Corn ears are thought to be most susceptible to infection by Stenocarpella maydis, the fungus that causes Diplodia ear mold, in the 3 weeks after silking (milk stage), when the silks begin to die. Fungal spores are rain-splash disseminated onto the crop from residue from previously infected crops. In 2015 in western Illinois, corn planted between April 5 and 15 reached silking at the end of June or the beginning of July after which periodic rains brought between 9 and 11 inches of rain - perfect conditions and timing for Diplodia ear mold.
Symptoms and signs. S. maydis can infect the plant by growing down the silks, infecting the husk near the base of the ear or penetrating the ear shank, growing up the cob and between kernels. Infections that take place earlier-on can result in the whole ear becoming diseased. Early infections can result in bleached or tan-colored husk tissue, rotten, lightweight kernels that are dull-gray to brown in color and in vivipary, kernels that germinate before harvest and while still attached to the plant. Growth of the pathogen can result in husks hanging on tightly to the ear.
Later infections may result in only portions of the ear being infected, often towards the base of the ear, with no outward symptoms or signs.
Signs of the fungus can be conspicuous and include thick, white, moldy growth on or between kernels (Figure). Later in the growing season small, black asexual reproductive structures of the fungus (pycnidia) may be visible to the naked eye (Figure).
Yield loss. Yield can be lost through smaller kernel size and lower test weights. This, along with moldy or damaged kernels and fine materials can result in discounts at the grain elevator.
Management considerations this year. Be aware of the potential for Diplodia ear mold this year and adjust plans accordingly. Diseased kernels may be more brittle than healthy kernels, increasing the possibility of breakage and the amount of fine materials. Adjusting combine settings can help to maximize grain cleaning and minimize breakage. Additional cleaning of harvested grain is recommended before sale or storage to minimize discounts and to improve aeration in short-term storage. Drying and cooling the grain as soon as possible is recommended to stop the growth of both S. maydis and other opportunistic fungi. Storing diseased grain separately and for only short periods of time is recommended to reduce the chance of additional losses.
On a positive note: unlike some of the other ear molds, in the U.S. Diplodia ear mold is not associated with mycotoxin contamination.
Management considerations in future years. Hybrids vary in their genetic susceptibility to Diplodia ear mold. Select hybrids with high levels of resistance to plant in fields that have had a history of disease.
S. maydis has a host range of one - corn – and survives in infested residue on the soil surface. This is why both crop rotation and tillage are important management strategies.
Severns, D.E. and White, D.G. 1999. Diplodia Ear Rot of Corn. Proceedings of the IL Crop Prot. Technol. Conf. pp 106-107.
Steckel, S. 2003. Biology and Management of Diplodia (Stenocarpella maydis) Ear and Stalk Rot. J. Nat. Resour. Life Sci. Educ. 32:5-7.
Water and Atmospheric Resources Monitoring Program. Illinois Climate Network. (2014). Illinois State Water Survey, 2204 Griffith Drive, Champaign, IL 61820-7495.
White, D.G. ed. 1999. Compendium of Corn Diseases. 3rd ed. APS Press. St. Paul, MN.
Woloshuk, C. and Wise, K. 2009. Diseases of Corn: Diplodia Ear Rot. Purdue Extension Bulletin BP-75-W.