Agronomy: Steps to make an accurate diagnosis

The key to pest management and many other agronomy decisions is to make a proper diagnosis before taking corrective action. Keep an open mind. Consider various possibilities and work through a process of elimination, scouting for symptoms and patterns. Use lab tests when available and other steps are inconclusive. Once you identify the likely cause, take action – if there’s an economic justification to do so.

Use the following list to work through a diagnosis.

Look at the recent weather record. Weather stress – including heat, drought, excess moisture, frost and hail – can create symptoms, like stunting and discolouration, that may look like nutrient deficiency or herbicide damage symptoms. Hot weather can cause hormone responses that trigger a wide range of symptoms, including flower abortion, missing pods, empty pods, stunted growth as well as bizarre growth patterns. Sometimes a process of elimination is required before settling on hot and dry as the cause. The Canola Encyclopedia has more on these environmental effects.

Look for insect damage. Insects can feed on canola from before emergence (cutwoms, for example) and all the way through to harvest (bertha armyworms, for example). Insects often leave fairly obvious feeding patterns and they are usually found at or near the scene. Some can work virtually unnoticed (thrips, for example) and some need to be controlled before they do their damage (cabbage seedpod weevil, for example.) The Canola Encyclopedia has scouting and management tips for all major insects.

Look for common disease symptoms. Disease symptoms can show up from emergence (seedling diseases) to harvest (alternaria on pods, for example) and every stage in between. Look for discoloured lesions or spots and rots on roots, stems, leaves and pods. Clubroot is one disease that can cause major yield loss without any clear above ground symptoms. You need to dig up roots and check for galls. Other major diseases have fairly distinctive symptoms, although in some case symptoms are similar enough to cause confusion. For example, blackleg and verticillium can cause darkening through the stem, although blackleg tends to be dark and in wedges. Verticillium is lighter in colour (grey) and throughout the stem cross section. See the table at the bottom of this article for quick comparisons. The Canola Encyclopedia has scouting and management tips for all major diseases.

Look at the herbicide history. Herbicide carryover damage, caused by residual from past applications, can result from slow breakdown in the soil (due to dry conditions) or from misreading of a label (some have restrictions on how long to wait before planting canola on a field that was sprayed). Carryover damage can sometimes occur in patterns. 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. Note that with herbicide carryover, plants need to be growing to take up herbicide, so it will usually be a case of damaged plants rather than missing plants. Herbicide carryover symptoms.

Consider sprayer contamination, misapplication, drift issues. In the case of drift or spray boom contamination, damage can show specific patterns. If a field is sprayed late or with the wrong product, for example, damage can occur throughout. Check herbicide records and labels to look for clues. Applying on a hot day can increase damage, so weather can be a factor here as well. Diagnostic tips.

Review fertilizer application records and cross-reference that with soil test results. Fertilizer deficiency symptoms can show up after emergence in severe cases but are more obvious closer to maturity when plants are growing quickly and major shortages become quite obvious. To give a diagnostic example, excess heat at flowering can lead to empty pods, but so can sulphur deficiency. Nitrogen, phosphorus, potassium, sulphur and micronutrient chapters of the Canola Encyclopedia include deficiency symptoms for each.

Consider fertilizer toxicity. This is a bigger issue at emergence, especially from higher rates of fertilizer in the seed row. 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. Safe rates of seed-placed nitrogen.

Look at field variability for clues. If symptoms seem related to field variability in topography, drainage, soil type, soil quality, this may provide some clue as to the cause. 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) that reduce boron solubility. Acid soils with pH below 6.0 can also increase the risk of carryover for some Group 2 herbicides. 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.

Check the history of the land. Recently broken forage land can 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. With long-term use, residual herbicides can stack up in the soil, creating odd symptoms – especially after a dry year or years. Tight canola rotations can increase clubroot and blackleg severity.

Look for obvious patterns in the field. Patterns of damage that are confined to hill tops, side hills, water runs or field edges often suggests environmental, disease or insect problems. For example, frost and excess water will both tend to collect in low spots. For herbicide carryover, hilltops are more susceptible since they can be lower in organic matter, reducing the buffering capacity of those areas, and they remain drier, resulting in less opportunity for breakdown to occur. Repeatable patterns in rows or groups of rows usually suggest a mechanical issue with the drill. Causes can include: Single rows plugged. Whole manifolds plugged. Some openers running too deep or too shallow. Excessive seeding speed causing variable seeding depth and/or undesirable mixing of seed and fertilizer. High fan speed causes cracked seed. Seeding rate is too low, resulting in lower than expected plant counts.

Look at other fields for similar symptoms. It often helps to look at other fields nearby, especially if you think the cause might be environmental. Are neighbouring fields seeing similar symptoms that could be the result of hot weather, frost or insects, for example? Or if the farm has chronically under-applied potassium, for example, the deficiency could be more obvious in a cereal crop. 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. If the problem is not evident in other nearby fields, this gives more evidence that the problem is specific to that field, such as disease, nutrient, or herbicide issues.

How bad is it? Perspective is an important consideration before making a management step. An issue confined to a few plants in a corner of the field may not be worth the time and cost, unless that issue is a patch of herbicide resistant weeds or clubroot. Then swift early action definitely pays off.

Get a test. Soil tests, disease identification tests and herbicide carryover bioassays can help with the diagnosis. Check the Canola Watch labs list. How to use tissue tests for nutrient deficiency diagnosis.


Quick comparison of stem diseases (for pre-harvest survey)