As we turn the calendar to summer, it’s time to start thinking about foliar diseases in corn and foliar fungicide applications. As you make fungicide decisions this year, keep these considerations in mind:
1. Scout your fields. This will help determine what disease or diseases (if any) are present. Remember the disease triangle, meaning in order for disease to surface, three components must occur: the pathogen must be present, the hot must be susceptible, and the environment must be favorable for infection and disease development.
2. Identify diseases(s) you see in the field. While most of our foliar corn diseases are caused by fungus, there are some diseases caused by other pathogens, such as bacteria or a virus.
In Iowa, two bacterial diseases that can be confused for fungal diseases are bacterial leaf streak and Goss’s wilt. A fungicide will not control a bacterial disease. Feel free to reach out to your local Iowa State University Extension field agronomist if you aren’t quite confident on the disease identification.
3. Keep your eyes open for corn stunt. This disease has not been found in Iowa, but it has been found in states to the south, including Missouri.
Corn stunt is transmitted to corn by corn leafhoppers. The responsible pathogen is a bacterium. However, other pathogens, including a phytoplasma and two viruses, also can be transmitted with the bacterial pathogen.
Infected corn plants may show symptoms of small chlorotic stripes at the base of the leaves, and over time, the leaves will become reddish or reddish-purple. Additionally, look for shortened internodes and smaller ears that don’t fill properly. If you suspect you may have corn stunt, reach out to Alison Robertson, ISU Extension plant pathologist, at alisonr@iastate.edu.
4. Consider the weather outlook. This goes back to the disease triangle mentioned above, and specifically favorable environmental conditions. While favorable conditions may have occurred for a disease to occur, we may not see the disease progress if the environment changes.
For instance, if it turns hot and dry, we are less likely to see diseases continue to develop, compared with when we have more moderate temperatures and leaf wetness due to rain, dew, humidity or irrigation.
On the other hand, if we continue to have a favorable environment, a disease could really take off and cause a more significant impact on yield. One new helpful resource that takes into consideration the weather/environmental conditions is the Crop Disease Forecasting Tool on the Crop Protection Network. Currently, it includes tar spot and gray leaf spot in corn, as well as white mold and frogeye leaf spot in soybeans.
5. Note disease incidence and severity. While we don’t have specific economic thresholds, it is important to note the incidence (number of plants with symptoms) and severity (amount or percent of leaf tissue) of disease in the field. This guideline also includes where you see it in the corn canopy and the growth stage of the crop. The ear leaf and upper corn canopy are the leaves we really want to protect.
6. Pay attention to VT to R2. This is the general window we see most foliar fungicides applied. However, for gray leaf spot, it may be more advantageous to spray earlier based on research ISU has done looking at application timings and fungicide products. While a V12 fungicide application usually reduces GLS better than an R1 application, applications at VT-R2 still are recommended for GLS.
In contrast with tar spot, we tend to see a greater reduction in disease severity with an R1 application. Regardless of the fungal disease present, a fungicide application after R3 is not likely to provide a return on investment. The Crop Protection Network has a Corn Fungicide ROI Calculator that can be used to help with fungicide decision-making.
For a list of fungicides effective against diseases like tar spot, gray leaf spot and northern corn leaf blight, the publication “Fungicide efficacy for corn diseases” is available from the Crop Protection Network. This publication is updated annually by corn pathologists across the U.S. and Ontario, Canada.
7. Don’t forget fungicide resistance is real. There is documented resistance with the pathogens that cause frogeye leaf spot and Septoria brown spot to the quinone outside inhibitor or strobliruin fungicides.
Moreover, fungicide resistance has been documented in pathogens that cause respiratory infections in humans. To help reduce the risk of fungicide resistance from developing, it is important to use integrated pest management, mix and alternate fungicides (use multiple effective modes of action or Fungicide Resistance Action Committee codes), and follow label directions.