Crop Scene Investigation - 40: Getting to the root of Jereme's failing soybeans

Monday, December 3, 2012

by BERNARD TOBIN

When he's teaching crop diagnostic class at Ridgetown College, Ontario agriculture ministry field crop plant pathologist Albert Tenuta has a clear strategy he shares with his students.   

He asks them first to establish the facts, including the field's cropping, fertility and herbicide history. Then it's time to identify what's happening in the field and eliminate causes through reason and intuition. "It's important to make decisions based on what the field is telling you," explains Tenuta. "Don't make your diagnosis fit the symptoms. It has to be based on what you see."

To illustrate his point, Tenuta tells the story of a cash cropper named Jereme and his soybean field near the Kent and Essex County border in Ontario.

"When he called, he said he had poor weed control and part of the field was dying off and going backwards, particularly after a recent rain," recalls Tenuta. Jereme also said that several people who had looked at the field suggested that it was likely a soil compaction problem because the poor-performing plants were located in a high traffic area at the front of the field. Others thought it was a herbicide issue, reasoning that leftover spray in the tank or hoses would have been applied to this area.

When Tenuta arrived to inspect the field in mid-June, he quickly established that neither compaction nor herbicides were the cause.

Tenuta noted there was no hardpan or compacted layer of soil. "The roots showed good growth and there was nothing from a soil structure perspective limiting the movement and development of those roots, so I ruled out compaction."

He didn't see evidence of spray tank contamination or drift. The affected area did not follow any spray pattern and there was no evidence of spray overlap. Tenuta also noted that Jereme had planted a glyphosate-resistant soybean variety and sprayed glyphosate. A quick examination of the roots effectively ruled out spray injury. "With herbicide damage, you typically get stubby or distorted roots and I saw none of that."

Having ruled out Jereme's suspected causes, Tenuta needed to focus on establishing the rest of the facts, using the strategy he preaches to students. He observed a circular area in the front of the field with significant weed escapes. Many of the plants were stunted and brown or bronze in colour. He concluded that the weed pressure was simply a result of the plants not developing properly and failing to fill the rows. Without a full canopy, there was plenty of sunlight to bring on weed flushes and escapes.

He also identified varying soil types in the field, with the affected area a little lighter than other areas. The field had adequate moisture and fertility appeared to be good. Jereme had recently limed the field and pH levels were sufficient.

Tenuta noted that circular field damage patterns can sometimes be linked to insects, but he crossed those culprits off his list of suspects. "Looking at the roots, we didn't see any evidence of seed corn maggot, wireworm or any insect that could have damaged the roots. There wasn't any pruning or typical feeding injury."

The one thing that kept nagging at Tenuta was Jereme's observation that the field appeared to go backwards after a recent rainfall. That comment had the pathologist thinking that the answer had to be in the soil.

When he examined the roots of affected plants, they appeared relatively normal. There was some discolouration on the roots and some browning and blackening, which would indicate some fusarium root rot and rhizoctonia, but not enough to cause the damage in Jereme's field.

"The nodules were quite abundant and when I cut them they were pink in colour, which indicates they are actively growing. That means they're doing everything they're supposed to be doing in terms of fixing nitrogen."

But as he examined the roots more closely, Tenuta noticed fine little white nodules on the roots. The mystery was solved.

Do you know the real cause of the soybean damage in Jereme's field? Send your solution 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. BF