ack is an experienced grower who farms 2,000 acres of canola, wheat, peas and oats southeast of Yorkton, Sask. He called me late last July to ask for suggestions on a fungicide application for his ailing canola. He told me he thought a disease was causing his canola to look sick and deformed.
When I examined Jack’s field, it was true his crop looked unhealthy and a bit deformed. The flowers were small and pale with poor, underdeveloped pods. The leaves of the crop were cupped and purpling. The symptoms were consistent throughout the whole field and some low-lying areas had no crop at all. The canola, in the 75% bloom stage, was stressed, but what was causing that stress?
When a crop is stressed, the field’s history becomes very important to determining the cause of that stress. Jack and I discussed the history of his field in detail. The rotation of Jack’s field is wheat, canola, peas, and oats. The previous crop was spring wheat and he had applied a Group 4 herbicide for in crop weed control.
Jack had been testing his soil every fall for five years in order to maintain his soil performance, and he applied fertilizer based on the recommendations of the soil tests.
Jack’s fertilizer blend and application rates were right on for maintaining the correct balance of nutrients for his field. Together we ruled out stress due to herbicide injury by the previous application of Group 2 and Group 4 herbicides, since the symptoms exhibited in Jack’s field were not consistent with damage caused by either of these herbicide groups.
Another consideration for the cause of the unhealthy canola was weather conditions. His field accumulated over 20 inches of moisture during the growing season, but when we examined other fields in the area they did not show signs of stress.
I thought one other potential source of the stress could be sulphur deficiency, but I considered this unlikely because Jack had applied ample amounts of sulphur according to his soil tests. Jack’s crop did exhibit most of the symptoms of sulphur deficiency— such as cupping, purpling, and interveinal yellowing of leaves, pale yellow flowers, poor pod development, delayed maturity, and spindly plants—but at 24 pounds/acre, Jack had applied normal, if not above normal, nutrition for his crop. I recommended we take tissue tests just to be sure.
When the results came back it was determined that, in fact, Jack’s soil was deficient in sulphur. Confused about why the symptoms didn’t show up earlier, I explained to Jack that if a field is mildly deficient in sulphur there may be good vegetative growth, but flowers and pods will be underdeveloped. Taking into consideration the extreme amount of moisture during the growing season, and that sulphur is mobile in the soil, it is possible for sulphur to leach through the soil profile under the root zone creating a lack of available sulphur. In Jack’s case, tissue tests were needed to confirm this diagnosis.
Unfortunately, since top dressing fertilizer up to the flowering stage is the only recommended practice to rectify sulphur deficiency, the diagnosis came too late for Jack’s canola—his crop was just past that stage. Foliar application of sulphur can be more effective than top dressing if applied in the late growing season, but the problem is when it contacts the leaves it is not mobile in the plant.
The use of fertilizer applied in the fall always leaves you at risk of leaching the following spring. When snow and moisture is excessive, consider corrective measures to maintain appropriate levels of macronutrients.