Despite all the rain in West Tennessee, little foliar disease has been seen in corn fields; but as some of the earliest planted fields are at tassel  – it is time to make decisions on fungicides.

As always, for a disease to be a PEST and require a fungicide to protect yield, the main factors that have to be considered can be remembered using PEST as an acronym for them:

• Pathogen presence – scouting is key as well as records on what was in the field previous seasons. Continuous corn or soybean can increase disease risk.
• Environment – there has to be the right conducive environmental conditions for the pathogen to infect and disease to further develop. Usually the same weather the crop will do well in, so do the diseases – good moisture and heat.
• Susceptible host – many hybrids and varieties that have high yield potential are also susceptible to various foliar pathogens, while others have build in resistance or tolerance into the genetics of the hybrids/varieties. It’s important to know this going into the season to help plan you disease management strategy.
• Timing – the previous 3 factors have to occur early enough and at a growth stage that will impact yield for disease to be a PEST and require a fungicide to protect yield. Many times I still have disease develop in my research plots and rate differences in treatment efficacy, but only to find no impact on yield occurred due to the disease developing late enough in the season and at a later growth stage where the disease does not have enough time to rob any yield.

These are the main factors that need to be assessed when deciding of fungicide applications in corn, as well as the economics behind the commodity price, input cost, and potential yield of the crop.

Outside of knowing what diseases you may have in your field currently or in previous seasons, certain pathogens have to be blown into the area from more southern climates each year – such is the case with Southern corn rust. Hence, plant pathologists monitor its presence each season and report  it on the Crop Protection Network – Southern Rust of Corn Map (scroll down to see map after clicking on link). At the top of the map page, a table shows the crop stages when Southern rust is first detected and if fungicide application could possibly be beneficial (from tasseling to R3).

While Southern rust has the highest potential to cause yield loss and require a fungicide application, it is dependent each year on wind currents and storms for it to be brought up from the south in time to cause yield loss. Spore disposition to symptom development takes about 7 to 14 days, depending on the conditions. Right now Southern rust has been reported in Washing, Issaquena, and Leflore Counties in MS and Geneva County in AL.  Another foliar disease of corn that could require a fungicide to protect yield is grey leaf spot, which can overwinter on corn debris as well as blow in from other fields/areas.

So scouting fields for Southern rust and grey leaf spot are needed to help make fungicide decisions – included are images of Southern rust, not be confused with common rust in corn that does not usual require a fungicide to protect yield. While rusts can be found in middle to upper canopy leaves, grey leaf spot begins in the lower canopy as smaller lesions, expanding into rectangular lesions and moving up into the middle and upper canopy throughout the season. Make sure to check your hybrid to know its tolerance to Southern rust and especially grey leaf spot, as some hybrids will have high resistance to grey leaf spot.

So we’ve covered pathogen identification, wet/humid, hot weather will favor both major diseases (too much heat and drought can negatively affect them); and know your hybrid’s susceptibility to these diseases. For timing, tasseling to brown silks (VT-R1) has had the most consistent efficacy for managing these diseases, when present, as well as protecting yield (when called for). From both regional data (table below) and TN data (from 2022-2024 trial data from Milan, TN mean yield for fungicide applied at V10 = 206 bu/a, VT-R1 = 209 bu;a, and R3 = 202 bu/a), standard VT-R1 or VT-R2 are the target for fungicide application when an application is called for (disease risk/presence, conducive environment, and susceptible hybrid).

Lastly, the economics … while it is simply put in this article, there are economists I will be continuing to work with to better utilize my data in economic models. Below I’ve inserted a simple ‘break-even scenario’ of the bu/a that has to be protected/gained from a fungicide based on corn price and application costs, to break-even on the input. Additionally, the Crop Protection Network has a Corn Fungicide ROI Calculator, where both expected yield, commodity price, and cost per acre of different fungicides can be adjusted to run a model providing back a breakeven probability and expected net benefit/acre – be mindful this is based on data from from university uniform corn fungicide trials conducted across 19 states and Ontario, Canada between 2019 and 2022. Primary diseases in this data set were tar spot and southern rust. Diseases, such as gray leaf spot, northern corn leaf blight, and others, were observed at lower frequencies in this dataset.

Based on the disease you are trying to manage, fungicide product selection should be based as well as the economics. For this, there are multiple tools that you can use from the Crop Protection Network: the Fungicide Efficacy Tool (where you select your crop and disease) as well as fungicide efficacy table. From field trials, partially supported by the Tennessee Corn Promotion Board, conducted at the Milan AgResearch and Education Center from 2022-2024 – in a continuous no-till corn production field (>15 years of corn), under irrigation and planted to a highly susceptible grey leaf spot hybrid – 4 fungicides were compared to non-treated check and while the yield differences were not statistically significant (p-value = 0.16), numerically all products had greater yield than the non-treated check (figure below). In general, in my trials at the Milan AgResearch and Education Center I have a disease nursery set up with grey leaf spot – so if yield response from a fungicide is to be had, I will get it there, and I do not consistently see the yield statistically greater with a fungicide compared to non-treated checks; and with this year’s tight margins if a field does not have good yield potential and does not meet when disease is a PEST factors, money can likely be saved by not putting out a fungicide in corn this year.