Delayed Senescence in Soybeans

According to the article Delayed Senescence in Soybean: Terminology, Research Update, and Survey Results from Growers that was published in Vol. 17 of Plant Health Progress in 2016, the terms used to describe symptoms of delayed senescence in soybean are inconsistent and do not adequately describe the varied symptoms observed in the field. The following sections review these terms and their definitions as presented in that and other articles.

Green Stem Syndrome. This term is used to describe soybean plants that have green or immature stems at maturity, along with nonsenesced leaves and petioles. Affected plants also have fewer pods per node, and smaller seed.

Green Bean Syndrome. This term has been used primarily in the southern US to describe the condition in which maturity of the whole plant, including pods, is delayed. It is likely that this condition is related to direct damage by an agent such as stink bugs that disrupts the normal plant maturation process.

Green Plant (or Stem) Malady. Symptoms of this condition usually include green leaf retention, green stems, and delays in seed maturation that results in poor seed quality. The primary symptom of this condition is stems remaining green after pods mature. Results from studies in Louisiana determined that this condition was primarily associated with cultivar, with stink bug damage and Headline (pyraclostrobin) fungicide application also contributing. Application of paraquat as a harvest aid is relatively ineffective in alleviating this condition.

Green Stem. This term has generally been used as a catch-all term for delayed senescence because it also refers to plants with or without leaves still attached and with pods turning brown or remaining green.

Green Stem Disorder. This term defines plants with non-senescent stems that have mostly unattached petioles and mature pods and seed that are normal at maturity. This disorder is not associated with viruses or feeding by insects. In fact, research results suggest this disorder may be associated with lack of fungal parasitism. Results from research also indicated that there was no correlation between green stem disorder and yield. There is variability among varieties in sensitivity to this disorder.

Greening Effect. This term has been used to describe the greening effect that is often associated with the application of strobilurin foliar fungicides. The result of this malady is a delay in the natural maturing process, which causes plants to maintain green leaf longer. However, once the plant has reached full maturity, it dries down normally. This normal drying down process of the entire plant at maturity delineates this condition from that of green stem disorder, where plants do not usually retain leaves and do not delay pod and seed maturity even though stems remain green.

Take-Home Message. 1) In the southern US, the two above maladies most likely to occur or be identified as occurring are green stem syndrome and green bean syndrome. 2) Some pests of soybeans, both disease and insect, can cause green stem syndrome by causing pod loss. 3) Some pests, mainly insects and especially stink bugs, can cause both green stem and green bean syndrome by causing both pod loss and damage to immature seed. 4) Green stem syndrome and green bean syndrome are likely two separate maladies of soybeans. 5) Check preferred varieties for ratings (both originating company and variety trial data) associated with the above maladies. 6) The remedy for both green stem and green bean syndromes of soybeans is to select less susceptible varieties, and monitor and control environmental stresses and disease and insect infestations to prevent pod loss and damage to seed.

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Composed by Larry G. Heatherly, June 2016, larryheatherly@bellsouth.net