STRATEGIC FARMING: LET’S TALK CROPS! SESSION TALKS WHETHER FOLIAR DISEASES OF CORN WILL CONTINUE TO THREATEN MINNESOTA’S YIELD POTENTIAL
On March 4, Dean Malvick, UMN Extension corn and soybean plant pathologist and Ryan Miller, UMN Extension crops educator joined UMN Extension crops educator Liz Stahl to talk about whether the foliar disease pressure experienced by the 2025 Minnesota corn crop is a ‘new normal’. This was an episode of the 2026 Strategic Farming: Let’s talk crops! webinar series.
Fungal diseases of corn made considerable news in southern Minnesota in 2025, with tar spot and southern rust epidemics occurring in fields. While the fungus that causes tar spot is now endemic to many regions of Minnesota, the fungus that causes southern rust cannot survive the winter. Spores from southern rust epidemics to the South need to travel into Minnesota early enough for yield potential to be at risk. With the orange rust pustules (an eruption of spores through a leaf’s epidermis) on upper leaf surfaces characteristic of southern rust evident as early as mid-July, the periodic rains, warm temperatures and high humidity combined to create a southern rust epidemic in southern Minnesota, resulting in significant yield losses in some fields.
While tools have been developed to help predict tar spot risk using weather data, ground-truthing has found that models can overestimate risk in Minnesota. Malvick shared that although considerable effort has gone in to understanding and learning to manage tar spot since its arrival in Minnesota in 2019, “we still have a lot to learn about the specific conditions needed for tar spot to develop.” This and the fact that both tar spot and southern rust fungi can cause multiple cycles of infection throughout a growing season mean that there is no substitution for scouting corn fields and making a timely fungicide application should one be warranted.
Working in southeast Minnesota where tar spot has become an endemic risk to corn production, Miller worked over the last three growing seasons with a local hybrid seed company (Wyffels) to research corn fungicide timing for tar spot (and in 2025, southern rust) management. Hybrid selection is helpful for tar spot management tactics as well. Miller found that even in a lower tar spot-pressure year, a foliar fungicide application at tasseling to one of the hybrids resulted in significantly higher yield compared to untreated corn. Finding that the combination of scouting and a fungicide application at tasseling when disease is present can protect yield potential, Miller identified additional risks that come with later timed or sequential applications, “4 to 5 percentage points higher grain moisture and the additional expense that comes with two applications; it adds risk.”
The benefit to attending Strategic Farming webinars live is that the presenters answer audience questions. For those that missed this session, it is now available to view on You-Tube at z.umn.edu/Strategic-FarmingRecordings. For more information and to register to attend other weekly session through the end of March, visit z.umn.edu/strategic-farming.
