As the 2014 growing season enters the homestretch, you are reminded to scout for stalk and ear rots of corn. Stalk rots are likely to be a problem this year particularly in fields where leaf diseases occurred and fields with high yield potential. Diplodia ear rot is already prevalent in some fields. The following information and update on the current situation, and some scouting tips, are provided by Alison Robertson, Iowa State University Extension plant pathologist.
Stalk rots are beginning to show up in some fields
Stalk rots may result in premature death of plants (See Figure 1) or reduced yield potential. Diplodia stalk rot has already been observed in some fields in central Iowa in early September.
Figure 1: Stalk rot results in premature plant death.
This stalk rot, known as Diplodia stalk rot, is easily recognized:
1) Look for tiny, minute black specks (called pycnidia) buried in the rind of the lower nodes of the corn stalk (See Figure 2). In moist conditions, millions of spores may be extruded from the pycnidia (See Figure 3).
2) Splitting the stalk will reveal shredded pith.
Figure 2: Tiny black specks (known as pycnidia) embedded in the rind are a diagnostic symptom for Diplodia stalk rot.
Figure 3: Diplodia spores are extruded from pycnidia when humidity is high.
Gibberella stalk rot may also be prevalent this growing season since conditions have been favorable for infection and disease development. This stalk rot is best identified from the pink-to-red shredded pith tissue. Black specks may also be observed at the lower nodes, but these are slightly larger than those observed with Diplodia, and can be easily scraped off the rind surface using a fingernail.
Identifying fields that are "at risk" from stalk rot
Northern corn leaf blight and Goss's leaf blight are widespread throughout the state this year in early September, particularly on susceptible corn hybrids. These fields should be scouted for stalk rot around the time when corn reaches the "black layer" stage of growth, which is when the corn plant reaches physiological maturity.
When photosynthesis is compromised as a result of reduced green leaf area due to leaf disease, stalk rots are often a problem. This is because the corn plant is dedicated to filling grain and will cannibalize carbohydrates from the stalk if necessary. For that reason, a good place to start scouting for stalk rots is in fields that have leaf blights.
Fields with high yield potential may also be at risk for stalk rots. The cool wet conditions that we had from blister (R2 growth stage of corn) onwards have favored grain fill this year. Since kernel abortion is unlikely after dough stage is reached (R4 growth stage), the corn plant will do everything to finish off the grain. And it will do this at the expense of using carbohydrates stored in the stalk, consequently leading to increased risk of stalk rots.
You should also be watching for ear rots showing up
Ear rots result in reduced grain quality. Diplodia ear rot is favored by cool, wet weather during early ear development. This disease is easily recognized. Look for a dead ear leaf while scouting fields (See Figure 4). Oftentimes, a white mold or "black pepper" may be visible in the husks at the base of the ear. Peeling back the husks will reveal the same dense white mold growing up from the base of the ear (See Figure 5). No mycotoxins have been associated with Diplodia ear rot in the U.S.
Figure 4. A dead ear leaf is symptomatic of Diplodia ear rot.
Figure 5: This is Diplodia ear rot. Note the dense white mold growing from the base of the ear. The tiny black specs, called pycnidia, may also be present on the husks.
Cool, wet weather after silking also favors development of Gibberella ear rot. When the husks of the ears are peeled back, a pink-to-red mold developing from the tip of the ear is diagnostic for this ear rot. Mycotoxins (such as DON and zearalenone) are associated with Gibberella ear rot.
Management of stalk rots and ear rots
Fields in which greater than 10% of the corn plants have stalk and/or ear rots should be scheduled for early harvest. Identifying these diseases can also help with management for future years. Since stalk and ear rot pathogens survive in infested crop residue, rotation to a non-host crop such as soybeans may help reduce the inoculum. Corn hybrid susceptibility to stalk rots and ear rots differs among hybrids.