Foliar Fungicide Decisions for Late-Planted Crops

July 17, 2019 2:15 PM
Blog Post

A new article recently released through the Crop Protection Network (CPN) discusses how to make fungicide decisions in corn and soybean, given the delayed planting scenarios across the Midwest. The article takes into account yield potential at various planting times throughout the North Central region, as well as the previous wet and cool conditions when evaluating for a positive return on investment, in 2019. Also considered in the article is the potential damage that frost can do to overall yield, which also impact your return on investment. 

Of particular interest to Iowa farmers, CCAs and crop managers is the threat of diseases during reproductive growth stages, as much of Iowa's corn and soybeans are entering those stages: 

"Disease development prior to grain fill will have a greater impact on yield reduction than when disease develops later in the cropping season. In a typical year, crops are usually at a more mature stage of development when diseases become prevalent, and thus there is less impact on yield. This year however, with delayed planting conditions and reduced growing degree day accumulation, corn and soybean development in many areas is two to four weeks behind normal. Disease development prior to or during early grain fill could significantly affect yield, thus scouting fields for yield limiting diseases prior to reproductive stages will be crucial in determining whether or not to apply a fungicide. 

Diseases like gray leaf spot and northern corn leaf blight (NCLB) have already been confirmed on corn prior to tassel in some areas. Gray leaf spot is favored by warm temperatures, high humidity, and heavy dews. NCLB is favored by cool temperatures and wet leaves. Both of these diseases typically start in the lower canopy, and if favorable conditions continue, they can quickly spread to the upper canopy and be very damaging. Similarly, the development of tar spot before R2-R3 may cause considerable damage and yield loss. University research indicates that fungicide applications occurring at tasseling/silking (VT-R1) are most effective at minimizing the impact of gray leaf spot and NCLB and protecting yield in susceptible hybrids. Southern rust could also be problematic in late-planted corn in 2019. This disease moves northward each year and may impact yields in late-planted corn. Fungicide applications prior to milk stage (R3) may be needed to limit yield impact in certain cases. 

Frogeye leaf spot and Septoria brown spot are two foliar diseases that are prevalent in wet and humid weather. Septoria brown spot generally is not an economic threat to soybean, but may cause yield reductions if infection reaches the upper canopy. It is important to begin scouting soybean fields for frogeye leaf spot and Septoria brown spot around beginning flower (R1) to help make a foliar fungicide decision. Generally, fungicide applications for management of frogeye leaf spot on susceptible varieties are made at beginning pod (R3). It is not likely that foliar fungicide applications to soybean prior to reproductive stages will be economical. If using foliar fungicides to manage these diseases, use products that contain multiple fungicide classes, as resistance to strobilurin (quinone outside inhibitor) fungicides has been observed in multiple states by both the frogeye leaf spot and the Septoria brown spot fungi.   

In the northern soybean-production regions (I- 80 and north) white mold (Sclerotinia stem rot) may be of concern. All stages of soybean are susceptible to infection by the white mold fungus, but most infection occurs through open flowers during periods of cool and wet weather. If we continue in a cool, wet weather pattern this year, late-planted soybeans will flower further into the growing season due to less accumulated growth days. Plants will be more susceptible to infection for a longer period of time when the weather is very conducive to disease."

Whether or not to use a fungicide will have to be considered on a field-by-field basis. The article goes further in depth on what field conditions have the potential for higher returns on investments. CPN also have valuable resources in the form of disease encyclopedias, disease management publications and CCA exams for continuing credits — all for free.