The terms used to describe symptoms of delayed senescence in soybean often are used inconsistently or interchangeably and do not adequately distinguish the observed symptoms in the field. Various causes have been proposed to explain the development of delayed senescence symptoms. In this article, we review published reports on delayed senescence symptoms in soybean, summarize current research findings, provide examples of terms related to specific symptoms, and present an overview of the results of a multi-state survey directed to soybean growers to understand their concerns about delayed soybean senescence. Some of these terms, such as green bean syndrome and green stem syndrome, describe symptoms induced by biotic factors, while other terms describe symptoms associated with abiotic factors. Some delayed senescence terms involve the whole plant remaining green while other terms include just the stem and other plant parts such as pods. In the grower survey, 77% reported observing soybean plants or plant parts that remained green after most plants in the field were fully mature with ripe seed. Most respondents attributed these symptoms to changes in breeding and choice of cultivars. At the end of this article, we standardized the terms used to describe delayed senescence in soybean. RightsWorks produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted.

Radiation use efficiency (RUE) is difficult to estimate and unreasonable to perform on a small plot scale using traditional techniques. However, the increased availability of Unmanned Aerial Vehicles (UAVs) provides the ability to collect spatial and temporal data at high resolution and frequency, which has made a potential workaround. An experiment was completed in Iowa to (i) demonstrate RUE estimation of soybean [Glycine max (L.) Merr.] from reflectance data derived from consumer-grade UAV imagery and (ii) investigate the impact of foliar fungicides on RUE in Iowa. Some fungicides are promoted to have plant health benefits beyond disease protection, and changes in RUE may capture their effect. Frogeye leaf spot severity did not exceed 2%. RUE values ranged from 0.98 to 1.07 and 0.96 to 1.12 across the entire season and the period post-fungicide application, respectively, and fell within the range of previously published soybean RUE values. Plots treated with fluxapyroxad + pyraclostrobin had more canopy cover (p = 0.078) compared to the non-treated control 133 days after planting (DAP), but yields did not differ. A “greening effect” was detected at the end of the sample collection. RUE estimation using UAV imagery can be considered a viable option for the evaluation of management techniques on a small plot scale. Since it is directly related to yield, RUE could be an appropriate parameter to elucidate the impact of plant diseases and other stresses on yield.

Frogeye leaf spot, caused by Cercospora sojina K. Hara, is a major soybean (Glycine max L. Merr.) disease that has become more prevalent in the upper Midwest and can be managed with foliar fungicides. Incorporating disease severity into a parameter directly related to yield may better relay the impact of disease on yield and yield components than severity alone. Experiments during the 2018 and 2019 growing seasons in fields located in north central and southwestern Iowa were completed to (i) determine how foliar fungicides affected frogeye leaf spot, remotely sensed plant health indicators, and soybean yield, and (ii) compare the relationship and impact of foliar fungicides and frogeye leaf spot on radiation-use efficiency (RUE) estimated using unmanned aerial vehicle reflectance data. Fungicides affected frogeye severity and yield in one of the three locations; in Lewis 2018, the flutriafol + fluoxastrobin treatment reduced frogeye leaf spot severity by over 50% and increased yield by 19% compared to non-treated controls. Applications of foliar fungicides increased canopy coverage compared to non-treated controls (p = 0.012), but NDVI, SPAD values, and RUE values did not differ between fungicide treatments at all three locations. Estimated soybean RUE values (1.05 to 1.66 g Mj−1) were within the range of known values. Overall, this study indicates that RUE can be a valuable resource to estimate the impact of the disease on yield, however, additional research will be needed to use RUE within certain pathosystems.

