Tar spot Again

​Tar Spot of corn became an important problem in Northern USA corn belt states in 2021.  It has drawn the attention of several corn pathologists that summarized their observations of the past few years in Oct. 2020 publication of Plant Disease Vol.104, No. 10.  The main pathogen causing the disease is an obligate pathogen, meaning that it can not be cultured on artificial media.  This characteristic hinders rapid investigations of the disease.  Its spread appears to be related to temperatures and humidity, again making intense annual studies difficult because of inconsistent occurrence.
 
The spores of this fungus are easily spread in wind, apparently for several miles.  Spores can overwinter in corn debris, infect corn pants at all stages and set up spread within and outside the field if humidity is relatively high for prolonged times.  There appears to be no complete resistance among today’s corn hybrids but a range of susceptibility from those that seem to have few lesions to those that are completely killed before normal black layer.  Uneven and somewhat unpredictable spread of the disease has complicated the evaluation of disease resistance.  We at PSR attempted to rate hybrids for resistance among hybrids submitted by seed companies for evaluation of resistance to other diseases.  In 2021, our nursery seemingly suddenly showed most plants with Tar Spot.  The black spots on most leaves in many hybrids often interfering with other disease symptoms.  Leaves appeared to die prematurely.  Those with mostly green leaves were tentatively appearing as more resistant to tar spot with the potential that relative maturity was complexing the ratings.  Checking those that had green leaves 7 days later, remained green, consistent with actual resistance and not maturity as the main factor in reaction to this disease.
 
This 2021 small nursery was isolated from other corn for at least a mile.  We saw tar spot in the field 2 years ago but very little last year, supporting the hypothesis that this year’s infection was mostly from spores spread from some distance.  Although our field has been in corn for several years, it had very little of last year’s debris before planting.  
 
This disease was identified in corn in Mexico in 1905, but, I think, only identified in corn in the Midwest area in 2015. Hopefully more knowledge will be gained from the 2021 experience, and we will find more on how to protect corn.  Corn genetics, weather, pathogens and agriculture are in constant change resulting in new disease occurrences.