What are the steps to get to the bottom of plant illnesses? There are key questions that can help point you in the right direction in your diagnostic process. Below you will find some key questions and considerations to help you narrow down which production problem you may be encountering.
1. What patterns, symptoms and damage do you observe?
Scout and document any problems you observe, and note any evidence you find that can help diagnose the plant disease. Is there fuzzy growth on the leaves? Are the leaves turning chlorotic (yellowing), necrotic (black, brown, or tan spots), or have random spots (are these spots circular, angular, or water-soaked)?
Also, when looking for symptoms, note the patterns you see—and where. A quick and dirty test is to fold the leaf over and look whether the pattern is similar on both sides of the leaf. If they are dissimilar, it is more likely a biotic cause such as a disease or insect or mite pest (Fig. 1, above). Disease spread and insect and mite feeding are random, so the pattern tends to vary over the plant’s surface or randomly through the plant population. Also note it is important to view the pattern on a leaf where the symptoms recently developed. For example, with powdery mildew, the initial spotting can develop anywhere on the leaf, and over time, it will spread over the entire leaf. Also, as insect or mite populations increase during an infestation, their symptomology will engulf the entire leaf, such as webbing with spider mites (Fig. 2, below).
If the pattern is similar on both sides of the leaf, this more strongly suggests it was caused by a non-living (abiotic) problem such as a nutritional disorder, chemical damage, or physiological disorder (Fig. 3, below). Symptomology with abiotic disorders tend to manifest on the leaves in a more even pattern or predictable fashion.
For nutritional issues, the problem’s location can help further refine your diagnosis. Symptoms appearing on the older leaves suggest a disorder caused by a deficiency of a mobile element such as nitrogen, phosphorus, potassium, and magnesium. Disorders that appear on the younger foliage or developing leaves are most likely a result in low levels of immobile elements such as calcium, boron, copper, iron, manganese, and zinc. Symptoms in the mid-section of the plant are more likely to be a sulfur or molybdenum deficiency. The location of the symptoms on the plant and which leaves are showing symptoms are key pieces of information to help you determine the cause of the problem.
2. Where is it occurring?
Is there a specific location or zone in the growing area in which the symptoms appear? Are infected plants closer to an air intake fan or on the hotter end of a greenhouse?
Symptoms that appear scattered across the growing facility more strongly suggest a biotic problem (Fig. 4, below). Diseases usually begin on one plant and can then spread over time. Insect infestations tend to be in pockets. As insects and diseases spread, continual monitoring of the crop will help you spot issues and patterns early.
Problems occurring in the entire growing area or at one end of a plant canopy more strongly suggest an abiotic problem. If a nutrient is not being supplied at adequate levels, then the problem will tend to appear on a large number of plants at once (Fig. 5, below). Excessive temperatures will also affect multiple plants at one end of the growing area or over the entire setup. The extent of the problem can help you solve the mystery of what is ailing your plants.
3. When did it occur?
How long did it take for symptoms to appear? Did they appear overnight or in a matter of days? Or did they gradually appear over two weeks? Symptoms that seem to appear quickly are typically caused by chemical phytotoxicities, a nutritional overdose, air pollution, environmental stress, or chemical drift.
Some problems that appear over a slightly longer timeframe (three to 14 days) could be a calcium deficiency or a disease such as botrytis. Problems that take a few weeks to gradually appear are most likely to be a nutrient deficiency, insects or mites that take time for populations to explode, or slow-spreading diseases such as leaf spots or root rot.
4. What tools/analysis are needed to confirm suspected problems?
Answering the questions above will help you narrow down the possibilities of what the problem could be, and help determine the next step you should take.
If you suspect a disease issue, you may still need to submit a sample to a diagnostic lab to correctly confirm the problem and specific disease. This is especially true with many of the leaf spot and root rot disease issues that can mimic one another, and knowing the specific malady will help determine the proper treatment to use.
For suspected nutritional issues, confirm the pH and electrical conductivity (EC) of the fertilizer solution to make sure they are on target. To confirm the problem, submit a leaf tissue sample, substrate sample, and fertilizer sample for analysis to confirm your diagnosis.
A 20X hand lens can help growers diagnose insect and mite problems. This makes seeing larger insects such as aphids, spider mites, and thrips possible. A 100X microscope is needed to identify broad mites and russet mites.
Diagnosing physiological disorders can be more challenging. If the problem was caused by a weather event, there is little testing that can be done to confirm it. Refer back to greenhouse records and talk to your team about what might have caused the issue.
You can feel a lot like Sherlock Holmes when diagnosing plant problems. Answering the above questions will help point you in the right direction and enable you to ask additional questions to help figure out the problem. Be curious and observant, and you will more quickly catch production issues before they become large-scale issues.