Ag Weather Forum              12/09 09:33

   Forecast Cold Wave Has Variable Impact on Corn Diseases

   Despite arctic cold, tar spot has adapted to survive Midwest winters.

Bryce Anderson
Ag Meteorologist Emeritus

   One of the effects of the oncoming series of cold waves in the central 
United States during the next five to seven days may be the effect on leftover 
remnants of corn diseases which likely caused some yield loss along with extra 
expense for control measures during the 2025 growing season.

   These costs can be significant. Top-quality fungicides have an average cost 
of about $20 per acre. Application charges are about $10 per acre, so the total 
cost for corn fungus control is about $30/acre. These charges have shown little 
change during the past two to three years. (Details courtesy of Minnesota-based 
farm management analyst Kent Thiesse.)

   During this past season, the impact of disease, either in the cost of 
control or the effect on yield, were conversation topics across the U.S. Corn 
Belt. On a national level, disease impact may have been low; after all, U.S. 
corn production is estimated at a new record of 16.8 billion bushels. But on a 
local and area level, disease presence was a problem -- one with a price tag.

   So, how will this cold period affect those microorganisms still out there 
following harvest? As with many issues in crop production, the impact varies 
depending on the particular disease. A look at two of the headline corn 
diseases of this past season, southern rust and tar spot, illustrates that 
variability.

   In the case of southern rust, central and northern U.S. winters are 
detrimental to its spores and keep the disease as an annual threat. For 
example, "This pathogen does not overwinter in Nebraska. Disease occurrence in 
Nebraska is dependent upon wind dispersal of urediniospores from southern 
states in early to mid-July," noted a University of Nebraska-Lincoln extension 
bulletin by plant pathologists Jim Stock and Tamra Jackson-Ziems. So, for this 
disease, summertime temperature, humidity and south winds will be notable 
features for its re-occurrence.

   Another major feature in this past season's corn disease spectrum, tar spot, 
shows a more resilient characteristic. Tar spot, which notably showed up in the 
Midwest about 10 years ago, is a disease which goes dormant during the winter 
and thus survives on crop residue in extreme cold. The tar spot pathogen has 
adapted to the point where viable spores have been noted all the way into 
Canadian corn fields along with the U.S. Corn Belt.

   One of the main features of crop diseases repeatedly identified in research 
projects is the ability of some pathogens to cope with adverse conditions. That 
shows up again here; the arctic cold may bring some natural corn disease 
control, but the cold will not wipe the slate -- or the fields -- clean of the 
presence of every pathogen ahead of the 2026 growing season.

   Bryce Anderson can be reached at bryce.anderson@dtn.com




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