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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. Accepted for publication 23 March 2016. Published 15 April 2016.

A major constraint in breeding for resistance to soybean rust has been the virulence diversity in Phakopsora pachyrhizi populations. In greenhouse experiments, reactions of 18 soybean genotypes to 24 U.S. isolates from 2007 and 2008 and 4 foreign isolates were compared. Reactions of four differentials (Rpp1 to Rpp4) to these U.S. isolates were also compared with reactions to nine foreign isolates and three U.S. isolates from 2004. Principal component analysis (PCA) of the reaction types grouped the U.S. isolates into a single virulence group, whereas each of the foreign isolates had a unique virulence pattern. In another experiment, reactions of 11 differentials to the 24 U.S. isolates were compared and significant interactions (P < 0.001) were found between the isolates and host genotypes for rust severity and uredinia densities. PCA of these two measures of disease placed the 24 isolates into seven or six aggressiveness groups, respectively. In a third experiment, evaluation of 20 soybean genotypes for resistance to the previously established aggressive groups identified 10 genotypes resistant to isolates representing most of the groups. This study confirmed the pathogenic diversity in P. pachyrhizi populations and identified soybean germplasm with resistance to representative U.S. isolates that can be used in breeding.

The effects of resveratrol and pterostilbene on in vitro growth of three soybean pathogens were tested to determine whether these stilbenic compounds could potentially be targets to increase innate resistance in transgenic soybean plants. Growth of Macrophomina phaseolina, Rhizoctonia solani, and Sclerotinia sclerotiorum was measured on solid and in liquid media amended with resveratrol and pterostilbene (concentration in the media of resveratrol at 100 μg/ml and pterostilbene at 25 μg/ml). All three fungi were very sensitive to pterostilbene in potato dextrose agar (PDA), which reduced colony area of each of the three pathogens to less than half of the control 3 days after incubation. The three fungal pathogens were less sensitive to resveratrol compared with pterostilbene; however, area under the curve (AUC) calculated from colony areas measured over 3 days was significantly (P < 0.05) less than the control for S. sclerotiorum and R. solani on PDA with resveratrol or pterostilbene. AUC for M. phaseolina on PDA with pterostilbene was significantly (P < 0.05) lower than the control whereas, on PDA with resveratrol, AUC for M. phaseolina was lower than the control but the difference was nonsignificant (P > 0.05). AUC for all three fungi was significantly lower (P < 0.05) on PDA with pterostilbene than with resveratrol. In potato dextrose broth (PDB) shake cultures, AUC for all three fungi was significantly (P < 0.01) lower in pterostilbene than in the control. AUC for R. solani and S. sclerotiorum was significantly lower (P < 0.01) in resveratrol than the control, whereas AUC for M. phaseolina in resveratrol was lower, but not significantly (P > 0.05) different from the control. AUC in pterostilbene was highly significantly (P < 0.01) lower than in resveratrol for M. phaseolina and significantly (P < 0.05) lower for R. solani but the difference for S. sclerotiorum was nonsignificant (P > 0.05). There was a trend for lower mass accumulation of all three fungi in either pterostilbene or resveratrol compared with the control during the course of the experiment; however, S. sclerotiorum appeared to recover from the effects of pterostilbene between days 2 and 4. Results of biochemical analyses of the PDB over time indicated that the three fungi degraded resveratrol, with nearly 75% reduction in concentration in M. phaseolina, 80% in S. sclerotiorum, and 60% in R. solani PDB cultures by day 4 of fungal growth. M. phaseolina and S. sclerotiorum were able to resume growth after early inhibition by resveratrol after its concentration was reduced in the cultures through degradation, whereas R. solani was less efficient in resveratrol degradation and was not able to overcome its inhibitory effects on growth. The capacity to degrade pterostilbene was lowest in M. phaseolina compared with S. sclerotiorum and R. solani and the recovery of M. phaseolina cultures after initial growth inhibition by pterostilbene was minimal. The potential products of resveratrol and pterostilbene degradation by fungi were identified to be dimers and various oxidation products.

