By Mark Johnson
Environmental conditions influence which corn diseases may be present each growing season. Moreover, the severity of the diseases is affected by various factors. Early-, mid- and late-season weather; the crop’s disease levels in the South, and winds out of the right direction at just the right time are just a few.
Then there is the resistance level of any given corn hybrid to any specific disease. So it is hard to predict which corn diseases we need to be on the lookout for later this growing season.
Having said that, there are some newer diseases of corn you may not have had much experience with to date. Watch for these as you do your scouting. Two new diseases for Iowa are bacterial leaf streak and tar spot. A third is physoderma brown spot and node rot.
Bacterial leaf streak
Bacterial leaf streak is caused by the bacterium Xanthomonas vasicola. This pathogen was identified with the disease observed in Nebraska in 2014 and Iowa in 2016. Effect on yield and quality are unknown. According to USDA’s website, “there is no evidence of adverse impact on corn yield or quality from this plant disease.” However, since it is new in this part of the world, the verdict may not be in yet for us.
Where did BLS come from? This disease had not been previously identified in the U.S. It was first reported in South Africa in 1949 and still affects crops there. Since it was identified in nine of the major corn growing states (South Dakota, Minnesota, Colorado, Nebraska, Iowa, Illinois, Kansas, Oklahoma, Texas) during the 2016 growing season, it may now be widely distributed throughout the Corn Belt.
This disease was observed in six counties in Iowa in 2016. ISU Extension worked with USDA, Iowa Department of Agriculture and the Iowa Crop Improvement Association to determine how widespread BLS was at the time. Some counties in Iowa were surveyed. ISU tested suspected samples for confirmation. The six counties with confirmed identification were Hancock, Webster, Grundy, Buchanan, Marshall and Tama.
In corn’s early vegetative stages, the lesions appear on lower leaves. With favorable conditions or during a corn plant’s reproductive stages, lesions occur in the upper canopy.
Lesion characteristics vary when BLS symptoms show up on corn leaves. Narrow lesions with wavy margins are evident. Yellow, tan, brown or orange lesions occur between and along leaf veins. Lesions occur on the leaf blade or clustered around midrib. When you hold the leaf up to sunlight, bright-yellow halos extend from the ends of the lesions.
What we know about BLS
Three things must occur for a disease to show up in a crop. You must have a pathogen, a host and a favorable climate: the disease triangle. Here’s what we know about the disease triangle for bacterial leaf streak:
• Pathogen. Likely survives in crop residue, likely spread by rain or irrigation splash, no wounding needed for infection; the pathogen enters the corn plant through the stomates. Is it seedborne? Does it have alternate hosts? We don’t know.
• Host. No resistance is known. Corn hybrids appear to vary in susceptibility. Field corn, seed corn, sweet corn and popcorn are all hosts.
• Environment. This disease is more common in continuous corn, but is observed on corn after soybeans, wheat and fallow. Overhead irrigation, or rainfall during hot weather, appears to increase severity.
For more information, see this publication from the Crop Protection Network.
Tar spot minor problem
Another disease to watch for is tar spot. Caused by the fungus Phyllachora maydis, tar spot is usually a minor problem. It was first reported in 2015 in Indiana and Illinois, and in 2016 in Florida, Iowa, Indiana, Illinois, Michigan and Wisconsin. Prior to the Indiana finding, tar spot was known to occur only in cool, humid areas at high elevations in Latin America. The initial confirmed case in Iowa was last year in Jones County; there have been no additional Iowa fields or counties since. Tar spot symptoms on corn leaves include small, raised, black spots scattered across the leaf surface.
TAR SPOT: Symptoms of this disease are small, raised black spots scattered across the leaf surface.
Sometimes tar spot may be further colonized by a second fungus, Monographella maydis. When that happens, the disease is referred to as tar spot complex, and corn yield loss can occur. Fortunately M. maydis hasn’t been identified in the Midwest. When M. maydis infects P. maydis, the tar spots resemble fish eyes, and can enlarge and cause severe blight. Tar spot complex has led to yield losses of 30% to 70% in Mexico and Guatemala.
Physoderma brown spot, node rot
Brown spot is becoming more prevalent in Iowa. Most reports have been from northern Iowa, with some from southwest and southeast Iowa. Corn is most susceptible to infection during V5 to V9 vegetative stages. Symptoms include dark-purplish to black oval spots on the leaf’s midrib.
These spots may also occur on the stalk, leaf sheath and husks. Infected nodes become rotted and snap when gently pushed. Nodes often snap as you walk through the field. Since 2013, there have been increased reports of node rot. Other than these symptoms, affected plants may look very healthy and appear to have excellent yield potential.
BROWN SPOT: Physoderma brown spot and node rot symptoms include dark oval spots on midrib of leaf. Infected stalk nodes become rotted and snap off easily.
Management is somewhat difficult; corn hybrids vary in susceptibility. But not all hybrids have ratings for this disease. Rotating to soybeans or some other non-host crop is helpful. Residue management may reduce survival of the pathogen. Many fungicides are labeled for physoderma brown spot management; however, logistics of application are confounded by the fact that infection happens early in the season, and by the time symptoms appear, application wouldn’t be of much help.
If you suspect any of these diseases in a field, submit a sample to the Iowa State University Plant and Insect Diagnostic Clinic using this sample prep regimen:
• Send in six to 10 leaves that vary in disease severity (from a few to many lesions).
• Wrap leaves in a dry paper towel and place them in an envelope.
• Ship it overnight or drive the sample to the clinic.
• Avoid shipping on Thursdays and Fridays.
• Submit your sample with Form PIDC 0045.
Johnson is ISU Extension field agronomist in central Iowa. Contact him at [email protected].