Iowa State University Extension field agronomists around the state, as well as ISU Extension plant pathologists Daren Mueller and Alison Robertson on campus at Ames, are continuing to receive questions from farmers and others about Northern corn leaf blight, Goss's wilt and whether or not it will pay to spray corn this summer with a fungicide. Robertson and Mueller provide the following information and guidelines to help farmers and crop consultants make the correct decision for managing foliar diseases.
Parts of rural Iowa are abuzz about fungicide use to manage some emerging diseases, and we have received several questions about the basics of fungicides. A quick reminder, APS PRESS (American Phytopathological Society) recently published a book geared towards farmers and agronomists on the basics of fungicides. It is available in print, Kindle version, and online through the Plant Management Network. The Plant Management Network also has several webinars on fungicide use for managing leaf diseases on corn and soybean. Note: membership may be required to access some information on Plant Management Network.
How can you tell when it will pay to spray a fungicide?
Is there a threshold for Northern corn leaf blight (or other diseases for that matter)?
The answer is "No." Although research attempts have been made to establish a threshold, they have not been successful likely because the disease triangle drives disease development and even the best pathologist has little say regarding the prediction of upcoming weather conditions. Nevertheless, here is a stab at how we may approach deciding on spraying a fungicide:
1) Scout from your office
* Check past, current and future weather patterns. More rain equals more disease and temperatures in the mid-70s to 80s favor most pathogens
* Check relevant newsletters (or Twitter, blogs, etc.) from ISU Extension, seed companies, etc. Find tweets, articles on diseases that are showing up. (e.g., Northern corn leaf blight article earlier this week; Twitter @dsmuelle or @alisonrISU)
* Check susceptibility of hybrid/cultivar in seed company literature.
2) Scout at-risk fields. If the above three bullet points indicate a risk of disease, get out and scout those at-risk fields. It is important to look in many different places across the entire field since the environment may differ across the field, e.g. river bottom to the crest of a hill.
3) You many need to spray fungicide. If you find ANY of foliar fungal diseases (e.g., gray leaf spot, northern corn leaf blight, northern corn leaf spot, southern rust on corn; frogeye leaf spot, soybean rust on soybean) in these at risk fields, you should spray. Make sure to properly identify the disease. You don't want to be spraying a field that has Goss's wilt with a fungicide. The fungicide will not control Goss's wilt.
4) Is your corn hybrid supposed to be resistant? If you have a "resistant" hybrid and disease is showing up, collect some leaves and send them to the Plant and Insect Diagnostic Clinic (call you sample to the attention of: Alison Robertson and Daren Mueller). If it is northern corn leaf blight you should probably consider spraying the field with a fungicide application because it could be a race that defeats the resistance gene in that corn hybrid. If it is gray leaf spot, a fungicide application is probably NOT necessary on a resistant hybrid. Gray leaf spot lesions on resistant hybrids tend to be smaller and develop slower than those on more susceptible hybrids.
5) If no disease one week, be sure to scout the next week. If there is no disease showing up in a corn or soybean field, consider delaying spraying one week until you scout the field again. If no disease occurs by brown silk for corn or middle of pod fill for soybeans, chances of getting a return on your investment (ROI) for a fungicide application are not as good.
Other considerations when deciding to apply fungicides:
* Research done by several pathologists across the Midwest suggests that return on investment for fungicide application is more likely when conditions are favorable for disease.
* Resistance to strobilurin fungicides has been reported in the fungi that cause frogeye leaf spot, and Cercospora leaf blight in several states. Although no resistance has been reported in Iowa or in corn pathogens across the Midwest, we continue to monitor fields.