Crop Scene Investigation - 19: What caused Roger's aphid infestation?

Monday, May 4, 2009

by SHAWN BRENNEMAN

The expression "more than meets the eye" often applies when there is trouble brewing in a soybean crop. This was certainly the case when Roger from Perth County called in late June.  

"My soybeans aren't looking good in one part of a field," he said. "The plants are discoloured. I haven't had a chance to scout very closely, but I can see some are yellow and some are purplish-black."

I agreed to head over after I'd finished another farm visit. So far, the growing season had been droughty and I'd been receiving lots of calls for agronomic advice.

Roger's problem area was easy to spot from the edge of the field – the affected plants were concentrated on a large knoll. The crop had reached the flowering stage and, for the most part, seemed to be developing normally.

As we walked the clay-loam field, I quizzed Roger on its history. "The yields have been great in the last two years. We've never had anything like this in our beans before," he said.

Yellowing could be caused by many things – disease, insects and nutrient deficiency all come to mind. I asked about the soil fertility. Roger admitted he hadn't done a soil test recently. "We cut down on our fertilizer application rates this year in order to reduce some input costs," he added.

One yield-robbing culprit became apparent as we climbed the hill. Soybean aphids were so numerous on the affected plants that they appeared to be almost "dripping" from the leaves. We scouted the discoloured plants on the knoll as well as several healthy-looking plants in the lower-lying areas of the field. I estimated about 700 to 1,000 aphids on the plants in the high areas and 50 to 250 in the fringes.

The fact that there weren't significant numbers of the small insects in the remainder of the field told me the problem wasn't just aphids. There was an underlying issuewhich needed to be addressed.

I gave Roger an explanation for the purplish-black plants – the waste secretions of the tiny sap-sucking aphids caused the sooty mould on the leaves. "But there's more to it than meets the eye," I said. "Something besides the aphids is contributing to the yellowing of the plants." I took a soil test to prove my theory.

Do you know the root of Roger's problem? Why were the aphids attracted to plants on the knoll? Send your solution along with your name, address and telephone number, to Better Farming at: rirwin@betterfarming.com or by fax to: 613-678-5993.

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 next issue of Better Farming. BF

Shawn Brenneman, CCA, is a Sales and Agronomy Manager for NK® Brand, Syngenta Seeds. He is based near Tillsonburg.