Postmortem: Corn Diseases in 2015 - Old Nemeses and New Foes

Timely planting in April and May was followed by record-setting rains in June. This led to ponded water that remained for long periods of time in some areas. Research has shown that flooded, anaerobic soil conditions can be devastating for young corn plants, resulting in suffocation death after only 3 to 4 days. Flooded soil conditions can also favor water mold pathogens like Pythium. Water molds are fungal-like organisms that form swimming spores called zoospores that can swim toward plant roots. Indeed, the University of Illinois Plant Clinic diagnosed multiple Pythium-caused root disorders this year. Research regarding the management of those Pythium species involved in stand losses, including sensitivity to commercial seed treatment fungicides, is taking place at Iowa State University (ISU) and other Midwestern universities.

When water sits in leaf whorls, a water mold pathogen called Physoderma maydis can infect young leaf tissue to cause a disease called Physoderma brown spot (PBS). Small, roundish yellow lesions turn brown over time and can occur in bands across leaf blades (Figure). PBS symptoms in 2015 were not always limited to leaves, but included stalk rot and breakage in certain very-susceptible hybrids. Although some fungicides are labeled for PBS management, research regarding product efficacy and optimal application timing is needed. Research to answer these questions is underway at ISU.

Another disease favored by long periods of leaf wetness, and prevalent throughout Illinois in 2015, is northern corn leaf blight (NCLB), caused by the fungus Exserohilum turcicum. Symptoms of NCLB include 1 to 7 inch-long lesions that taper at each end and run parallel to leaf veins (Figure). Multiple lesions may form on a leaf, resulting in blighted or dead tissue. Research at the University of Illinois has shown that foliar fungicides are most likely to result in a profitable yield response when disease is present before tasseling and conditions favor the development of additional cycles of infection (ie: susceptible hybrid, residue is present from previously-infected crops, weather favors disease).

In some Illinois regions in 2015 favorable weather conditions occurred when plants were most susceptible to Stenocarpella maydis, the fungus that causes Diplodia ear rot (DER). Spores are rain-splashed onto plants from residue of previously-infected corn. Ears are thought to be most susceptible to infection in the 3 weeks after silking. Purdue University researchers suggest that poor fungicide-pathogen contact at the site of infection may explain why, regardless of application timing, fungicides labeled for ear mold did not consistently reduce disease or increase yield in field trials.

At the beginning of September, Purdue University plant pathologists confirmed that a disease called tar spot had been found for the first time in the U.S. in Indiana. Caused by the fungus Phyllachora maydis, this disease was also confirmed in three Northern Illinois counties. P. maydis produces raised, black structures on leaves that resemble small drops of tar (Figure). Before its 2015 Midwestern appearance, tar spot was only found on corn grown in the cool, humid conditions that occur at high elevations in Latin America. Research at the University of Illinois is underway to determine whether P. maydis is capable of surviving an Illinois winter.

One thing that all of these diseases have in common is that the organisms that cause them survive in residue of previously-infected corn. Provided conditions favor infection, those corn fields that were heavily diseased in 2015 will supply plenty of inoculum to cause disease in 2016. Actions that allow time for plant residue, and the pathogen inoculum surviving on it, to degrade before corn is again grown can reduce disease risk. These actions can include rotating away from corn and managing residue through tillage. Knowing which diseases were present in their fields in 2015 can help producers to target future disease resistance needs and hybrid selections.