By Travis Faske, Extension Plant Pathologist
Frogeye leaf spot (FLS) is one of the most common foliar diseases of soybean in Arkansas. As with all foliar disease, some degree of leaf wetness and favorable temperatures are needed for disease development. As expected, such conditions do not occur statewide and each year, FLS is more active in one area of the state than others. This year, conditions have been more favorable in the northern part of the state, but given the forecast for widespread rain, the level of disease incidence maybe be more widespread in the new few weeks. Here are some important topics when considering a fungicide to control FLS on susceptible varieties.
Climatic conditions that favor FLS development consist of warm temperatures (77 to 86ºF) and prolonged dew periods or light rain. These conditions enable the fungus to infect expanding soybean leaves, which are the most susceptible leaves on a susceptible soybean. As these conditions persist, multiple infection and sporulation events can occur in a few days, thus increasing disease severity. Symptoms are often observed after 7 days, so FLS can develop relatively rapidly when conditions are favorable for disease development. Alternately, dry and hot conditions suppress disease development, which has been the case for the majority of the state. Typically, various stages of FLS maturity can be detected in the field (Fig. 2). Immature lesions on expanding leaves look similar to mature lesions; however, they lack the pronounced maroon or purple edge surrounding the lesion (Fig. 2) 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. 2). 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.
Based on the results from the 2016 UA variety testing program, 75% of soybean varieties are resistant or moderately resistant to frogeye leaf spot. However, in-season decisions are concerned with protecting those susceptible and moderately susceptible varieties from excessive disease development. Given that strobilurin-resistant FLS is widespread across the state and Mid-South, a triazole fungicide is needed to manage these strains. There are several options when it comes to soybean fungicides (too many to list here), so consult the MP 154 and 2016 fungicide efficacy table for the most up to date information on fungicide efficacy. Unfortunately, there is no economic threshold to trigger a fungicide application, but decisions should be made on a field by field bases. Fungicides have been effective at protecting yield potential on susceptible or moderately susceptible varieties 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. If FLS is relatively uniform in the field and conditions are reported to favor disease development most fungicides are more effective when applied preventatively. In contrast, if hot dry conditions are in the forecast continue to scout fields with FLS and monitor disease development. Finally, refrain from applying fungicides in the absence of disease as such practices lead to the development of fungicide-resistant pathogens.