risk of the disease.
If you suspect Goss’s bacterial wilt may be present in a corn field and want to have the field diagnosis confirmed, you can submit a sample to the University of Missouri Plant Diagnostic Lab (for information visit the website—http://plantclinic.missouri.edu/). For the $15.00 fee samples will be checked for symptoms and microscopically examined for bacterial streaming. For the remainder of the 2014 season, samples suspected of having Goss’s wilt will be further tested by culturing and serological testing with no additional charges. After December 31, 2014, normal fees for culturing and serological testing will be charged.