Fungicide-resistant frogeye leaf spot confirmed in North Carolina soybeans

Resistance of the frogeye leaf spot fungus (Cercospora sojina) on soybean to strobilurin type fungicides has now been identified in North Carolina, US.

Resistance of the frogeye leaf spot fungus (Cercospora sojina) to strobilurin fungicides (FRAC code 11; Headline, Quadris, Evito, and Approach) has been reported from the Mississippi Delta and other areas in the past several years.  Most finds of resistant fungus strains have been confined to the Mississippi river valley as far north as Illinois.

Some growers are reporting that management of frogeye leaf spot with fungicides in North Carolina has been poor this year.

This may be a result of: 1) applications made in an untimely manner, 2) applications of a less than labeled rate, and (or) resistance of the fungus to the fungicides used.  If an application of a strobilurin type fungicide has been made, then a traizole fungicide (FRAC code 3) should be used if a second application is necessary.

As a result of reports of poor control of frogeye leaf spot on soybean in Beaufort County, County Extension Director Rod Gurganus collected three soybean leaf samples which were sent overnight to the University of Illinois for evaluation of resistance to strobilurin type fungicides.  Dr. Carl Bradley of the Crop Science Department of  the University of Illinois extracted DNA  from the lesions and performed a Q-PCR assay to determine if the G143A mutation is present (this is the mutation that confers resistance to strobilurin fungicides in Cercospora sojina).

The Q-PCR assay was positive for two of the three samples submitted from North Carolina indicating that some strains of strobilurin resistant frogeye leaf spot are present in North Carolina.  We are now one of ten states with confirmed resistance of frogeye leaf spot to the strobilurin fungicides.

Fungicide recommendation changes:

Since resistance of Cercospora sojina to strobilurin fungicides has been confirmed  in North Carolina, we recommend that this disease be managed using either triazole fungicides (Group 3) such as Folicur, Domark, Topguard, Proline or combinations of these fungicides with strobilurin fungicides such as Stratego Yield, Quadris Top, or Fortix. Frogeye leaf spot is caused by the fungus Cercospora sojina.  Many soybean varieties currently grown are resistant to this disease, and the use of resistant varieties is the preferred method of control.

Although frogeye leaf spot is seed borne, it tends to be worse in fields of continuous soybean. Only newly formed leaves are susceptible to this disease, and fully expanded leaves are resistant until they start to senesce. Immature leaves become infected with periods of rain or high humidity, but infection will be limited by dry weather.

So, as the soybean plants put on new layers of leaves, frogeye may be present or absent depending on weather conditions during leaf expansion. This can lead to a situation where frogeye is layered in the canopy at different levels.

Frogeye has caused yield losses of 30% in some fields, so the general recommendation for susceptible varieties is the application of a strobilurin type fungicide, especially if continued wet and/or humid weather is expected.

We do not have a threshold for number of spots or percent leaf area affected to justify fungicide application. If wet and/or humid weather persists as plants start to senesce, older leaves become susceptible again, and the plant may defoliate early. Early defoliation can result in smaller seeds which will translate into yield loss. Also, pod infection can cause a reduction in seed quality or contribute to seed rot.