Agronomy •  2022-03-23

Tar Spot

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What is it

First discovered in the United States in 2015, tar spot (Phyllachora maydisis) is a fungal disease of increasing agronomic importance in corn. As of 2020 it has been confirmed in Minnesota, Iowa, Missouri, Illinois, Wisconsin, Indiana, Michigan, Ohio, Pennsylvania, Florida and Ontario. 

Significant yield loss can occur under favorable environmental conditions for disease development. Tar spot can be identified by small black and circular lesions. They are slightly raised bumps that can be felt on affected plant tissue. Lesions can appear on leaves, husks or stalks and are often surrounded by a light tan-colored halo. Lesions are caused by fungal structures called stromata that cannot be rubbed off affected plant tissues.

Symptoms appear most frequently from silking through late grain fill. Under severe infestation, where most of a field shows 50% severity on the ear leaf during grain fill, significant yield losses of 20-60 bu/ac have been reported. 


Corteva scientists have been studying tar spot and screening hybrids to develop a rating scale to communicate a hybrid’s level of tolerance.  Significant genetic response to tar spot has been observed and quantified by Corteva. Brevant products in 93 CRM and later have been assigned a tar spot score.  


Tar spot management is a function of both in-plant tolerance and fungicide application. Under high disease pressure, crop response to fungicide application will be influenced by genetic tolerance (Table 1). 

In Plant Tolerance

Level of Control

Crop Response

Score =/< 4 


Fungicide treatment unlikely to maintain plant health  

Score 5 


Hybrid is viable when treated.  Premature death still likely.  

Score =/>6 


Hybrid benefits from fungicide application and will retain most yield and plant health 

Table 1. Crop response to fungicide application under high disease pressure by Brevant tar spot score

Scouting is recommended to determine the presence and severity of tar spot to make fungicide application decisions.  Areas with high inoculum and high pressure may benefit from more that one fungicide application.  

There are currently two products registered for tar spot in Canada; Veltyma® (Mefentrifuconazole & Pyraclostrobin) and Delaro® Complete (Prothioconazole, Trifloxystrobin & Flupyram).

Corteva’s large network of PKP and IMPACT research plots will enable Brevant to characterize more products in 2022 as the pathogen is observed in Eastern Canada.