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Favorable Conditions. Stalk rot development is favored by late-season stresses such as an excess or lack of moisture, a nutrient deficiency or imbalance, excessively cloudy weather, and invasive injury to the leaves, stalks, or roots by insect feeding, foliar diseases, wind, or hail. Diplodia overwinters on corn debris; therefore, corn-on-corn fields managed with reduced tillage have an increased potential for Diplodia stalk and ear rots.
Symptoms. Diplodia stalk rot commonly occurs around mid to late ear fill. Diplodia stalk rot causes a straw-brown discoloration of the lower nodes and internal disintegration of the pith, leaving only vascular tissues intact (Figure 1). Stalks will feel squishy and break easily. Small black dots, called pycnidia (fungal spore-producing structures), appear embedded around the lower nodes of the infected stalks (Figure 3).
Favorable Conditions. Dry conditions during early vegetative growth stages followed by warm, wet weather within the first 3 weeks after silking favors the development of Diplodia ear rot. Greatest losses may occur when rainfall is above average from silking to harvest, or when insects or birds damage the ear during development. Corn products vary in their level of susceptibility to Diplodia ear rot; however, any product can be infected under favorable conditions.
Symptoms. Ears infected with Diplodia ear rot may first be noticed by a premature tan or bleached appearance on the base of the husk (Figure 2). Infected ears develop a white to gray mold that grows between the kernels beginning at the base of the ear and progressing toward the tip (Figure 1). Diplodia continues to develop on infected ears until corn is harvested and dried. If left in the field, particularly when weather is rainy and humid, light infections may progress and ears can become completely mummified by the fungus. Pycnidia, similar to those seen with the stalk rot, can also be found on the husks, cobs, and kernels.
Losses. Diplodia stalk rot may reduce yield potential by more than just the loss of harvestable ears due to stalk lodging. As plants die from infection, the normal grain filling process stops. This can result in a reduction in kernel size and grain weight. Grain quality can also be affected by ear rots after the ears on lodged plants come into contact with the soil and crop residue.
Ears infected with Diplodia are lightweight and subject to breakage and losses during harvest. Infected kernels will be lightweight and have reduced nutritional value. Unlike some ear rots, Diplodia does not produce a mycotoxin harmful to livestock, but will result in lower quality feed.
Scouting and Stalk Quality. Scouting for stalk rots is recommended as corn reaches the dough through dent stage. Fields with heavy infestations of leaf diseases should be watched closely for stalk rots as this can lead to cannibalization of carbohydrates from the stalks in order to fill the grain.
Evaluate stalk quality by either pushing stalks to a 45 degree angle to see if the stalk breaks or by pinching the lower internodes between your thumb and finger to see if the stalk collapses. Conduct either test on 10 plants in a row at several locations throughout the field. If stalk quality has been compromised in more than 10% of stalks, then the field should be slated for early harvest.1
In-season options for managing Diplodia are limited and fungicides are not effective at controlling the disease, but proactive practices can help manage both Diplodia stalk and ear rot the next time corn is planted.
Minimize Stress. Minimizing stress during the growing season can help maintain stalk quality and minimize the effect of stalk rots.
Grain Drying. Consider the following management practices for harvesting and storing grain from fields with established ear rot: