How to Treat Your Sick or Dying Plant

In a garden she developed by the corner of a building on Purdue University’s campus, in West Lafayette, Ind., Janna L. Beckerman invites trouble.

The goal of this “disease garden,” as Dr. Beckerman, a professor of plant pathology, calls it, is to get botany and horticulture students up close and personal with what she refers to as “the problem children.” In order words, to help them get to know what ails troubled plants.

This is hardly the ornamental landscape most gardeners strive for: Disfigured hollyhocks, roses and peonies that have seen better days are among the plants filling the rows — sometimes growing alongside healthy, disease-resistant varieties, for a comparison that underscores the value of making the right plant choices from the beginning.

Many of these plants suffer from hand-me-down pathogens that come in on samples from the university’s nearby Plant and Pest Diagnostic Laboratory, where people send sick plants for analysis, providing no shortage of fresh material for this living show-and-tell.

There is no shortage of insect pests, either, making the disease garden an instructive stop for Clifford Sadof, an entomology professor, and the students in his course on pests of urban landscapes.

But Dr. Sadof and Dr. Beckerman imagined something bigger than that small plot of land: a sort of virtual disease garden, with a wider audience. Their idea became the Purdue Plant Doctor website, introduced in September.

They figured that users of the photo-driven site, which features 325 ornamental plants grown in the Midwest and East, could sharpen their observational powers — “calibrating their brains,” as Dr. Beckerman put it — just as their students had.

Black spots on a rose leaf, for example, are not always a sign of black spot disease. So the Plant Doctor helps users distinguish between look-alike problems and come up with a solid diagnosis and management plan.

Neither scientist has any shortage of tales, some with tragic endings, of amateur and even professional gardeners jumping to conclusions too quickly.

Haste can result in near misses or worse. Take the orange-and-black insects that were sitting on curled viburnum leaves and about to be exterminated. It turns out they were ladybug larvae — an earlier life stage of that familiar beneficial insect — that had just polished off an aphid infestation and should be thanked, not killed.

“That’s why we included a whole section of beneficials on the website,” Dr. Sadof said. “So people don’t assume every single insect will cause a problem.”

They shudder at a story Dr. Beckerman tells of dawn redwoods (Metasequoia glyptostroboides) that were presumed dead and cut down when they lost their needles for the winter. The person in charge knew they were conifers, and therefore assumed that if they were alive, they would be evergreen — but dawn redwoods are among the few deciduous conifers. R.I.P., trees.

Although most hasty conclusions don’t come to that, it’s a cautionary tale. Imprecise diagnoses can prompt treatment that is ineffective, costly and can cause harm. The key, these scientists agree, lies in cultivating our curiosity about what’s going on — even the nasty-looking stuff.

“If you go in there with this attitude of ‘yuck,’” Dr. Beckerman said, “that’s the kryptonite of curiosity.”

Instead of rejecting the ugly, try embracing it: Have a closer look.

The process isn’t linear or always the same, but there are some basic questions you can ask and thought processes to follow.

Just as weed identification is essential before tackling anything undesirable, diagnosing your ailing plant starts with accurately identifying it. That sounds simple, but it’s often where things go wrong. Try uploading photos to the iNaturalist or PictureThis apps, or to Google Lens.

Conifers, in particular — and trees, in general — may confound gardeners. Too often, the professors said, a “pine” called out as a problem proves not to be a pine after all, but a spruce or fir. Hint: True pine trees (genus Pinus) have bundles of two, three or five needles. The needles of spruce (Picea) and fir trees (Abies) are attached individually to the twig.

Without the correct identification, Dr. Sadof said, you can’t investigate the problem. To describe what’s wrong, you have to know what the plant is supposed to look like.

“It really starts with the idea of normality,” he added. “What’s drawing your attention to your plant — what’s abnormal about it, or unusual? If you know what the plant should look like, you have a better chance of recognizing that something is not right.”

Next, he seeks the details of the abnormality, and also the extent. Take note (and take photos): Is the problem on just one plant or on several?

Or maybe it’s across various species? If so, that may suggest that it’s not a pest or a disease you’re dealing with, but an environmental stressor like drought.

Does the problem appear to be more systemic, affecting the whole plant, or is it limited to one part? After you have identified the plant, the website prompts you to identify the affected portion and then narrows the possibilities from there.

Pest damage may be limited to the foliage, like leaf mines in a columbine (Aquilegia) leaf, or to flower buds. Take your time and look for signs of the trouble (Japanese beetles on a leaf, for example) and symptoms (the holes beetles have made in the leaves).

Are there eggs, perhaps beneath the leaves? Or sawdust, insect excrement or the shed exoskeletons of earlier stages of a pest? On the other hand, if the clues include oozing, wetness, fungal fruiting bodies or an “off” smell, Dr. Sadof said, they are more likely to hint at disease.

In that case, look farther back on the plant for more evidence, Dr. Beckerman suggested. Is there a canker on the stem or some other disfigurement? “Work your way down from what you think is the primary problem, to make sure that is the primary problem,” she added.

A simple 10x or 15x magnifying hand lens used properly can reveal a lot more than looking unassisted.

Remove a representative sample from the plant and then place the lens directly in front of your eye, Dr. Beckerman said, moving the sample toward the lens until the image comes into focus. Although many pathogens are microscopic — elusive even at that magnification — details may emerge: What would otherwise read as brown spots, for instance, could be revealed as having dark or angular margins, or perhaps concentric rings inside.

Other examples: A sucking insect like an aphid may cause the yellowing of a leaf; mosaic-patterned discoloration is more likely from a virus. Any additional observations like that can help refine your search.

And sometimes, nothing is wrong. Conifers with discolored or dropping needles may sound a gardener’s alarm bell, but they all shed some of their innermost needles every year, often in late summer or fall, with the rate of shedding particular to each species.

Browning at the tips may be of more concern: What time of year the loss is occurring, and where the discoloration is, can be informative.

Maybe your diagnosis is clear-cut, and you caught spongy moth caterpillars or Japanese beetles in the act. If not, share all the information you’ve gathered, along with a plant sample, with the nearest lab to confirm a diagnosis. The National Plant Diagnostic Network website lists links to labs around the country.

A lab report will give targeted treatment recommendations, as the Plant Doctor web pages do. If application of a product is recommended, Dr. Sadof said, be sure to take those specifics with you to the garden center and carefully match them to the label ingredients.

Also, follow the instructions on when and how to use the product. Pesticides are not interchangeable or equally effective against all pests, or even every life stage of the ones they can control. Insecticidal soaps or horticultural oils are smothering agents, for example, effective when sprayed on soft-bodied insects. Neem oil repels adult insects, but can kill younger ones as they molt into more mature stages. Bt (Bacillus thuringiensis) kills young caterpillars that ingest foliage sprayed with it, but not adult moths or butterflies.

A final piece of advice: Keep a dated journal as issues arise, as it may prove helpful in anticipating repeat performances in years to come.

The whole process, from your first observation to the diagnosis, is like doing a crossword puzzle, Dr. Sadof said. You’ll never know all the words — or the dozens of pests and diseases of each of the thousands of plants in the nursery trade — but little by little, your vocabulary will grow.

“If you like it, and just enjoy the process,” he said, “it’s like, ‘Oh, wow, I missed that — but I’ll remember it next time.’”

Dr. Beckman added: “One of the most gratifying things is when a student is like, ‘I look around now, and everywhere I see beetles and rusts and galls.’ It was always there, but they just never noticed because they didn’t know how to see.”

Margaret Roach is the creator of the website and podcast A Way to Garden, and a book of the same name.

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