Diplodia Stalk and Ear Rots in Corn

  • Diplodia stalk and ear rots are caused by the fungus Stenocarpella maydis, which infects the stalk and ears after silking.
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  • Stalk and ear rots can lead to reduced grain quality, stalk lodging, and premature plant death, all of which can impact yield potential.
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  • Proactive strategies that can help manage both Diplodia stalk rot and ear rot next season include: tillage, crop rotation, managing foliar diseases, and minimizing other forms of stress.
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Topic

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Figure 1. Diplodia grows around the kernels and infects the pith, resulting in reduced grain quality and compromised stalk integrity.
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Diplodia Stalk Rot

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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.

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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).

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Diplodia Ear Rot

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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.

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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.

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Effect

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Figure 2. Bleached husks from Diplodia ear rot.
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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.

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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.

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Action

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Figure 3. Pycnidia (fungal spore-producing structures).
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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.

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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

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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.

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Product Selection. 

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  • Choose corn products with greater levels of resistance to Diplodia stalk and ear rots and with good standability ratings.
13;10;Reduce Inoculum. 
  • The fungus that causes Diplodia ear and stalk rots only infects corn and survives only on corn debris; therefore, rotation to crops other than corn for 1-2 years is very effective at reducing the inoculum in the field. 
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  • Tillage that buries infected residue speeds decomposition and reduces inoculum.

Minimize Stress. Minimizing stress during the growing season can help maintain stalk quality and minimize the effect of stalk rots.

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  • Plant products with insect protection traits to minimize damage from stalk boring insects and ear feeding insects that may compromise husk coverage.
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  • Apply fungicides when foliar diseases are present at high levels to help maintain healthy photosynthetic leaf area and minimize stalk cannibalization during grain fill.
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  • Plant products with different relative maturities and/or different GDU requirements to flowering so that the entire crop does not flower during peak environmental conditions for Diplodia ear rot infection.
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  • Use recommended planting populations to decrease plant stress.
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  • Maintain balanced soil fertility.
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Grain Drying. Consider the following management practices for harvesting and storing grain from fields with established ear rot:

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  • Harvest early to prevent further spread of ear rot if weather conditions have been favorable or if stalk lodging is a concern.
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  • Allow corn to dry in the field to 23 to 25% moisture and dry grain to 13 to 14% moisture prior to storage.2
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  • Store grain at cool temperatures between 36 and 44°F after drying.
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  • Limit storage to cold weather and do not store through the next summer.
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  • Store diseased grain separate from healthy grain.
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  • Check grain periodically for temperature, wet spots, and insects.
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  • Clean bins thoroughly before storing.
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