Severe thunderstorms early Monday, July 11, flattened over 100, 000 acres of corn from southern Story County to Dubuque County in east-central Iowa. Much of the corn was at late vegetative stages, which are critical stages of development. There have been several questions about possibly applying fungicide to these fields. There isn’t much data to reference, but since one of our fungicide trials was extensively damaged in the storm, maybe this time next year we will have some data since we intend to carry on with the trial. Alison Robertson, Iowa State University plant pathologist offers these thoughts:
As ISU field agronomist Jim Fawcett points out in his weekly newsletter, the first thing to do is determine how much green snap has occurred and how much damage is root lodging.
Both Elmore and Fawcett refer to research done in Wisconsin, which is summarized in an ICM newsletter article that found yield losses of about 10-20% when corn was totally flattened at V14-V18.
Flattened corn may provide a better microclimate for disease development because leaves are closer to inoculum source, less wind movement through canopy, higher humidity and longer periods of leaf wetness that may favor infection.
Physical damage to the plants may lead to increased bacterial disease – which a fungicide would not control – or smut for which no products are labeled.
The chances of a fungicide application breaking even or being profitable is reduced in fields with lower yield potential. Fungicide applications should be targeted to fields with high yield potential.
Fungicide applications prior to tasseling can affect ear formation, particularly when an adjuvant is used.
Fungicide coverage of flattened corn will be more sporadic because penetration through this altered canopy will not be as thorough.
Strobilurin fungicides have been reported to improve host plant tolerance to yield-robbing environmental stresses, such as drought, heat, cold temperatures and ozone damage. Does flattened corn count as such a stress? I honestly don’t know.
Since lodging reduces the ability of the plant to take up water and nutrients, these fields may be more at risk for stalk rot issues towards the growing season. Improved stalk quality with fungicides is more common when foliar disease severity is high; although there are a few reports of reduced stalk rot with a fungicide application in the absence of severe foliar disease.
Applying a fungicide to flattened corn to reduce stalk rots later in the season is likely a high risk decision.