The concern is that we are starting to see diseases such as Northern corn leaf blight earlier than normal on corn this year. There have been reports of Gray leaf spot as well. Northern corn leaf blight symptoms will develop at lower temperatures (65 to 80 degrees F), when compared to gray leaf spot; whereas, Gray leaf spot requires warmer temperatures (75 to 85 degrees F) for disease infection to occur and, if conditions stay conducive, symptoms will develop after several weeks. Therefore, we are less likely to find Gray leaf spot in the Hughes footprint because it requires warmer temperatures for symptoms to develop.
|Gray leaf spot lesions|
Gray leaf spot typically appears first on lower leaves and can move up the plant, because this disease pathogen overwinters on residue. A wound is not needed for the infection of gray leaf spot or northern corn leaf blight disease. Northern corn leaf blight causes larger “cigar shaped” lesions (below), when compared to gray leaf spot symptoms (above), which are small, tan, and rectangular in shape.
|Northern corn leaf blight|
Northern corn leaf blight also overwinters within corn residue and in 2013 and last year, we saw a lot of Northern corn leaf blight in corn, so I fear there could be a build-up of inoculum. Northern corn leaf blight spores can blow into fields from very, long distances, then infect susceptible hybrids. Be sure to scout the entire field and not just field edges, as spores and infection may be more prevalent where spores are entering a field.
Based on disease scouting during pollination, disease pressure, and forecasted wet weather, the grower can make the decision if a fungicide application is needed.
If the disease is present, access disease pressure and continue to monitor disease spread, especially if wet weather persists. If 50% of the corn plants in the field are showing signs of this disease, on the third leaf below the ear or higher, for a period of time before and after corn tassel, you may want to consider a fungicide application.
When trying to decide if foliar fungicides are warranted, here are some questions you need to answer:
1.) What growth stage is the corn plant? Corn should be scouted for disease at pollination (2 weeks before and after tassel). Is your corn experiencing high disease pressure within this growth stage range? If so, yield loss could result.
2.) Have you had issues with corn leaf disease in the past or is there a high amount of residue in the field (corn on corn/no- till) that could be harboring the gray leaf spot pathogen? If so, you may be more at risk for gray leaf spot.
3.) Are disease symptoms beginning to appear near the ear of the plant (1 to 3 leaves below the ear) during pollination? This is a signal that disease could be a threat to corn yields.
4.) How susceptible is your hybrid? Every corn hybrid has a disease rating. Check the disease rating for your hybrid and if it is a low number, you may be more likely to use a fungicide if disease pressure is high.
"Do your other seed suppliers provide ratings for probability of needing to apply fungicides? At Burrus, we provide this information to save you time, if you to scout your most susceptible hybrids, and it they don’t need a fungicide application, you don’t have to scout every field." - Tom Burrus
5.) What is the future weather outlook? If wet weather is in the forecast, this may be another indicator that fungicides may be needed if your hybrid is susceptible and has high disease pressure appearing within at least half of the field.
For more information on Fungicide Efficiency of for Control of Corn Diseases, check out this chart from Purdue: https://www.extension.purdue.edu/extmedia/BP/BP-160-W.pdf