Corn Fungicide Considerations
BY Dairyland Seed Agronomy Team
As corn continues to mature into late vegetative stage and in some cases, tassel, fungicide application becomes a common topic of discussion. Disease infection of any kind requires three things often referred to as the disease triangle. The following is not a comprehensive list of corn foliar diseases, but these are the most common for our region.
- Pathogen: fungus, bacteria, or virus that causes disease.
- Host: crop susceptible to the pathogen
- Environment: weather, moisture, and soil conditions favorable to cause disease infection and lead to symptoms.
Tar Spot
Identification:
Small black spots that occur on the top or bottom of a corn leaf that cannot be removed through rubbing or scraping the spot. These spots called “stromata” can be found on leaves, stalks, and husks of corn plants. Tar spot infection can begin on any part of the corn plant. It was originally thought to start at the lower part of the plant, but it has been noted to start at the top of the corn plant as well.
Environment:
Leaf wetness has shown to be favorable for the spread of tar spot. Areas of high humidity, steady rainfall/irrigation and moderate temperatures are environments that favor tar spot infection. Infection occurs between V11 (pre tassel) through R5 (dent).
Treatment:
The first line of defense is planting hybrids that show high tolerance of tar spot. Once symptoms occur, it is important that the field be treated as soon as possible. Tar spot is a polycyclic disease, which means that infection will continue to worsen as long as the environment stays favorable. Fungicides containing a both a Group 3 DMI Triazole and Group 11 Qol Strobilurins should be applied between V11 and R4 for the best control. Specific timing depends on environment and if/when symptoms occur. It is important to note that fungicide applications do not cure a corn plant of tar spot. Fungicide only pauses further infection until the residual activity of the fungicide ends which is around two weeks.
Grey Leaf Spot
Identification:
Grey leaf spot infection will appear on lower leaves and work its way up the canopy. The lesions of GLS are rectangular or blocky and two inches in length. Early lesions will be interveinal or between the veins of the corn leaf but as the disease progresses, the lesions will join and can cover the entire leaf surface. The smaller lesions present early can be mistaken for multiple other foliar diseases. When a leaf is held up to light, a GLS lesion will have a yellow halo around it.
Environment:
Warm and humid conditions and periods of fog are conducive for GLS infection. The GLS pathogen overwinters in corn residue so corn on corn acres may be at a higher risk. Later planted corn is at higher risk as leaf area is destroyed prior to or during grain fill when infection occurs.
Treatment:
Hybrid tolerance to GLS is the first line of defense. If infection is noted or the environment conducive to infection, an application of fungicides containing a both a Group 3 DMI Triazole and Group 11 Qol Strobilurins may be warranted. Best application timing is VT-R1.
Northern Corn Leaf Blight
Identification:
Lesions of NCLB can first be found on lower leaves of the corn plant and will progress up the plant. Lesions are one to six inches long, elliptical or cigar shaped, and are grey or tan. Spores can be seen in the lesions which can give the lesion a grey or dirty appearance.
Environment:
Heavy dews with leaf 6 to 18 hours of leaf wetness, and moderate temperatures between 65°F- 80°F favor infection. Prolonged periods of overcast skies that prevent leaf drying can also increase chances of infection and reinfection. Later planted corn and areas of corn on corn are most susceptible.
Treatment:
Hybrid resistance is first line of defense. If lesions found on the third leaf below the ear on 50 percent of plants, application of fungicides containing a both a Group 3 DMI Triazole and Group 11 Qol Strobilurins show the best control of NCLB.
Summary
There is no substitute for scouting, having a handle on the severity of disease pressure in a field help to make timing decisions easier. Timing and specific fungicide products to use are the two most common questions debated when talking about disease control. University and our own PAR research says that the best timing for fungicide application is VT or 80 percent tassel emergence. This is based on multi-year data and every year is different. Generally, if conditions for disease infection are conducive at VT then an application of fungicide at this time makes sense. We have also seen where disease infection was not conducive at VT and an application at brown silk or R2 was more beneficial.
Each year varies, and if you have questions about fungicide applications and regional foliar disease progression in your area contact your local Dairyland Seed Dealer, DSM or Regional Agronomist.
Brian Weller
Western Region
507.456.3034
Rod Moran
Western Region
507.456.3034
Dan Ritter
Central Region
219.863.0583
Branden Furseth
Northern Region
608.513.4265
Mark Gibson
Eastern Region
260.330.8968
Amanda Goffnett
Eastern Region
989.400.3793
Ryan Mueller
Eastern Region
989.400.3793