Green-Stem Syndrome In Soybean

DR. D.B. EGLI

LEXINGTON, KENTUCKY

No producer wants to see soybean plants with green stems in a field that is ready to harvest. What’s to be done, or, more importantly, can they be prevented? Understanding the green-stem syndrome will help us answer these questions.

Green stem has been attributed to virus diseases, insect feeding and the old catch-all – environmental stress. To learn more about the causes of green stem, Bill Bruening and I investigated the effect of pod removal on the appearance of green stems.

We applied two depodding treatments [25 and 50% pod removal at the beginning of seed filling (beginning of growth stage R6)] for two years to soybean varieties from Maturity Groups III, IV and V (three in each maturity group). We monitored the appearance of brown stems and pods as the plants matured and we measured seed moisture and carbohydrate and N levels in the stems at maturity. 

Our depodding treatments produced the green-stem syndrome – fully mature brown pods on plants with green stems – 50% removal produced more green stems than 25% pod removal.

Stems on the depodded plants eventually turned brown (unless they were killed by freezing temperatures) with the delay, compared to the control, varying from as little as 4 days to more than a month. One variety still had green stems when a killing freeze occurred 46 days after the control reached 65% brown stems. We couldn’t reach any conclusions about variety or maturity group effects because of extreme variability between years and the fact that depodded plants (especially in maturity group V) were often killed by frost before the green stems turned brown.

Pods on depodded plants turned brown only a few days after the control plants with no apparent differences among varieties or maturity groups. Seed moisture levels when pods on green-stem plants turned brown were the same as the seeds on control plants.

The green stems contained much higher levels of soluble sugars, starch and N than the control plants when 95% of the pods were brown. The 50% depodding treatment always had higher levels (often more than twice as much) than the 25% treatment. 

Reducing the pod load produced classic green-stem symptoms – the remaining pods matured at roughly the same time as the control, but the stems stayed green. To understand this phenomena, we have to consider what goes on in a normal plant during seed filling.

Soybean plants start to senesce early in seed-filling. The enzymes in the leaves responsible for plant growth are destroyed and the ammo acids are shipped out to the growing seeds. Sugars and nitrogen stored in the stems are also translocated to the developing seeds. The plant basically destroys itself  to support seed growth, leaving very little sugar and N in the abscised leaves and stems.

The lack of pods reduced redistribution of sugars and N to the seeds, so the stems stayed green. This same process can cause the leaves to stay green instead of turning yellow and falling off the plant.

The cause of green stem seems to be an abnormal reduction in the pod load that prevents the normal senescence processes from occurring. So, the question – what causes green stem - is replaced with the question - what reduces the pod load?

Plants that are infected with certain viruses (e.g., bean pod mottle virus or tobacco ring spot virus) often exhibit reduced pod loads. Insects feeding on pods (stink bugs are a classic example) obviously reduce the pod load. A variety of environmental stresses (e.g., moisture stress) will increase flower, pod abortion, and reduce the pod load potentially causing green stem. Any stress that reduces the pod load can cause green stem.

The variety of causal agents makes managing green stem a real challenge, especially when the plants are set up for green stem well before green stems are visible.

What should a producer do when faced with a field full of green stems? Since the seed moisture in the pods on the green stem plants is similar to those on non-green stem plants, one option is to grit your teeth and grind those green stems through the combine. 

Another option is to wait for the green stems to turn brown (or for a frost to kill them) before harvesting. Unfortunately, the delay may increase pod shattering, or disease may infect seeds before harvest, causing reductions in yield and/or quality. Obviously, neither option is very attractive. 

There is no neat simple solution to the problem of green stem. Keeping your fields as disease and insect free as possible will help, but it won’t protect you from the environmental aspects of this problem. When faced with green-stem syndrome it pays to remember that “The art of life is to know how to enjoy a little and endure much” William Hazlett (essayist, 1778 – 1830).

Adapted from Egli, D.B. and W. P. Bruening. 2006. Depodding causes  green-stem syndrome in soybean. Crop Management. doi:10.1094/CM-2006-0104-01-R5.   ∆

DR. D.B. EGLI: University of Kentucky