Frogeye leaf spot (FLS) activity as been increasing on soybean in Arkansas, but not all leaf spots are FLS. In some situations, herbicide injury looks similar to FLS. 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 (Fig. 1). Spots uniformly distributed across a field are good indicators of a non-disease injury to soybean.
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; however, small, dark “spots” (called pycnidia) can be found in the center of these spots (Fig. 2). 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.
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. Misidentification may result in a fungicide being applied too early, which may require a second application before soybeans reach physiological maturity. Contact your local county agent about submitting questionable samples for diagnosis to the Plant Health Clinic in Fayetteville.