B. Rogers Leonard, Donald J. Boquet, Boyd Padgett, Jeffrey A. Davis, Raymond Schneider, James L. Griffin, Rodrigo A. Valverde and Ronald J. Levy Jr.

In recent years, soybean growers have experienced significant problems with mature plants retaining green leaves, green stems and green pods in soybean fields. The occurrence of these symptoms either alone or together has been termed the "green plant malady," which is also known as the "green bean" or "green plant" problem.

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The green plant malady of soybeans is defined as an abnormal physiological condition when, after crop maturity should have occurred, main stems, leaves or pods remain immature. This problem can delay crop harvest and decrease the quality of harvested seed. In some cases this problem has prevented soybean harvest entirely. In other situations seed quality reductions can cause significant losses in value, or an entire load of soybeans can be rejected at the storage elevator because of excessive moisture or damaged seed.

Research scientists and soybean production specialists believed this condition could be induced by numerous biological agents or other factors, but no definitive research demonstrated these effects within current production systems. The only biological agent that had been associated with green plant malady symptoms was stink bugs, which caused severe pod and seed injury. During the past few years, however, the green plant malady has occurred in soybean fields where stink bugs have not been an issue. A few studies with fungicide efficacy have suggested that green leaf retention and green stems beyond normal crop maturity were related to the improved plant health caused by the fungicides.

Multidisciplinary research was initiated in 2008 and continued for three years to investigate factors that may contribute to this late-season green plant malady. Individual factors including insects (stink bugs), fungicide use, plant pathogens, variety genetics, herbicide applications and stress (moisture deficit) were evaluated for their effect on green leaf retention, green stems and green pods. Agronomists, weed scientists, plant pathologists and entomologists participated in a series of coordinated experiments to recreate the green plant malady in complex field experiments. The goal of the collaborating scientists was to gather information to better understand the issues associated with these problems and determine if selected production strategies can mitigate the green plant malady and its effects on soybean yield and quality.

In field experiments at several Louisiana locations, the suspected causal agents were evaluated as single factors and in combination with other possible causes. These trials included several specific objectives. One was to survey entries in the commercial soybean variety trials for genetic differences in the incidence of green stems, leaves and pods at crop maturity. Another was to evaluate the effects of water deficiency, stink bugs, fungicide treatments and selected plant pathogens on the incidence of the green plant malady symptoms and seed yield. A third objective evaluated the use of herbicidal harvest aids to overcome the incidence of green leaves, green stems and green pods influencing normal crop maturity and harvest efficiency.

During the three years of research, a common observation was the association of weather-related plant stresses to this malady in which moisture deficits restricted plant growth, reduced insect pest infestations and limited the severity of plant diseases. The overall effect reduced the frequency of plants exhibiting green plant malady symptoms. Other observations were as follows:

• Selected entries in the LSU AgCenter soybean variety trials exhibited large differences in green stems ranging from zero to 100 percent. Varieties exhibiting 100 percent green stems were common, and there appears to be a genetic predisposition to this specific symptom of the green plant malady. Incidences of green leaf and green pod retention were not as consistent or common. Funding from the Louisiana Soybean and Feed Grains Research and Promotion Board suggested these results to be included in the LSU AgCenter soybean variety recommendations. 
• Results from several trials associated moderate drought stress with green plant malady symptoms. Extreme drought stress, however, caused plants to die prematurely and was inconsistent in producing the same symptoms. These results varied by location and soybean variety.
• Selected strobilurin fungicides, such as Headline, increased green leaf retention, green stems and green pods at harvest compared with plants in nontreated plots. This result should be expected because fungicides promote healthy plants, and nontreated plants may have been infected with plant pathogens that caused premature leaf aging or crop maturity. However, applications of this fungicide to some varieties resulted in green stems even though pods had matured and leaf aging was complete. This was unexpected and is not normal. There is a genetic component to this relationship because not all varieties responded in the same manner.
• Soybean plants exhibiting green stems were collected from various locations and analyzed for the presence of viruses, including the soybean mosaic virus, bean pod mottle virus and cucumber mosaic virus, which are associated with this problem in other states. AgCenter scientists, however, found no relationship between these viruses and the green plant malady.
• Stink bugs have been the most yield-limiting insect pest in Louisiana soybeans during the past few years, and infestations were associated with severe instances of the green plant malady. Moderate populations near the treatment action threshold that persist for an extended period or reach outbreak levels two to three times the action threshold can severely damage seeds and result in green leaves, green stems and green pods in defined field areas or across entire fields.
• Leaf retention, the incidence of green stems and increased seed moisture in field plots associated with the green plant malady were reduced when the herbicide paraquat was applied as a harvest aid.

This research has provided essential information that can be used to better understand the issues associated with the soybean green plant malady. Farmers who want to minimize the symptoms of this soybean disorder can select varieties that do not express green stems at harvest. However, there is some evidence that high yields correlate with green stems, and this needs further evaluation. Several of the highest-yielding soybean varieties displayed 70-100 percent green stems.

Green stems can be considered a normal genetic characteristic of some varieties. Excessive green leaf retention or green pods after maturity are an abnormal response of a variety and indicative of the soybean green plant malady. Dates of expected crop maturity are included in the soybean variety trial information and should be used to determine if the presence of these symptoms indicates abnormal crop maturity.

Producers should use all available production practices to reduce the effects of environmental stresses on soybeans. Fungicides may be necessary to reduce the yield–limiting effects of pathogens, but these treatments can increase the incidence of the green plant malady. Cur rent stink bug treatment action thresholds and chemical control strategies are sufficient to manage insect pests and reduce the incidence of excessive green stems, green pods and delayed crop maturity. However, multiple treatments may be required to manage persistent and high infestations of insect pests.

Delayed crop maturity can cause poor harvest efficiency. Immature pods, green stems and green leaves allow more foreign matter to be collected during harvest, which increases harvest difficulty. A chemical harvest aid such as paraquat can be an important production tool in some situations to overcome the negative consequence of the green plant malady. Application timing of harvest aid is critical to managing harvest efficiency.

B. Rogers Leonard, Professor and J. Hamilton Regents Chair in Cotton Production; Donald J. Boquet, Professor and Jack and Henrietta Jones Professor; and Boyd Padgett, Professor, Macon Ridge Research Station, Winnsboro, La.; Jeffrey A. Davis, Assistant Professor, Department of Entomology, LSU AgCenter, Baton Rouge, La.; Raymond Schneider, Professor, Department of Plant Pathology and Crop Physiology, LSU AgCenter, Baton Rouge, La.; James L. Griffin, Professor and Lee Mason LSU Alumni Association Professor, School of Plant, Environmental and Soil Sciences, LSU AgCenter, Baton Rouge, La.; Rodrigo A. ValverdeProfessor, Department of Plant Pathology and Crop Physiology, LSU AgCenter, Baton Rouge, La.; and Ronald J. Levy Jr., Professor, Dean Lee Research Station, Alexandria, La.

(This article was published in the spring 2011 issue of Louisiana Agriculture Magazine.)