By Jeremy Ross, Extension soybean agronomist
With soybean harvest in Arkansas just beginning, we are receiving many reports of poor seed quality in many of these fields (Figure 1). Many are placing the blame for this poor seed quality on the tremendous number of Redbanded stink bugs we have experienced this growing season. We have seen some damage in these early-planted fields from Rebanded stink bug, but most of the seed damaged observed is caused by adverse environmental conditions seen in Arkansas over the last four weeks, which is also associated with late-season fungal diseases. Yield reports from these fields are better than expected with many in the upper-70’s to over 100 bu/ac, but many are reporting damage in excess of 10%.
Diseases that cause seed damage consists of purple seed stain, pod and stem blight, and Phomopsis seed decay. All of these diseases can be found in Arkansas to some degree and more than one disease is often associated with poor seed quality. Cercospora leaf blight (CLB) and purple seed stain are caused by the same fungal pathogens; however, CLB symptoms are not always associated with purple seed stain symptoms. The onset of CLB occurs during stages R4-R5. Initial leaf symptoms consist of a light purple discoloration on the upper leaf surface that can expand to cover the entire upper leaf surface (Figure 2). Severe and advanced leaf symptoms include foliar blighting (Figure 2) and defoliation. Cercospora leaf blight can cause elongated, irregular shaped lesions on petioles. These lesions may coalesce and completely discolor the petiole when conditions favor disease development. Severity of symptom development can vary among soybean varieties. Purple seed stain is characterized by irregular light to dark purple blotches on the seed that may cover some or all of the seed coat. Infection by the pod and stem blight pathogens occurs earlier in the cropping season, but symptoms are not obvious until growth stages R6-R8. The most characteristic sign of the pod and stem blight is black, raised fungal reproductive structures (pycnidia) in linear rows on stems of mature soybean plants (Figure 3). Pycnidia can be found on mature pods, but may not follow the linear orientation as seen on stems (Figure 4). Seed damaged by Phomopsis seed decay are cracked, shriveled, and light weight seed with chalky, white fungal mold on the seed surface (Figure 5).
Damage caused by Redbanded stink bug is identifiable by the dark, uniform legions on the seed coat (Figures 6 and 7). Stink bug feeding on seed not fully developed could cause abortion or shriveled seed. Seed fed on at R6 stage and beyond typically do not shrivel or have a reduction in seed size, but have a feeding lesion on the seed.
Many of the fields where poor seed quality has been seen are fields that were planted during April. From conservations with consultants and producers, many of these fields received a single fungicide application around the R3 growth stage. Even though a fungicide was applied, this application at R3 was too early and was not giving any control when conditions were favorable for disease development later in the season. We are currently investigating if any of these April planted fields received two fungicide application to see if any quality differences are seen.
We are hoping that the later planted soybean crop will not experience the poor seed quality seen with the early planted crop. Factors including the weather and how long the soybean crop remains in the field after maturity will influence the seed quality of the remaining soybean crop.