Core Ideas Removing 25 to 75% of pods increases green stem disorder. Some QoI fungicides combined with pod removal further increases green stem disorder. Cultivars differ in response to changes in source‐sink balance. Green stem disorder (GSD) is a type of delayed maturity of soybean [Glycine max (L.) Merr.] when stems remain green and fleshy after pods reach physiological maturity and are ready to harvest. Although GSD has not been shown to directly impact yield, the immature green stems may lead to harvest issues. The objectives were to determine how GSD is affected by (i) cultivars and levels of pod removal, (ii) foliar fungicide applications, and (iii) a combination of pod removal and foliar fungicides. A source‐sink imbalance caused by abiotic and biotic sources (e.g., drought or pathogen infection) may be a contributing factor to the development of GSD. To simulate a sink‐limiting stress and induce GSD, pods were removed from soybean plants, at magnitudes typically not seen in normal field conditions. Experiments were completed during 2014 and 2015 in Iowa. Higher levels of GSD were observed in 2015 across most locations. Cultivars varied in their response to pod removal within specific levels. Combining pod removal and fungicide application indicated that certain quinone outside inhibitors (QoI)‐containing fungicides may result in more GSD with and without pod removal under specific conditions. However, this result was inconsistent, especially with no soybean pods removed. Soybean farmers should be aware that prophylactic applications of foliar fungicides might increase GSD incidence, especially in cases that plants experience a sink‐limiting stress.

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Farmers may adjust seeding rate across or within fields for a variety of reasons. The objectives of this study were to determine if seeding rate affected two late-season stem diseases, anthracnose stem blight (ASB) and pod and stem blight (PSB), and yield of soybean. Disease incidence, severity, and yield were collected from eight field trials in Iowa and Wisconsin during 2016 to 2018. ASB incidence exceeded 80% in most of the trials and significantly differed across years with the highest incidence in 2016 and 2018 compared with 2017, the year with the least precipitation. A similar trend across years was observed for ASB severity. The incidence and severity of PSB also differed across years, although PSB incidence and severity were always less than ASB. PSB was greatest in 2017 compared with the other two years. Seeding rate did not affect either disease. A seeding rate of 198,000 to 395,000 seeds/ha provided statistically similar yield. Weather conditions each year affected late-season stem diseases.

Green stem disorder (GSD) of soybean (Glycine max L. Merr.) is when stems remain green and fleshy after pods reach physiological maturity and are ready to harvest. Little is known about GSD and its causes. Although GSD has not been shown to directly impact yield, the immature green stems may lead to harvest issues. During the 2014 and 2015 growing seasons in fields located in central Iowa, experiments involving fungicides and pod removal took place. The objective of the first study was to investigate how different cultivars get GSD after varying levels of pod removal, which was used to simulate the result of a sink limiting stress. A second study was conducted at seven locations across Iowa to examine how foliar fungicides affected GSD. A third study determined how a combination of pod removal and foliar fungicide applications affected the incidence of GSD. Foliar fungicides applied in these studies were applied during reproductive stages. Results indicated that cultivars varied in their response to pod removal and that pod removal in combination with fungicides of certain strobilurin chemistries produced higher incidences of GSD (P=0.06). Results varied across years, as GSD was significantly higher in all treatments in 2015 (P<0.001). Soybean farmers should be aware of the added possibility that prophylactic fungicide applications may increase GSD incidence especially if plants experience a sink limiting stress.

The purple symptomology we are seeing on both mature and green soybean stems is most likely caused by elevated anthocyanin levels. Elevated anthocyanin levels are more common in soybean cultivars with purple flowers, but can occur in cultivars with white flowers as well. Soybean plants with purple stems have been common across Indiana in 2006. The extent to which we are seeing this phenomenon does correspond with the growing season we are having. As the soybean plant reaches maturity the source-sink relationship for carbohydrates is disrupted. In 2006, we have seen delayed maturity (green stems and prolonged leaf retention) coupled with mature pods. So simply the plant is still producing carbohydrates (sugar) and has nowhere to put it. These sugars are being converted to anthocyanins and expressed in the stem. If you look at the stem, the purple color it is usually occurring on ½ to ¾ of the stem. The back-side (usually north) however remains its normal tawny color. This consistent symptomology suggests that this is a physiological response and not a disease or nutrient deficiency response.