Tar Spot can cause severe yield loss.
Like other corn diseases, the pathogen causing tar spot overwinters in infested corn residue. When conditions such as high relative humidity and prolonged leaf wetness are present the likelihood of tar spot is greater.
Tar spot appears as small, raised, black spots scattered across the upper and lower leaf surfaces. These spots are ascomatum (fungal fruiting structures). If viewed under the microscope, hundreds of sausage-shaped asci (spore cases) filled with spores are visible. When severe, ascomatum can even appear on husks and leaf sheaths.
Tar spot is sometimes confused with common and southern rust. Common and southern rusts’ orange-red spores turn to black spores, resembling tar spot. These spores can be scraped away from pustules on the leaf surface, tar spot can not.
Practicing residue management, rotation, and staying clear of susceptible hybrids may aide in avoiding development and severity.
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