Several sections of Iowa are reporting Diplodia ear rot, a disease that causes kernels to be lightweight and have reduced nutritional value. This pathogen can be a problem in storage if grain moisture is 20% of above. Ear and stalk rots are likely to become prevalent in Iowa as the growing season draws to a close, says Alison Robertson, Iowa State University Extension plant pathologist. "This past week, we have had a few reports of Diplodia ear rot from the northeast, central and southeastern parts of the state. This ear rot is not as common as Fusarium or Gibberella ear rot in Iowa," she says. Diplodia ear rot is caused by the fungus Stenocarpella maydis (Diplodia maydis). The same fungus also causes Diplodia stalk rot, explains Robertson. The fungus survives in corn residue and seed, and tends to be more of a problem in corn-following-corn fields. Robertson, and Fanny Iriarte, a pathologist in the Plant and Insect Diagnostic Clinic at ISU, offer the following information and advice.
Cool, wet weather at grain fill favors ear rot
Diplodia ear rot is favored by cool, wet weather during grain fill. Infection occurs through the silks and/or ear shank, or via the base of the husks of the ear.
Symptoms of Diplodia ear rot can be striking – a bleached ear leaf and husk (Figure 1). When the husk is peeled back, a dense white to grayish white mold which starts at the base of the ear is visibly growing between the kernels. Often the husks of the ear are difficult to remove and appear "glued" to the ear by the mold. Very small, black fruiting bodies can be found scattered on husks or embedded in cob tissues and kernels (Figure 2).
Although S. maydis does not appear to produce mycotoxins in the grain under typical Iowa field conditions, infected kernels are lightweight and have reduced nutritional value. Damage caused by Diplodia ear rot is usually limited to the field, but the pathogen can be a problem in storage if grain moisture is 20% or above.
Options for managing Diplodia ear rot are limited. Rotating the field out of corn is recommended since the fungus survives in crop residue. Corn hybrids do differ in their susceptibility to Diplodia so talk with your seed dealer.
Figure 1. A bleached leaf is associated with Diplodia ear rot. Alison Robertson.