Green stem disorder of soybean (Glycine max) has increasingly become a nuisance for soybean producers. The disorder is distinguished from other manifestations of delayed plant maturity by the delayed senescence of stems only, with normal pod ripening and seed maturation. The primary objective of the first study was to determine whether green stem disorder increased with a fungicide treatment. Field cages to isolate soybean plants to prevent insect interactions were used and treatments included maturity group (MG) II insensitive and sensitive soybean cultivars with or without fungicide applications. A secondary objective was to determine fungi potentially associated with the disorder. The results indicated significant elevation of the incidence of green stem disorder when using a fungicide. Species of Diaporthe or Phomopsis and Macrophomina phaseolina were more frequent in stems without the disorder, whereas species of Colletotrichum were found mostly in stems with the disorder. In another study, field experiments were conducted without cages in replicated field plots to compare the effects of fungicides with different chemistries and timing of fungicide application on incidence of green stem disorder using green stem disorder MG II- and MG III-sensitive and insensitive soybean cultivars. There was a significant increase in percentage of green stem disorder due to fungicide application, depending on fungicide chemistry, timing of application, year, location, and cultivar sensitivity to green stem disorder. Generally, Headline and Headline-Domark applications resulted in higher incidence of green stem disorder than Domark alone or the nonsprayed control, with over 50% incidence in many cases. Higher percent green stem disorder was significantly (P < 0.05) associated with higher yields in 11 of the 28 trials. From the results of this research, soybean producers should be aware of the possible risk that fungicide application may have in increasing incidence of green stem disorder. In addition, producers can help manage green stem disorder by selecting soybean cultivars reported to be consistently insensitive to the disorder.

Charcoal rot of soybean is caused by the fungal pathogen Macrophomina phaseolina. Effective and reliable techniques to evaluate soybean for resistance to this fungus are needed to work toward a management scheme that would utilize host resistance. Three experiments were conducted to investigate the use of a cut-stem inoculation technique to evaluate soybean genotypes for resistance to M. phaseolina. The first experiment compared aggressiveness of M. phaseolina isolates collected from soybean on different soybean genotypes. Significant (P < 0.05) differences among the isolates and genotypes for relative area under disease progress curve (RAUDPC) were found without a significant isolate–genotype interaction. The second experiment compared 14 soybean genotypes inoculated with M. phaseolina in multiple trials conducted in two environments, the greenhouse and growth chamber. Significant (P < 0.05) differences among environments and highly significant (P < 0.001) differences among soybean genotypes for RAUDPC were found. The environment–genotype interaction was nonsignificant (P > 0.05). Soybean genotypes DT97-4290, DT98-7553, DT98-17554, and DT99-16864 had significantly (P < 0.05) lower RAUDPC than 7 of the 14 genotypes. The third experiment evaluated resistance in selected Phaseolus spp. and soybean genotypes. The range of RAUDPC for Phaseolus spp. was similar to that of soybean. The Phaseolus lunatus ‘Bush Baby Lima’ had significantly (P < 0.05) lower RAUDPC than P. vulgaris genotypes evaluated. The cut-stem inoculation technique, which has several advantages over field tests, successfully distinguished differences in aggressiveness among M. phaseolina isolates and relative differences among soybean genotypes for resistance to M. phaseolina comparable with results of field tests.

Macrophomina phaseolina is the cause of charcoal rot of soybean (Glycine max). Resistance to M. phaseolina in commercial soybean cultivars is not common but is needed in locations where the disease is chronic and severe. The objective of this study was to develop a reliable method to produce sufficient M. phaseolina conidia that can be used to inoculate soybean plants in a high-throughput resistance-screening program. Production of pycnidia is not common on most culture media, such as potato dextrose agar, but can be produced on media containing plant parts or oilseed extracts. Seven semi-defined media were tested to induce pycnidia production. Results indicated that the number of pycnidia that were produced by eight M. phaseolina isolates was dependent on induction medium; however, peanut butter extract-saturated filter paper placed over soynut butter extract agar (PESEA) allowed for greater pycnidia and conidia production than the other media tested. Production of pycnidia on PESEA ranged from 269 to 1,082 per plate. There were no differences among isolates in germination of conidia produced on PESEA, which averaged 83 ± 2% germination. A conidial suspension from one M. phaseolina isolate produced on PESEA and inoculated onto soybean radicles significantly distinguished (P < 0.01) ‘DT97-4290’, a soybean genotype with partial resistance to charcoal rot, from a susceptible genotype, ‘LS98-0358’. Results of this study indicated that multiple isolates of M. phaseolina from soybean produced sufficient amounts of conidia on PESEA to use as inoculum. This conidia inoculum production method will facilitate soybean charcoal rot resistance screening evaluation with different soybean isolates.

