Crop Scene Investigation - 61: What took a bite out of Stan's Soybeans?
Tuesday, March 8, 2016
by BERNARD TOBIN
As Syngenta agronomist Shawn Brenneman drove up to Stan's soybean field near Hamilton, he could see that something wasn't right.
It was mid-September and Brenneman wanted to take a closer look at the soybean disease the farmer said had invaded his 50-acre soybean field, nestled in the city's shadow in the midst of urban sprawl.
As he scanned the field, the first thing the agronomist noticed was the difference in plant maturity. Some of the plants were completely brown and the leaves had dropped while other plants were still green with leaves clinging steadfastly to the stem. The phenomenon appeared to be spread throughout the field.
As he examined the green plants, he observed the symptoms that Stan had described. "The pods were covered with small, circular discolorations. When I opened up the pods, the seeds were shrivelled and shrunken," recalls Brenneman. "At that point, it was pretty clear that something was actually feeding on the pods."
He explained to Stan that the culprit was unlikely to be disease. "Most diseases affect the leaves and the stalk. I checked the plants as well as the roots and the symptoms were strictly on the pods. There are a few diseases that could affect the seeds in severe infestations, but there was no sign of white mould or other disease that could have had that impact." Brenneman also checked the soybean variety Stan had planted and noted that it featured strong disease resistance. No red flags, in other words.
Stan wondered whether nutrient deficiency may have played a role, but Brenneman pointed out that the symptoms never really appeared until late in the season. Typically, nutrient problems are observed earlier, in mid-season.
The variation in maturity was puzzling. Brenneman told Stan that this phenomenon is often observed in fields that have varying soil types. "Sometimes you will see fields go from a loam to sand to gravel to clay across a field. In those cases, you will see maturity differences." But here the immature plants were widespread throughout the field and there was no correlation with soil type. "Overall, plant height was normal and there were no stunted plants. That likely means that the problem is not environmental."
Brenneman quickly concluded that Stan's suspected disease was actually the work of an insect. A thorough search of the trash across the field failed to turn up any pests. There had been reports of aphids in the area, but a quick check of the undersides of the plant revealed no evidence of them. Brenneman also considered spider mites, but these insects tend to concentrate efforts on the edges of the field. That wouldn't explain widespread damage across Stan's 50-acre field.
Taking a closer look at the scarred soybean pods, it was evident that the pest had done some significant feeding. Brenneman had seen this kind of feeding in the past. It could only be the work of one pest. He then asked Stan if he noticed anything while scouting the field. The farmer indicated that he walked the field in mid-July, but during the hot, dry days of August he merely stopped at the head of the field and observed from the cab of his truck.
As the sun faded in the September afternoon, Brenneman couldn't help but take note of the creeping urban development surrounding the farms in the area. He told Stan the pests had likely done the damage in late July and August before seeking a place to overwinter. And they may very well return next year.
Do you know what was feeding on Stan's soybeans? Send your solution to Better Farming at rirwin@betterfarming.com or by fax to: 613-678-5993. Please include mailing postal address and phone number.
Correct answers will be pooled and one winner will be drawn for a chance to win a Wireless Weather Station. The correct answer, along with the reasoning followed to reach it, will appear in the April issue of Better Farming. BF