Just ahead of the 2025 growing season, many in the crop protection industry expected tar spot to be the primary disease concern in corn. Instead, southern rust emerged as the dominant threat across much of Iowa, forcing growers to quickly evaluate their management plans as the season unfolded.
Todd Cogdill with FMC says even those watching the disease closely were surprised by how quickly southern rust took over.
Instead of tar spot, southern rust became the disease that separated growers who stayed disciplined with their fungicide plans from those who chose to wait. Cogdill says that the difference showed up clearly as the season progressed.
Southern rust is not a disease Iowa typically plans for. It does not overwinter locally, and outbreaks this far north are usually uncommon. However, Cogdill says a unique set of weather patterns lined up in 2025 that allowed the disease to move north rapidly and establish itself in corn fields.
Looking back on the season, Cogdill says the key takeaway is not trying to predict which disease will dominate from year to year, but recognizing the early indicators that can shift disease risk quickly. For Iowa growers, 2025 served as a reminder that disease pressure can escalate fast, and having a fungicide plan in place before symptoms appear can make a meaningful difference in protecting yield.
