Crown rot developed into a bigger problem than Iowa State University extension agronomists anticipated during the 2026 growing season, according to Meaghan Anderson, who tracks field conditions across Iowa for ISU Extension.

Anderson, speaking with Brownfield, described overall disease pressure in the state as a mixed bag for Iowa farmers throughout 2026 — with some pathogens arriving too late in the season to matter much, and others, like crown rot, proving more significant than field scouting earlier in the year suggested.

The pathogens that grabbed attention late were southern rust and tar spot. "Southern rust and tar spot were two of the bigger ones that came in really late," Anderson said. "Like, so late that I question whether or not they had any effect" on final yields.

That timing distinction carries weight for growers deciding how to budget fungicide applications in future seasons. A disease that arrives after the crop has largely filled means a late-season fungicide pass may return little, while an underestimated disease such as crown rot — which affects the plant's structural base rather than its leaves — sits outside the window where a foliar fungicide would do any good at all.

Crown rot's emergence as a bigger-than-expected issue in 2026 matters for Iowa growers because the disease works below the canopy. By the time symptoms are visible — lodging, premature senescence, compromised stalk integrity — management options have largely closed. That shifts the economic question from in-season treatment toward hybrid selection, residue management and rotation decisions made well before planting.

For an agronomist with ISU Extension to flag crown rot as exceeding expectations is itself a data point worth noting. Extension field observations across Iowa anchor much of what growers and crop advisors use to calibrate risk from year to year, and a season that resets expectations on a soil-borne and residue-borne disease changes how agronomists will advise scouting priorities next spring.

The 2026 pattern Anderson describes — significant late flights of southern rust and tar spot with questionable yield effect, alongside a crown rot problem that grew beyond forecasts — underscores the difficulty of translating disease presence into bushels lost. Disease identified in a field is a condition observation, not a harvested result, and the two can diverge sharply depending on when infection arrives relative to crop growth stage.

Anderson's assessment that southern rust and tar spot came in "so late" that she questions whether they had any effect is the kind of caution that matters when growers evaluate whether their fungicide programs paid for themselves. Late infections are easy to see and easy to attribute damage to; separating what a disease actually cost from what it merely looked like it cost requires comparing treated and untreated fields at harvest.

Going forward, Anderson's read of the 2026 season suggests Iowa growers and their advisors will be watching crown rot pressure more closely in 2027, while weighing whether late-season leaf diseases justified treatment at all in the year just finished.