Conditions favorable for foliar diseases of corn
Rainy, humid weather has increased the likelihood of foliar diseases developing in corn in 2013. Gray leaf spot is appearing in the lower canopy of susceptible hybrids across Indiana, and northern corn leaf blight has also been observed in the lower canopy of fields in northern Indiana. The bacterial disease Goss’s wilt has been confirmed in popcorn and hybrid corn in northeast Indiana. Many fields across Indiana are currently at a younger growth stage than normal due to delayed planting, and therefore may be at greater risk for yield loss due to disease development.
Fungicides are available to manage foliar diseases such as gray leaf spot and northern corn leaf blight. Research in Indiana indicates that fungicides are most effective at preventing yield loss due to disease when applied at the tasseling to early silking (VT-R1) growth stage. Scouting fields around V14, or just prior to tassel emergence, can help determine the level of disease pressure in a field.
Recommendations for In-season Management of Gray Leaf Spot:
Iowa State University developed guidelines to determine when a fungicide may be necessary to prevent yield loss from gray leaf spot. These thresholds incorporate hybrid susceptibility ratings to gray leaf spot and disease levels prior to tasseling:
1. Consider a fungicide application if:
The hybrid is rated as susceptible or moderately susceptible AND 50 percent of the plants in a field have disease lesions present on the third leaf below the ear leaf or higher prior to tasseling. Please see the following video for identifying the area on the plant to scout for disease threshold decision-making: <http://www.youtube.com/watch?v=OCpwhvw9W_Y&feature=c4-overview-vl&list=PL1DCC2F39021955FF>.
2. Consider a fungicide application if:
The hybrid is rated as moderately resistant AND 50 percent of the plants in a field have disease lesions present on the third leaf below the ear leaf or higher prior to tasseling AND additional factors or conditions that favor disease development are present (residue present, favorable weather conditions)
Scout resistant hybrids for disease problems, but in general, fungicide applications to resistant hybrids are not recommended and will not consistently result in increased yield. For more information on gray leaf spot, please read Purdue Extension bulletin BP-056-W: <http://www.extension.purdue.edu/extmedia/bp/BP-56-W.pdf>.
Recommendations for In-season Management of Northern Corn Leaf Blight:
Currently we do not have thresholds available for determining when fungicide applications are needed for northern corn leaf blight, but we follow similar guidelines as gray leaf spot, in that fungicides will typically be more beneficial when applied to susceptible hybrids in high-risk fields for disease development, such as those that are in minimum or no-till. For fungicide recommendations, please see the updated fungicide efficacy table for management of corn diseases, which is developed by the national Corn Disease Working Group: <http://www.extension.purdue.edu/extmedia/BP/BP-160-W.pdf>. For more information on northern corn leaf blight, plese read Purdue Extension bulletin BP-084-W: <https://ag.purdue.edu/btny/Extension/Pages/ExtPubs.aspx>.
Recommendations for In-season Management of Goss’s Wilt:
Goss’s wilt is a bacterial disease, and therefore fungicide applications will not be effective at reducing disease development. There are several products promoted for in-season management of Goss’s wilt, however, Indiana research indicates that these products do not consistently reduce disease once symptoms are present on the plant. If symptoms of Goss’s wilt are suspected in a field, please remember to have the sample confirmed by a diagnostic lab. Fields with confirmed Goss’s wilt should be planted to a hybrid that is more resistant to Goss’s wilt in subsequent years. Tillage and crop rotation will also help reduce the amount of residue that can harbor bacteria for the next corn crop. For more information on Goss’s wilt, please read Purdue Extension Bulletin BP-081-W: <https://ag.purdue.edu/btny/Extension/Pages/ExtPubs.aspx>.
Before Deciding on a Management Strategy, Keep This in Mind:
The thresholds and recommendations available for in-season management decisions for any disease are not hard and fast rules. It is important to remember that disease severity can be unpredictable in Indiana, even when factors favoring disease are present. Before deciding on in-season management of any foliar disease, consider threshold guidelines, cropping practices, planting date, predicted weather conditions and economic factors, and manage expectations for what type of yield response a in-season fungicide/bactericide application will provide.