For several years, Louisiana soybean growers have experienced significant problems with plants that retained excessive green leaves, green stems and/or green pods after they have reached normal maturity. These symptoms – either alone or together – have been termed the "green plant malady" or "green bean malady."
LSU AgCenter researchers define the green plant malady of soybeans as an abnormal physiological condition that appears after crop maturity should have occurred, said LSU AgCenter entomologist Dr. Rogers Leonard.
"This problem delays crop harvest and decreases the quality of harvested seed," Leonard said.
In some cases, green bean malady 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.
Previous work had shown that one biological agent associated with green plant malady symptoms was stink bugs that caused severe pod and seed injury. During the past few years, however, symptoms of the green plant malady have occurred in soybean fields where stink bugs have not been an issue.
"These symptoms are caused by a combination of factors," Leonard said. "In any given field, we cannot make a determination on what’s happened until we obtain a history of agronomic and plant protection information."
The research program began three years ago and finished last year.
"Cooperative studies with a multidisciplinary team of LSU AgCenter faculty provided a number of answers to a complicated problem," Leonard said.
The researchers identified a number of causal agents, and any combination of those can make the problem much worse, he said.
A series of coordinated experiments re-created the green plant malady in complex field experiments. The researchers wanted to get a better handle on the issues associated with these problems and determine if any strategies can be used to mitigate the green plant malady and its effects on soybean yield and quality.
They studied insects – primarily stink bugs – fungicides, plant pathogens, variety genetics, herbicide applications and moisture deficits for their effects on green leaf retention, green stems and green pods. Specific experiments examined:
The effects of varietal characteristics on symptoms (using the entries in the commercial soybean variety trials).
The interactions of water deficiency, stink bugs, fungicide treatments and selected plant pathogens on symptoms and seed yield.
The use of harvest aids to overcome the incidence of green leaves, green stems and/or green pods influencing normal crop maturity and harvest efficiency.
One common observation during this period was weather-related plant stress in which moisture deficits restricted plant growth, reduced insect pest infestations and limited the severity of plant diseases, Leonard said. The overall effect was fewer plants exhibiting the symptoms.
Results from several trials associated moderate drought stress with green plant malady symptoms, Leonard said. Extreme drought stress, however, caused plants to die prematurely.
For entries in the LSU AgCenter’s soybean variety trials, green stems ranged from 0 to 100 percent. Varieties with 100 percent green stems were common, and this appears to be a genetic predisposition. Green leaf and green pod retention were not as consistent or common, Leonard said.
Dr. Don Boquet at the LSU AgCenter’s Macon Ridge Research Station has characterized all the soybean varieties and included these results in the LSU AgCenter publication on soybean variety recommendations, which will provide additional information for variety selection in future years, Leonard said.
LSU AgCenter plant pathologists showed that selected strobilurin fungicides increased green leaf retention, green stems and green pods at harvest.
"This should be expected because fungicides promote healthy plants, and untreated plants may have been infected with plant pathogens that caused premature leaf aging and early crop maturity," Leonard said.
The entomologists confirmed that allowing stink bugs to cause excessive seed injury consistently generated symptoms of the green bean malady and that keeping infestations below the action threshold reduced the incidence of this problem.
In addition, LSU AgCenter plant scientist Dr. Jim Griffin and his students showed that excessive green leaf retention, the incidence of green stems and the high seed moisture in field plots associated with the green plant malady were reduced when the herbicide paraquat was applied as a harvest aid, Leonard said.
"The best preventative measures are any agronomic practices and plant protection strategies producers can do to minimize stress on a plant and greatly reduce the potential of this problem," Leonard said. –Rick Bogren
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