By Travis Faske, Extension Plant Pathologist
Frogeye leaf spot (FLS) has been increasing in activity over the past few weeks, which has many looking for the latest information in disease management. Here is a brief summary of three key points to consider about FLS in Arkansas: Distribution of strobilurin-resistant (S-R) FLS, conditions that favor disease development, and fungicide efficacy on S-R FLS.
In 2012, S-R FLS was first detected in Arkansas and now has spread across the state (Fig. 2). Currently, it has been confirmed in 25 Arkansas counties, which accounts for nearly 90% of the soybeans produced in the state. Fungicides that are no longer effective on these fungal strains include: Quadris, Headline, Aproach, Gem, and Evito, which are all classified as strobilurin fungicides, hence the name strobilurin-resistant FLS. Soybean varieties that are resistant to FLS are the best and most economical way to manage S-R FLS.
Recently, cooler temperatures have been more favorable for FLS sporulation and infection across the state. Sporulation occurs early in the morning (spores can often be seen on the underside of the leaf by 10:00 am), and if followed by cool temperatures (77 to 86ºF) and prolonged dew periods or light rain, conditions are ideal for the fungus to infect the plant. Symptoms are often observed after 7 days so FLS can develop relatively rapidly when conditions are favorable for disease development. Typically, various stages of FLS maturity can be detected in the field (Fig. 3). Immature lesions on expanding leaves look similar to mature lesions; however, they lack the pronounced maroon or purple edge surrounding the lesion (Fig. 3) and have yet to sporulate. Even earlier, these immature lesions start out as faint, water-soaked spots, which can be seen alongside immature lesions on some leaves (Fig. 3). Alternately, immature lesions on fully expanded leaves often appear as unexpanded smaller spots or purple specks with faint tan center. These small specks are observed on beans at later reproductive stages of growth.
Fungicides that contain a triazole are effective on S-R FLS but not all are equal in efficacy – see 2015 soybean fungicide efficacy table. An older, but still accurate efficacy table is also available in the MP 154 with some minor format changes. Fungicides are effective at controlling FLS; however, there is no economic threshold for a fungicide application. Fungicide thresholds are complicated by the differences among soybean susceptibility to FLS, time for soybean to reach maturity, yield potential, severity of FLS, and unpredictability in forecasting the weather. However, to provide some guideline, consider a fungicide on a susceptible variety when there is at least 9 FLS per leaf (tri-foliate) found at fairly regular stops in the field AND weather conditions favor disease development. Take into consideration immature lesion development as this is an indication that conditions were recently favorable for disease. Some triazole fungicides can cause phytotoxicity on soybean (see earlier report on this blog). Finally, be aware that harvest aids like Gramoxone herbicide look very similar to FLS (Fig 4). So, when considering a fungicide make sure it is the real deal and not a disease imposter!
Key points to identify Gramoxone injury:
- No sporulation on spots caused by Gramoxone.
- Gramoxone injury can be found on grasses and other nearby weeds in and along the edge of the field.
- Gramoxone injury seems to appear at once and may gradually decrease across the field.