Agronomy: Scouting with an open mind

May 9, 2018 - Issue 6

Crop scouting can begin the week after seeding to check on early emergence issues. The key with scouting at any crop stage and for any issue is to make a proper diagnosis before taking corrective action. Here are common early-season problems in canola and the set of symptoms often associated with each:

Seedling disease. Infected stems may look like frost damage, but look at the location. Stem and hypocotyl damage near the soil surface and below is more likely a seedling disease. Seedling diseases can be more common in canola seeded too deep into cool soils. That’s simply because the vulnerable period is longer and vulnerable plant surface is bigger: plant’s seeded deep take longer to emerge and the hypocotyl has to be an inch or two longer.

Insect chewing. Insect chewing will take chunks out of them stem or slice the stem right off. Seedling disease will produce a soft rot or lesions on or through the hypocotyl. Use a magnifying glass to verify disease or insect chewing.

Herbicide carryover. To identify a growing-point injury from Group 2 herbicide carryover, look for increased damage at the field every point where you’d expect sprayer boom overlaps and in the corners of the field. Where the sprayer makes a sharp turn, one end of the boom is going faster than the speed sensor and application rates would be a small fraction of normal, so injury symptoms should be minimal. Meanwhile rates would have been many times higher than normal at the other end, which would have been almost stopped while the machine made the turn. (The rate-change issues will not occur with sprayers using pulse width modulation systems.) Hilltops are a worst-case scenario for herbicide carryover because they typically have lower organic matter and lighter soils that reduce moisture availability and lack the clay and organic matter that provide more binding sites to hold the herbicide relative. This means the herbicide residue is more available for plant uptake. Sometimes a geo-referenced check plot with a quick off-on of a couple of nozzles or a section can be a useful diagnostic tool now that sprayers are leaving fewer gaps or overlaps in the field.

Fertilizer toxicity. This will occur in patterns, often worse in dryer or lighter parts of the field that don’t have the moisture to diffuse fertilizer concentrations beside the seed. You may also notice differences row to row if some openers are worn more than others and not providing required seed/fertilizer separation.

Flooding. Seedlings may emerge in flooded water, but lack of oxygen can impede root development and nutrient uptake, stalling growth. In the case of flooding, stands may emerge nicely on hill tops and not at all in low spots. Read more.

Frost damage. Look at the stem. Severe frost damage will cause stems to brown and dry up at the growing point, up from the soil surface. If there has been a frost but the growing point between the cotyledons stays or becomes green again 3-4 days after the event, the seedling should survive the frost. Read more.

Next steps in a diagnosis

Because symptoms like purpling, for example, can have many causes, further analysis may be required. Identification of symptoms is just one step. Here are the other steps to take to reach a proper diagnosis:

Soil test. Are any nutrient shortages evident? Other soil test parameters such as texture, pH and electrical conductivity may also provide clues in the diagnosis. To give a couple quick examples, boron deficiency is more likely to occur in sandy soils with low organic matter, or in high pH soils (8.0 or higher) which reduce boron solubility. Acid soils with pH below 6.0 can also increase the risk of imidazilonone (Odyssey, Solo, Ares, Assert) herbicide carryover damage. Herbicide molecules will bind with soil particles, but in acid soils, these molecules are more easily “washed off” the soil with a rain and can be taken up by canola plants. The opposite is true of residual sulfonylurea (SU) herbicides where high pH means slower breakdown and greater availability.

Fertilizer history. Look at what rates and sources have been used on that field the past 5 years. Have these rates have been adequate to match removal? Perhaps you have been applying adequate rates of nitrogen, phosphorus and sulphur, but what about potassium? Most Prairie soils still have adequate potassium reserves, but tight canola rotations and high canola yields will be mining the soil and deficiencies are starting to show up on some farms. Given that 75% of potassium is returned to the field in the straw, a rate of 30 pounds of actual potassium per acre would be enough to maintain soil reserves for a 40- to 50-bushel crop.

Tissue test. These can be useful tools, but only if used in context with all other diagnosis techniques — especially environment. A tissue test may show that the plant is deficient in phosphorus or calcium, for example, but heavy rains and saturated soils may have stopped nutrient uptake. The nutrients may be in the soil at adequate rates, but the plant simply can’t access them. So even if the tissue test shows deficiency, in this situation plants may recover on their own when soils dry out again. Comparative tissue tests that look at differences between healthy and poor areas of the field are often the most useful.

Herbicide history. Is there any chance of carryover from products applied one or two years ago? Have the field seen similar products used multiple times? In dry conditions or very wet conditions, herbicides can take longer than expected to break down to safe recropping levels for canola. Maybe a comment about submitting samples to a diagnostic lab for another opinion on the diagnosis. They are usually pretty inexpensive relative to the potential crop loss.

Environment. Cold, wet, hot and dry can all stress canola plants, creating symptoms that may look like nutrient deficiencies. If neighbors have similar symptoms, they are probably the result of frost, excess moisture or drought stress.

History of the land. Recently broken forage land is likely to be depleted in a lot of nutrients. Canola seeded into long-term alfalfa land is one example where you may see severe crop stunting and delayed maturity as a result of phosphorus deficiency.

Look at other fields for similar symptoms. When diagnosing for a specific nutrient, first look at a crop that tends to be most sensitive to that nutrient. For example, cereals are more likely to show potassium deficiency symptoms when soil levels drop below 300 lb./ac. Symptoms may not be obvious in canola until soil reserves drop below 150 lb./ac. Potassium deficiency symptoms in canola include a yellowish brown “scorched” look at leaf margins edges, yellowing of bottom leaves first, and uneven pod maturity.

Further reading:

Scouting toolkit
Canola Diagnostic Tool

Canola Watch