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Arkansas soybean disease update: Target spot and frogeye leaf spot
Author: Travis Faske, Extension Plant Pathologist

By Travis Faske, Extension Plant Pathologist

The spectrum of foliar diseases changes each year as environmental conditions change, which has a significant impact on disease development.  The weather pattern in early August was favorable for target spot (TS) and there have been several reports of TS in central and northcentral Arkansas.  Overall, conditions and variety selection have limited the widespread severity of this disease compared to what we observed during the 2016 cropping season.  Looking ahead, the long range temperature forecast is more favorable for frogeye leaf spot (FLS), so late season beans should be monitored over the next few weeks for FLS, TS and other foliar diseases of soybean.

Target spot symptom development begins in the lower canopy with large, circular lesions with concentric rings (Fig. 1).  With the right environmental conditions TS produces smaller lesions in the mid-to-upper canopy (Fig. 1).  These lesions can be confused with frogeye leaf spot (Fig. 2).  Target spot can also cause lesions on the petioles, pods, and stems. Environmental conditions that favor leaf infection are several consecutive days of rain and warm temperatures.

Figure 1.  Mature large target spot lesion from leaflet in lower canopy (left) and smaller lesions on leaflet in mid-canopy (right).

Figure 1. Mature large target spot lesion from leaflet in lower canopy (left) and smaller lesions on leaflet in mid-canopy (right).

Variety selection is the most economical approach to managing TS in soybean. Some of the more susceptible varieties observed in 2016 consisted of Armor 47R13, Asgrow 4632, Cradenz CZ 5150, Stine 47LF32 and Dyna Grow 48RS53, which may or may have not been available this season.  Target spot has been reported this season on Asgrow 47X6, P48T67, Stine 47LF32, and CZ 4748.  Fungicides are another tool to manage this disease, but like sheath blight in rice, timing is an important factor.  Given that this disease begins in the lower canopy and moves up on susceptible varieties and that fungicides do not move down into the lower canopy, fungicide timing is important. When fungicides are applied after canopy closure, the majority of the leaf protection will occur in the upper 1/3 of the plant canopy.  So, unless the disease is beginning to move up the canopy, a foliar application will do very little to protect the mid canopy.  Based on university lab trials, fluxapyroxad (SDHI in Priaxor) was among the most effective at suppressing fungal development.  Strobilurin were among the least effective in some isolates due to fungicide resistance. Several research trials supported by the Arkansas Soybean Promotion Board have been set up this year to determine the best timing for fungicide and variety susceptibility, which will be discussed at winter production meetings.

Frogeye leaf spot (Fig. 2) is one of the more common foliar diseases in the state and relatively easy to recognize.  Leaf spots (smaller lesions with dark brown to maroon margins) often develop in the upper canopy as emerging leaves are more susceptible than expanded soybean leaves.  Environmental conditions that favor FLS development consist of more moderate temperatures (77 to 86ºF) and frequent, prolonged dew periods or light rain with symptoms showing up 7 to 10 days after infection.  During the onset of disease symptoms is a good time to review variety susceptibility to FLS and determine if a fungicide will protect yield potential.    Some 75% of the varieties screened in 2016 had at least a moderate level of FLS resistance, which is the most economical way to manage FLS.  This information can be found on the Arkansas Variety Testing website.  Based on university trials, good yield protection was observed when a fungicide was used on FLS susceptible varieties WHEN conditions favored disease and FLS was observed in the field (at least 9 spots were observed on a tri-foliate leaf at relatively frequent stops in the field).  Alternately, yield protection was significantly lower when fungicides were applied on moderately resistant varieties compared to FLS susceptible varieties WHEN conditions favored disease and FLS was observed in the field.  Several fungicides are available that have good efficacy on FLS in Arkansas and optimum timing can prevent the need and expense of a second fungicide application.  Fungicides registered for use in Arkansas and efficacy on FLS can be found in the Arkansas Plant Disease Control Products Guide (MP 154).

Figure 2.  Frogeye leaf spot of soybean

Figure 2. Frogeye leaf spot of soybean


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