Frogeye leaf spot advancing in Southern soybean fields

In Delta Farm Press

Frogeye leaf spot (FLS) activity as been increasing in southern soybean fields. And with reports of resistance to some fungicides and difficulty in distinguishing frogeye from other leaf spots and disease, it is harder to control.

As a result, management of frogeye and other diseases will require more detailed field scouting by farmers, say plant pathologists.

Steve Koenning, North Carolina State University Extension Plant Pathologist, says some farmers in Beaufort County, N.C. are seeing fungicide resistance show up in the frogeye leaf spot pathogen, which may account for the need for additional fungicide treatments. “Resistance of the frogeye leaf spot fungus (Cercospora sojina) to strobilurin fungicides (FRAC code 11; Headline, Quadris, Evito, and Aproach) has been reported from the Mississippi Delta and other areas in the past several years,” he says.

“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 (2013).  This may be a result of applications made in an untimely manner, applications of a less than labeled rate and/or resistance of the fungus to the fungicides used.”

Koenning says that 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.

Travis Faske, University of Arkansas Extension plant pathologist, adds that FLS activity has increased in Arkansas, but not all leaf spots are FLS.  “In some situations, herbicide injury looks similar to FLS,” he says. “Glufosinate (Liberty) injury on Roundup Ready soybeans is a good example and has been reported in several fields across the state.

Initially, the spot is yellow and as it ages, the center becomes necrotic. “Spots uniformly distributed across a field are good indicators of a non-disease injury to soybean,” Faske says.

He says that in some situations, “saprophytic fungi (non-pathogenic) or weakly pathogenic fungic,” infect spots injured by herbicides that look similar to FLS.

“Opportunistic fungi like Peyronellaea spp., Phyllostricta spp., or Ascochyta spp. infect injured tissue and produce tan to gray lesions surrounded by a dark brown to purplish border,” he says. “However, small, dark spots called pycnidia can be found in the center of these spots.

“In contrast, FLS will not contain pycnidia in the center, but rather gray ‘fuzzies’ (conidia) may be seen with a hand lens on sporulating leaf spots.”

Faske says leaf spots on soybean varieties with resistance to FLS could be a result of herbicide injured leaf tissue that is infected by opportunistic fungi.  “Thus, not all leaf spots are caused by a fungal pathogen or recommended to be treated with a fungicide,” he says.  “Misidentification may result in a fungicide being applied too early, which may require a second application before soybeans reach physiological maturity.”

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