Plants recognize invading pathogens and respond biochemically to prevent invasion or inhibit colonization in plant cells. Enhancing this response in crop plants could improve sustainable methods to manage plant diseases. To enhance disease resistance in soybean, the soybean phytoalexin glyceollin was assessed in soybean hairy roots of two soybean genotypes, Spencer and PI 567374, transformed with either soybean isoflavone synthase (IFS2) or chalcone synthase (CHS6) genes that were inoculated with the soybean pathogens Diaporthe phaseolorum var. meridionales, Macrophomina phaseolina, Sclerotinia sclerotiorum, and Phytophthora sojae. The hairy-root-transformed lines had several-fold decreased levels of isoflavone daidzein, the precursor of glyceollin, and considerably lower concentrations of glyceollin induced by pathogens measured 5 days after fungal inoculation compared with the nontransformed controls without phenolic transgenes. M. phaseolina, P. sojae, and S. sclerotiorum grew much more on IFS2- and CHS6-transformed roots than on control roots, although there was no significant difference in growth of D. phaseolorum var. meridionales on the transformed hairy-root lines. In addition, glyceollin concentration was lower in D. phaseolorum var. meridionales-inoculated transformed and control roots than roots inoculated with the other pathogens. Glyceollin inhibited the growth of D. phaseolorum var. meridionales, M. phaseolina, P. sojae, S. sclerotiorum, and three additional soybean pathogens: Cercospora sojina, Phialophora gregata, and Rhizoctonia solani. The most common product of glyceollin conversion or degradation by the pathogens, with the exception of P. sojae, which had no glyceollin degradation products found in the culture medium, was 7-hydroxyglyceollin.

Green stem disorder of soybean (Glycine max) is characterized by delayed senescence of stems with normal pod ripening and seed maturation. Three different field research approaches were designed to determine the relationship of green stem disorder to Bean pod mottle virus (BPMV) and other potential factors that may be involved in causing this disorder. The first research approach surveyed green stem disorder and BPMV in individual plants monitored in several commercial soybean fields during three growing seasons. Leaf samples from maturing plants (growth stage R6) were tested by enzyme-linked immunosorbent assay (ELISA) for BPMV. The percentage of monitored plants infected with BPMV at growth stage R6 in some fields was higher than the incidence of green stem disorder at harvest maturity. Many plants infected with BPMV did not develop green stem disorder, and conversely, many plants that had green stem disorder were not infected with BPMV. According to a chi-square test of independence, the data indicated that green stem disorder was independent of BPMV infection at growth stage R6 (P = 0.98). A second research approach compared green stem disorder incidence in an identical set of soybean entries planted in two locations with different levels of natural virus infection. Despite differences in virus infection, including BPMV incidence, 20 of 24 entries had similar green stem disorder incidence at the two locations. A third research approach completed over two growing seasons in field cages showed that green stem disorder developed without BPMV infection. BPMV infection did not increase green stem disorder incidence in comparison to controls. Bean leaf beetle, leaf hopper, or stinkbug feeding did not have an effect on the incidence of green stem disorder. The cause of the green stem disorder remains unknown.

Soybean seed coat mottling often has been a problematic symptom for soybean growers and the soybean industry. The percentages of seed in eight soybean lines with seed coat mottling were evaluated at harvest after inoculating plants during the growing season with Bean pod mottle virus (BPMV), Soybean mosaic virus (SMV), and both viruses inside an insect-proof cage in the field. Results from experiments conducted over 2 years indicated that plants infected with BPMV and SMV, alone or in combination, produced seed coat mottling, whereas noninoculated plants produced little or no mottled seed. BPMV and SMV inoculated on the same plants did not always result in higher percentages of mottled seed compared with BPMV or SMV alone. There was significant virus, line, and virus-line interaction for seed coat mottling. The non-seed-coat-mottling gene (Im) in Williams isoline L77-5632 provided limited, if any, protection against mottling caused by SMV and none against BPMV. The Peanut mottle virus resistance gene Rpv1 in Williams isoline L85-2308 did not give any protection against mottling caused by SMV, whereas the SMV resistance gene Rsv1 in Williams isoline L78-379 and the resistance gene or genes in the small-seeded line L97-946 gave high levels of protection against mottling caused by SMV. The correlations (r = 0.77 for year 2000 and r = 0.89 for year 2001) between virus infection of the parent plant and seed coat mottling were significant (P = 0.01), indicating that virus infection of plants caused seed coat mottling.