By Travis Faske, Extension Plant Pathologist
Though soybean rust (SBR) has yet to be detected in 2015 in Arkansas, it has returned to the Mid-South. SBR has been found in several Mississippi counties over the past few weeks with the most recent find occurring just across the MS River (Fig. 1). Historically, SBR progresses up the MS River so, it is only a matter of time before it is detected in the state and moves progressively northward. For the past few years, when SBR has been detected in the state, the majority of the Arkansas crop was close to maturity and was unaffected by this disease. For this reason, fungicides are rarely recommended to protect the Arkansas soybean crop. In the past 10 years the UA Extension Service has only recommended a fungicide once to control SBR. The current weather forecast does not favor SBR development, which will suppress SBR movement. However, early detection is key to provide an early warning to producers; thus, now is a good time to scout for SBR.
When scouting for SBR select mature soybean fields (R6+) and scout areas in the field that that are difficult to cover with an aerial fungicide application (i.e. under a tree, along a power line, or near a tree line). Inspect leaves in the lower canopy as SBR develops first in the lower canopy then moves up the plant. Symptoms consist of small (2.0 to 5.0 mm) tan or reddish brown angular spots/lesions on leaves. These lesions are often observed first at the base of the leaflet near the petiole. Volcano-shaped pustules (Fig. 2) can be observed within the lesion on the underside of the leaf.
These pustules can be observed in the field with a 20x hand lens (Fig. 2 and 3) but may be misdiagnosed as bacterial pustule (Fig. 4) or one of many other numerous “SBR imposters” by the untrained observers. Questionable samples should be sent the Plant Health Clinic in Fayetteville through your county agent.
Conditions that favor SBR development consist of moderate temperatures (72 to 82 ºF) and several hours of heavy dew (12 hr leaf wetness). Given that SBR is present in the Mid-South, the return of favorable conditions will promote sporulation and subsequent infection of soybean leaves. However, the current weather forecast does not favor widespread SBR development.
Fungicides are effective and though there is no recommendation for their use at this time, a list of fungicides and their efficacy are listed in the Arkansas Plant Disease Control Products Guide (i.e. MP 154). As a general rule, strobilurin fungicides are protective and should be applied prior to disease, whereas triazole fungicides can be effective after disease has been observed in the field but are most effective when applied prior to disease development. Given SBR often arrives late in the growing season, if a fungicide is recommended it is often a triazole. Thresholds for SBR are complicated by yield potential, time needed to reach R6 (stage at which a fungicide is no longer recommended), and most importantly, forecasting the weather. Currently, there is no recommendation for the use of fungicides to manage SBR, but future recommendations will be posted as necessary on this website and IPM PIPE.