Crop advisor casebook: Why is this canola crop greener on one side of the field?

A Crop Advisor's Solution from the February 2, 2020 issue of Grainews

On the left of this NDVI (normalized difference vegetation index) satellite image is a 40-acre block recently purchased and cropped for the first time by the producer and on the right of the image is his previously cropped land.
Terry Moyer. photo: Supplied

David has a 2,000-acre farming operation in southeastern Manitoba where he grows corn, soybeans, canola, wheat and winter wheat. The producer had added to his farm the previous year by purchasing a 40-acre block from one of his neighbours. This was his first year cropping his newly obtained land.

Since the field was right beside another one he’d cultivated for the past two decades, David thought it best to farm both blocks of land as one large field for his canola crop. He was aware some challenges might arise from combining two side-by-side fields that had been managed differently over the years.

Although it’d been a drier year than normal, things were looking good for the crop, with little to differentiate the canola on one side of the field from the other — until just after the flowering stage in mid-July. That’s when David paid a visit to my office asking if I’d have a look at some satellite imagery taken of the field.

It was an NDVI map that indicated plant growth wasn’t consistent across the canola field and part of the crop was lagging behind. David told me he’d just driven by the field to see if he could spot the difference with his own eyes.

He was sure he could. “I can see a line where the crop is a bit greener on one side,” he said. “I wonder if it’s because of a fertility difference.”

David said he’d taken steps to try and prevent that from happening by performing soil tests in the fall and developing a fertility program he hoped would ensure nutrient levels were sufficient and consistent across the entire field.

David was worried his fertility plan was flawed, and he wanted to know if I could suggest a quick fix. “Is it too late to apply nitrogen?” he asked.

I told David I didn’t want to rush to conclusions and I’d come by his place to have a look. When I arrived at the farm, I was able to detect a very slight variation in vegetation colour at the division line of the two fields.

When I went in to have a closer look at the crop, however, there didn’t appear to be any differences in plant growth. The flowering and pod sets all looked very similar, as did the root structure of plants on either side of the dividing line. The plant density within the canola stand seemed to be consistent across the whole field as well.

This was turning into a real mystery, and my initial observations turned up little in the way of clues.

Crop advisor’s solution: Verticillium stripe present in areas under tighter rotation

I had an idea about what was happening, and it became clearer as I discussed the cropping history of the two fields with the producer. David had been growing wheat, corn and soybeans in rotation on his original piece of land and there’d been no canola planted there for seven years. This was in stark contrast to land previously farmed by David’s neighbour, where a tight two-year, wheat- canola rotation had been used for many years.

The tighter the rotation, of course, the greater the chance of a crop disease occurring. I suspected that verticillium stripe, a soil-borne disease that had just started making inroads into this part of Manitoba, might be to blame for the differences in plant growth within the canola field. I uprooted a few plants and sent them off for testing, and when the results came back, it confirmed my diagnosis.

Verticillium stripe was present, but only in the areas of the field where canola had previously been grown every other year. This allowed the pathogen inoculum to build up on that part of the field, while there was relatively little disease pressure on the other side, where seven years had elapsed since the last canola crop.

As the crop got closer to harvest, the disease symptoms worsened and became more visible as affected plants slowly dried up and started exhibiting chlorotic stripes along one side of their stems. In the end, there was a significant yield difference between the two parts of the field, with the side infected with verticillium stripe yielding 24 bushels per acre less than the other side.

Case studies have shown that the incidence of verticillium stripe declines dramatically when there’s a break of five years (or more) between canola crops. There are also no registered foliar fungicides or seed treatments for the pathogen. My advice to David was he’d be wise to work on a plan to reduce canola in his crop rotation on his newly purchased land, since this would bring the inoculum load to a more manageable level and offer him a much greater chance of success.

Terry Moyer, CCA, works for Richardson Pioneer Ltd. in Landmark, Man.

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