Diagnosing plant problems with a systematic approach
When analyzing a plant problem, it’s best to not immediately jump to conclusions based on the first thing you see. Jonathan Ebba, a UNH Extension landscape and greenhouse field specialist, recommends growers look at their affected plant from the broadest level first, and then get specific.
With this systematic approach, growers are able to push past their blind spots and “zero in on new and emerging issues and problems,” Ebba said.
A lot of times, growers will find that their plant problems were not caused by a single factor but were instead caused by multiple accumulating factors. Fixing the problem on the surface level will not save your plant if there’s a bigger problem hidden underneath.
Ebba had a three-step approach to diagnosing plants. The grower must first determine if the problem is abiotic (not living) or biotic (living). Abiotic damage patterns tend to be “fairly ordered and fairly symmetrical” across the plant, no matter the plant’s stage of growth. Biotic damage patterns tend to be chaotic and much more random-looking across the plant. The damage targets a certain portion of the crop and radiates out from the center point, showing more damage in the middle and less damage as you move out.
If the problem is biotic, the grower must determine what’s causing it – a pest or a disease? Keep in mind that not all cases are obvious, as some diseases create plant damage that look like insect feeding. If determined that the problem is a disease, you now have to determine if it’s a fungus, bacterium, virus, phytoplasma or nematode.
Although not 100% accurate, Ebba mentioned how fungi usually leave a rounded lesion with a bullseye pattern in the center; bacteria leave an angular/squared-off lesion; and viruses leave behind a “ghosting pattern,” which is discoloration inside of the plant tissue. If you determine that the problem is from a pest, you must analyze the damage to see if it’s a chewing pest (caterpillar, slug, snail, etc.) or a piercing/sucking pest (aphid).
Abiotic issues “can be a little harder to figure out,” Ebba admitted. Abiotic factors can be a one-time scenario, such as spray damage, but they can also be progressive, meaning it’s a continuing problem that’s causing damage over time, like nutrient deficiencies.
When analyzing nutrient deficiencies, it’s important to remember that “pH is often our first culprit in potting mixes,” followed by lack of airflow for nutrient uptake. If the plant damage is most pronounced in the older foliage, there’s most likely “a deficiency of mobile elements,” such as nitrogen, phosphorus, potassium, magnesium and molybdenum. On the other hand, if the old and new foliage are affected, there is likely a deficiency in immobile elements like iron, calcium, manganese, boron and sulfur.
With so many possible factors causing plant damage, growers can always send plant samples to labs be tested. The UNH Plant Diagnostic Lab recommends sending in the full affected plant, a list of observations and a description of the severity – how much of your crop is affected? Is the damage concentrated or uniform?
For more information, contact your local Extension office.
by Kelsi Devolve
