The majority of carbohydrates needed for grain fill in corn come from the foliage above the ear leaf, and the one or two leaves below the ear leaf. Experts estimate that nearly 90% of carbohydrates come from these tissues. Consequently, ensuring that these tissues are healthy and able to function optimally is an important factor in maximizing yields. This is a major reason that foliar fungicide applications to corn around the VT to R1 growth stages result in the best yield protection; however, the magnitude of the response can differ. Foliar fungicides can protect the foliage from foliar fungal pathogens that infect green plant tissues can reduce the total availability of carbohydrates for approximately a three-week period after application.
The weather before and during grain fill is a key factor in driving yield responses to fungicides. Air temperature, rain events, and relative humidity drive pathogen development and spore release. What weather is encountered can facilitate the development of some diseases over others in each location. For example, northern corn leaf blight (NCLB), common rust, and tar spot develop under cooler temperatures, whereas grey leaf spot (GLS) and southern rust thrive in warmer conditions. However, when it comes to moisture, high humidity environments are a common factor for foliar disease advancement and potential yield loss.
The more prolonged the elevated humidity in the canopy, the greater the likelihood is that foliar diseases will continue to develop. Consequently, yield response to fungicide applications tends to be higher under high moisture environments, where rainfall or irrigation is prevalent during the late vegetative to early reproductive stages. Differences in when high moisture occurred in relation to crop development is a major reason why we see varying yield responses to VT fungicide applications across the region. Some areas may receive higher than average rainfall during the late vegetative and/ or through the reproductive stages of development receive while others remain dry.
A second important factor that plays a major role in crop response to a foliar fungicide is field history and production practices. Diseases such as tar spot, NCLB, and GLS overwinter in the Midwest in and on the previous years infected corn residue. Therefore, fields with a history of these diseases, in continuous corn production, and minimal or no till systems have a greater likelihood of seeing a pronounced yield response to a fungicide as opposed to rotated, tilled acres. Basically, the more surviving corn residue that was infected with the pathogen in the field, the more potential pathogen inoculum is available to start a disease epidemic if the appropriate weather occurs.
The last component that impacts response to fungicides is the susceptibility of the crop to common, resident diseases. Across the region, GLS is by far the one of the most common disease that drives fungicide applications in corn. Hybrids rated susceptible or moderately susceptible to grey leaf spot tend to be more responsive to fungicide applications and result in a positive yield response compared to moderately resistant or resistant hybrids. In fact, university recommendations will abstain from encouraging fungicide applications on resistant hybrids because grey leaf spot severity will most likely not build to damaging levels. However, in situations when grey leaf spot is not the only fungal disease with the potential to damage yields, a fungicide application may still be of benefit. This is due in part to the fact that there are other diseases that can potentially build and develop, and high yielding hybrids likely are not resistant to all of the various foliar diseases that may be encountered, or few hybrids are available with resistance to the disease in question. A good example of this is in areas impacted by tar spot or southern rust, where a fungicide application during the early reproductive stages provides significant yield protection in areas with little or no incidence GLS or other fungal diseases. For more information on disease management including fungicide and hybrid selection contact your Southern States Cooperative Agronomist.