How to Treat Your Sick or Dead Plant
In a garden she developed at the corner of a building on the campus of Purdue University, in West Lafayette, Indiana, Jana L. Beckerman is stirring up trouble.
The goal of this “Disease Garden,” as Dr. Beckerman, a professor of plant pathology, calls it, is to bring botany and horticulture students closer to what she refers to as “problem kids.” Put the words in order, to help them recognize what the troubled plants are experiencing.
This is hardly the ornamental landscape most gardeners strive for: misshapen hollyhocks, roses and peonies that have seen better days are among the plants filling the rows—sometimes growing alongside healthy, disease-resistant varieties, a comparison that underscores the crop’s making value. Make the right plant choices from the start.
Many of these plants suffer from widespread pathogens that come from samples from the university’s nearby Plant and Pest Diagnostic Laboratory, where people send diseased plants for analysis, providing no shortage of fresh material for this live display.
There’s no shortage of insect pests either, making the Disease Garden a teaching stop for Clifford Sadoff, professor of entomology, and students in his course on pests of urban landscapes.
But Dr. Sadoff and Dr. Beckerman imagined something bigger than that small plot of land: a kind of virtual disease park, with a broader audience. Their idea became the Purdue Plant Doctor website, which was introduced in September.
They found that users of the photo-based site, which lists 325 ornamental plants grown in the Midwest and East, They can sharpen their powers of observation—“calibrate their brains,” as Dr. Beckerman puts it—just as their students did.
Black spots on a rose leaf, for example, are not always a sign of black spot disease. So Plant Doctor helps users distinguish between similar problems and come up with a solid diagnosis and management plan.
Neither scientist has a shortage of anecdotes, some with tragic endings, about amateur and even professional gardeners jumping to conclusions too quickly.
Rushing can lead to near accidents or worse. Take, for example, the orange-and-black insects that were sitting on the curled viburnum leaves and were about to be exterminated. It turns out they were ladybug larvae — an early stage in the life of that familiar beneficial insect — that had just rid themselves of an aphid infestation and should be thanked, not killed.
“That’s why we’ve included a whole benefits section on the website,” Dr. Sadoff said. “So people don’t assume that every insect is going to cause a problem.”
They shudder at the story Dr. Beckerman tells about dawn redwood trees (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, so he assumed that if they were alive, they would be evergreen, but dawn redwoods are among the few deciduous conifers. Rest in peace, trees.
Although most hasty conclusions fall short, it is a cautionary tale. Inaccurate diagnosis can lead to ineffective and expensive treatment and can cause harm. These scientists agree that the key is to develop our curiosity about what happens, even things that seem bad.
“If you go in there with this flawed attitude, that’s the kryptonite of curiosity,” Dr. Beckerman said.
Instead of rejecting the ugly, try embracing it: take a closer look.
For starters, what is normal?
The process is not linear or always the same, but there are some basic questions you can ask and thought processes to follow.
Just as identifying weeds is essential before treating anything undesirable, diagnosing your ailing plant begins with accurately identifying it. This may seem simple, but things often go wrong. Try uploading photos to iNaturalist, PictureThis, or Google Lens apps.
Conifers in particular and trees in general may confuse gardeners. Often, the professors said, the so-called problem “pine” tree turns out not to be a pine tree after all, but a spruce or fir. Hint: True pines (genus Pinus) have bundles of two, three, or five needles. The needles of spruce (Picea) and fir trees (Abies) are individually attached to the twig.
Without proper identification, you can’t investigate the problem, Dr. Sadoff said. To describe the problem, you need to know what the plant is supposed to look like.
“It really starts with the idea of normalcy,” he added. “What draws your attention to your plant – what is abnormal about it, or unusual? If you know what a plant should look like, you have a better chance of recognizing that something is not right.”
It’s all in the details
Next, he looks for details of the anomaly, as well as how big it is. Take notes (and take photos): Is the problem with just one plant or several?
Or maybe it’s across different genres? If so, this may indicate that you are not dealing with a pest or disease, but rather an environmental stressor such as drought.
Does the problem seem more systemic and affects the entire plant or is it limited to one part? After you’ve identified the plant, the site prompts you to select the affected part and then narrows down the possibilities from there.
Pest damage may be limited to foliage, such as the leaf mines of columbine (Aquilegia), or to flower buds. Take your time and look for signs of the problem (Japanese beetles on a leaf, for example) and symptoms (holes made by beetles in the leaves).
Are there eggs, perhaps under the leaves? Or sawdust, insect droppings, or fallen exoskeletons in the early stages of the pest? On the other hand, if evidence includes oozing, wetness, fungal fruiting bodies or a foul odor, Dr. Sadoff says, it likely indicates disease.
In this case, look to the plant for more evidence, Dr. Beckerman suggested. Is there a lesion on the trunk or other deformity? “Work on what you think is the core problem to make sure of that He is “The main problem,” she added.
A simple 10x or 15x hand magnifying glass used properly can reveal much more than just showing up without assistance.
Remove a representative specimen from the plant and then place the lens directly in front of your eye, moving the specimen toward the lens until the image is in focus, Dr. Beckerman said. Although many pathogens are microscopic – elusive even at this magnification – details may emerge: what can be read as brown spots, for example, may be detected as having dark or angular margins, or perhaps concentric rings. Inside.
Other examples: A sucking insect such as an aphid may cause leaf yellowing; The mosaic discoloration is most likely caused by a virus. Any additional feedback like this can help improve your research.
And sometimes there is nothing wrong. Conifers with discolored or falling needles may ring an alarm bell for a gardener, but they all shed some of their inner needles each year, often in late summer or fall, with a shedding rate specific to each species.
Turning brown at the tips may be more concerning: what time of year the color loss occurs, and where the color changes, can be telling.
Before you shop for a solution
Your diagnosis may be obvious, and you’ve caught sponge moth larvae or Japanese beetles red-handed. If not, share all the information you’ve collected, plus a sample of the plant, with the nearest laboratory to confirm the diagnosis. The National Plant Diagnostics Network website lists links to laboratories across the country.
The lab report will provide targeted treatment recommendations, as do the Plant Doctor web pages. If a product is recommended, be sure to take these details with you to the garden center and carefully match them to the ingredients on the label, Dr. Sadoff said.
Also follow the instructions for when and how to use the product. Insecticides are not interchangeable or equally effective against all pests, or even every life stage of those they can control. For example, insecticidal soap or horticultural oils are suffocating agents, and are effective when sprayed on soft-bodied insects. Neem oil repels adult insects, but can kill younger insects as they move to more mature stages. Bt (Bacillus thuringiensis) kills young caterpillars that ingest foliage sprayed with it, but not adult moths or butterflies.
Final tip: Keep a dated diary of when problems arise, as it can be helpful in anticipating repeat performance in future years.
The entire process, from first observation to diagnosis, is like solving a crossword puzzle, Dr. Sadoff said. You’ll never know all the words — or the dozens of pests and diseases for each of the thousands of plants in the nursery trade — but little by little, your vocabulary will grow.
“If you like it, and you just enjoy the process, it’s like, ‘Oh, wow, I missed that — but I’ll remember it next time,'” he said.
Dr. Beckman added: “One of the most satisfying things is when a student says, ‘I’m looking around now, and everywhere I see beetles and rust and galls.’ It was always there, but they never noticed it because they didn’t know how to see.”
Do you want more help? Try these online tools
Margaret Roach is the creator of the website and podcast Path to the parkAnd a book of the same name.
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