Early Scouting For Tar Spot

Identifying tar spot can be a challenge, especially in its early stages. The timing of its arrival, along with recent hot, wet and humid weather that promotes the diseases spread, can increase potential damage from tar spot.

Where to start:

  • Scout fields where the disease has occurred before and fields where neighboring fields had tar spot.
    • Inoculum overwinters in corn residue and can survive extreme temperatures. Spores are dispersed via wind and rain splash.
  • Scout susceptible areas in cornfields where tar spot is more likely to be present.
    • This includes areas where leaves may stay wet longer due to early morning fog, such as river bottoms, low-lying areas and near windbreaks. Check field edges as well.
  • Go low
    • Start by checking leaves in the lower part of the canopy and work up. Look for small, raised, irregular-shaped black spots (1/16-3/4 inch) on the top-side of lower leaves.
  • Confirm tar spot
    • Tar spots are firm, mostly smooth, and do not rub off or break open.
    • Not all black spots are tar spot. Turn over the leaf and see if the spot is visible from the underside of the leaf. If it is, it’s more than likely tar-spot.

Tar spot has a 14-day incubation period between infection and symptoms so it’s difficult to spot and know when to spray.

If you or a neighbor had tar spot in the past, scout and spray early with a systemic insecticide and keep scouting in case another application is required.

The best management practice is to select tar spot tolerant hybrids from Latham Hi-Tech Hybrids. Contact your local RSM to see what hybrids are best in your area.

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