Report: Tar Spot Could Cause Up to 38 Bushel Per Acre Losses

The 2018 distribution map of tar spot infections confirmed in the US ( (Image source: Kleczewski et al. 2019. )

A recent publication from Extension experts at University of Illinois, Michigan State University, Purdue University and University of Wisconsin details how tar spot impacted corn hybrid yields in 2018. 

Per the report, fields that experience 40-50% tar spot severity on the ear leaf by R5/R6 saw a loss of 16.8 to 38.7 bu/acre.

Tar spot was first observed in the U.S. in 2015 in Indiana and Illinois. To-date there has not been much field screening for corn germplasm resistance to the disease, according to the report. Other controls for tar spot include fungicide application, crop rotation and tillage. 

In 2018, tar spot was confirmed in Illinois, Indiana, Florida, Iowa, Michigan, Ohio and Wisconsin. Farmers have reported yield loss from tar spot as severe as 60 bu. per acre. 

The 2018 research effort coordinated by the four universities was the first time yield losses to tar spot were documented in the U.S. and evaluated. In this research, all hybrids in the trials were susceptible to infection by the tar spot pathogen. The test plots were non-irrigated sites with a randomized complete block design with three to four replications. Hybrids were rated at the R6 (physiological maturity) growth stage by estimating the amount of tar spot, which ranged from 15% or less leaf area where the stroma developed up to 50% of the leaf area affected. 

The researchers report that field observations indicated maturity may have influenced susceptibility and yield impact during the 2018 epidemic, so they reported results breaking down yield impact by relative maturity.

Here’s a summary of how tar spot has been evaluated to effect yield by hybrid maturity: 

  • For every 1% increase in tar spot severity on the ear leaf, there was a 0.48 bushel per acre yield loss for hybrids with 92-106 day relative maturity
  • For every 1% increase in tar spot severity on the ear leaf, there was a 0.83 bushel per acre yield loss for hybrids with 107-114 day relative maturity

For a detailed state-by-state breakdown of the results, click here.

In the 2019 season, farmers in the states where tar spot has been observed in the past as well as adjacent states are encouraged to scout. 

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