After working with greenhouse crops for 35 years, Brian Whipker has found there’s a system for detecting plant problems. Whipker, professor and commercial floriculture Extension specialist at North Carolina State University, addressed abiotic issues (caused by non-living factors) during the Northeast Greenhouse Conference webinar series.

Whipker said diagnostics involve a systematic approach, beginning with several key questions: Is there a pattern, and what’s the distribution in the greenhouse? When did the problem begin, and how quickly did it occur? Each answer should provide direction for next steps.

First, observe leaves for chlorosis (yellowing), necrosis (black spots), other spotting and any unusual appearances on leaves.

“When you look at the pattern of problems on leaves, if you fold the leaf in half, is there a mirror image on both sides of the leaves?” said Whipker. “The key is looking at leaves that are starting to develop initial symptoms.”

A leaf with mirror images is likely exhibiting an abiotic disorder.

Location of symptoms on the plant (flowers, leaves, stem, roots/crown) is key when assessing nutrition issues. Problems in younger plant material are likely due to immobile elements.

“If it’s a mobile element, symptomology begins in the lower leaves,” said Whipker. “A few elements come in both zones.”

Symptoms observed on older leaves include deficiencies in nitrogen, phosphorus, magnesium and potassium. Iron or manganese toxicity (low pH), boron toxicity and high EC typically occur on lower foliage.

Upper foliage deficiencies include calcium, boron, iron, manganese, zinc, copper and water stress. Middle foliage problems are usually due to sulfur and molybdenum deficiencies.

The next step is determining the distribution of the problem. If the symptoms occur on a bench of plants with problems on scattered plants in a non-uniform pattern, it’s likely a living factor – a biotic problem such as insects or pathogens. If the problem shows up on one end of a bench or throughout the entire greenhouse and is uniformly distributed, consider a mechanical, physical or chemical problem.

Timing and an estimated percentage of plants affected help diagnose abiotic issues. Did the problem happen quickly, within one to four days, or gradually, over one to four weeks?

Whipker noted that nutrition issues are usually easy to nail down, while physiologic disorders are more challenging. Spray burn or water stress is visible within a day or two, while an iron deficiency (high pH) will be vague at first then become more evident over time. Iron issues begin as slight interveinal chlorosis of recently matured leaves, then advance to bleaching over several weeks.

Interveinal chlorosis caused by Iron deficiency. Photo courtesy of Ansel Oommen,

Issues that show evidence quickly include chemical phytotoxicity, herbicide drift, air pollution (such as from furnace malfunction), environmental stress, temperature extremes and nutritional toxicity. Issues that occur over a moderate time period include calcium deficiency and botrytis. Diseases, insects and nutritional problems usually begin gradually.

“If you have a good plant today and a bad plant tomorrow, the timing factor can key you in as to what’s going on,” said Whipker. “Experience helps, and knowing what common problems look like.”

He added that familiarity with initial signs can help growers determine their next step – whether to submit a sample for insect or disease diagnosis or for plant nutrition. Talking with product reps also helps because they’re familiar with most problems.

Low pH is common in areas that are not over limestone. “Water quality matters,” said Whipker. “No alkalinity in the water means no buffering, no resistance to change. The type of fertilizer or the starting pH is going to drive substrate pH. You don’t want to be on the low pH side – it’s harder to fix and if the damage starts, it’s more difficult to turn the plant around. High pH can be treated in about a week if it hasn’t gone too far.”

Low pH problems show up in three ways: blackish-purple discoloration on lower foliage, although such patterns can also indicate manganese toxicity. Bronzing or stunted growth also indicate low pH. If plants are pH tolerant, they’ll fail to show physical symptoms.

“Any time you’re playing with pH, make sure that’s the problem,” said Whipker. “Don’t assume it. Do a pH test, then you can follow up with a tissue test.”

With high pH, plants will exhibit interveinal chlorosis on upper foliage. High pH is common in areas over limestone bedrock. This is due to water with high levels of dissolved limestone, which influences substrate pH.

Iron deficiency shows up as chlorosis on upper foliage due to iron being unavailable to the plant. “Some species don’t develop interveinal chlorosis,” said Whipker. “There’s complete yellowing. With overirrigation, iron uptake can be problematic.”

For growers who suspect a high pH problem, Whipker suggested a substrate test and examining the roots to see if they are oversaturated and hindering iron uptake.

Nitrogen deficiency (low EC) is the most common nutrient disorder. This is because nitrogen is the element most often added and is used by the plant in several aspects of growth.

“It’s lower leaf yellowing,” he said, describing typical symptoms. “With most plants you can get away with low nitrogen fertilization rates of 100 to 150 ppm nitrogen. If fertility is low, a single application of 300 ppm nitrogen will restore nitrogen levels.”

Phosphorus deficiency often manifests as purpling or olive-green spotting on leaves. This deficiency occurs with rot, cold conditions and overwatering.

“Holding back phosphorus is a method to control growth,” said Whipker. “But you don’t want to go to zero. With most species, if there isn’t a lot of fruit, you probably only need 10 to 15 ppm to maximize growth.”

Low phosphorus symptoms are evident in just a few weeks. Whipker said plants will hyper-accumulate phosphorus.

“If you apply it early, they will suck it up and store it, then it will translocate,” he said. “You could upload at the beginning then coast at the end. Look at the root system to make sure there’s no root rot and you aren’t overwatering or growing cold. A single application of 20-10-20 at 200 ppm nitrogen will provide 42 ppm phosphorus.”

Calcium deficiencies are usually due to environmental conditions. Typically seen is tip burn around the whorl on zinnias. Lack of air flow increases water loss, and leaves are bypassed in favor of the more important flower. Whipker recommends a single application of 15-0-15 at 200 ppm nitrogen to provide ample calcium for plant availability, and emphasized the importance of using good quality calcium.

Magnesium deficiencies are often seen on bedding plants, manifesting as interveinal chlorosis on lower foliage. “The good news is it’s easy to fix,” said Whipker. “Determine whether you get any magnesium from your water supply. You aren’t getting any magnesium from a 20-10-20.”

Distorted growth is difficult to diagnose due to numerous factors. Distorted upper leaves on gerbera may appear to be a boron or an herbicide issue but may be due to broad mites. Determining the cause of distorted growth usually requires submitting a sample to a diagnostic lab.

“I like diagnosing plant problems,” said Whipker. “It’s always something new.”

by Sally Colby