Green stem disorder (GSD) is a major field problem that has been problematic for soybean growers in recent years. GSD is characterized by delayed maturation of stems with normal senescence of pods. The delay in stem maturity makes harvesting beans difficult. In order for the crop to be harvested, combine adjustments are required to prevent blockage. These adjustments increase harvest time and fuel usage. In the past GSD was thought to be a result of viral infection; however, research has demonstrated that the disorder can occur in the absence of viral infection. So what’s the deal? Well, first of all, soybean varieties differ in their presentation of GSD, so there is likely some genetic basis for the disorder. Some have suggested that some fungicides can cause GSD. In particular, those with a group 11 (strobilurin) mode of action. These include fungicides such as Quadris, Headline, Quilt, Aproach, Quadris Top, Evito, Priaxor, and Stratego. It is suspected that GSD may somehow be associated with the, “greening effect” that can occur when these fungicides are applied to some soybean varieties.
A study recently published in Plant Disease examined the effects of fungicides and soybean cultivar on GSD in Illinois. I will briefly summarize part of that manuscript here. Researchers conducted field trials from 2007-2009 in a total of three different locations in the state. GSD sensitive and insensitive soybeans were planted at each site, and either Domark (group 3; tetraconazole) or Headline (group 11; pyraclostrobin) were applied to plots separately or as a tank mix at R2, R3-R4, or R5-R6. What did these studies show? As expected, GSD was greater overall in GSD sensitive varieties. In general, fungicide applications containing Headline (group 11) increased GSD incidence over unsprayed controls of plots receiving Domark alone. The amount of GSD was highly variable between years and locations and timing did not influence GSD incidence. Interestingly, GSD was associated with higher yields 39% of the time, but was highly variable and unpredictable. Yield gains for each incremental increase in GSD on a 1-5 scale ranged from 1.12 bu/A in one variety at one location in 2009 to 9.5 bu/A for another variety at a different location in 2008. Unfortunately, the authors did not report the yield data for each treatment and year. In addition, the economics associated with the GSD associated yield increase (i.e. did the yield increase pay for the cost of combine adjustments, fuel use, fungicide applications?) were not calculated.
The researchers hypothesize that the increase in yield and GSD was not due to a greening effect but rather, a reduction in stem colonization by fungi. GSD is often associated with lower levels of fungal colonization and the researchers suggest that strobilurin fungicides prevent colonization of stems by opportunistic or pathogenic fungi that lead to senescence of stems before harvest. In sum, the research illustrates the role of strobilurin fungicides in the development of GSD. It is important to note that Domark increased GSD in these studies as well, but not to the degree that sprays containing Headline did. Thus, the increase in GSD could be a result of the kinetics and fungicidal properties of the active ingredients, not associated stimulation of plant physiological processes. Thus, the role of fungicides in GSD may not be due to the infamous, “greening effect” but rather a consequence of the fungicide doing what it is intended to do, which is kill fungi. More research is required to better clarify the relationship between GSD and fungicides.