Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
This year looks like it will be a good one for our soybean crop. The vast majority of fields are clean with few, if any, major issues. I have seen a few fields where Frogeye leaf spot has taken off a bit (Figure 1). In terms of management, the fields I have looked at are at R5 or later which is past the point where chemical intervention is recommended. In addition, the severity of the disease has been low and unlikely to have an impact on yield. If you have noticeable amounts of Frogeye leaf spot and you already applied a fungicide, please contact me at nkleczew@udel.edu or send a sample to the UD diagnostic clinic. We are in the process of collecting samples to determine if our populations are developing fungicide resistance as has been observed in Midwestern states and North Carolina. For more information on Frogeye leaf spot, see my factsheet: http://extension.udel.edu/factsheet/frogeye-leaf-spot-on-soybean-2/.
We also have had a few reports of sclerotinia stem blight (white mold) on soybeans (Figure 2). This is a cool season disease that we see almost every year, but only to a very small degree. Often you will see it in high yield environments and in shaded areas of the field, such as along wood edges. If you notice sclerotinia stem blight (white mold) in your field it is likely there to stay. The best management practice at this point in full season beans is to schedule infested fields to be harvested last to minimize spread of the pathogen from field to field on farm equipment. In double crop beans planted into fields with a known history of sclerotinia stem blight, applications targeting the R1-R2 stage are most efficacious. Example fungicides for SSB suppression include labeled group 1s (Topsin), labeled group 3s (Proline), Fluazinam (Omega); labeled group 7s (Endura) and labeled group 11s (Aproach). Often fungicide applications are not justified in Delaware but they can be beneficial under some circumstances. As usual, preventative applications are most efficacious. For more information on fungicide ratings, please see the NCERA 212/218 fungicide efficacy tables: http://extension.udel.edu/factsheet/fungicide-efficacy-for-control-of-foliar-soybean-diseases/
For more information on Sclerotinia stem blight (white mold) on soybean, see my factsheet: http://extension.udel.edu/factsheet/sclerotinia-stem-blight-white-mold-on-soybean/
Figure 1. Full season beans with frogeye leaf spot on the upper canopy. In this particular example we are looking at roughly 1-3% leaf severity, which isn’t likely to impact yield to a noticeable degree.
Figure 2. A soybean stem with characteristic hard, black structures (sclerotia) located on the soybean stem.
We have started to see some SDS (Sudden Death Syndrome) popping up to a very low degree in full season soybeans. This is likely a result of planting full season beans into cool, wet soils which favors early season infection of plants by this pathogen. SDS can easily be confused with other diseases of the stem and other disorders. Early symptoms of the disease include mottling and crinkling of the leaves. As the disease progresses the leaf tissue between the veins turns yellow/brown, while the veins remain green. Soon thereafter the leaves shrivel and fall from the plant, but the petioles often remain intact. If the plant is removed from moist soil tiny blue structures may be visible at the base of the stem. In most cases diagnosis is completed by sectioning the lower portion of the stem lengthwise. The cortex of a stem infected with SDS will be streaked with tan/light brown lesions, and pith of infected plants retains a white coloration. There is nothing you can do for with season management, but SDS can be associated with SCN, so the presence of SDS may indicate the field also has soybean cyst nematode. See my factsheet on SDS for more information on identification and management: http://extension.udel.edu/factsheet/sudden-death-of-soybeans-sds/
Troubleshooting samples for Soybean Cyst Nematode
A reminder that if you have a field that you think has soybean cyst nematode issues, ask that your soil be assessed for SCN egg counts. The information from juvenile extractions is limited and can be easily misinterpreted. SCN egg assays are more representative of what your field “sees” during the course of the season and are much more sensitive than juvenile assays.