diagnosing plant troubles, with the purdue plant doctor

AS A GARDEN WRITER, I get a lot of questions every year basically asking this: What’s wrong with my (fill-in-the-blank) plant? An accurate diagnosis is the critical first step before taking any corrective action. But how do gardeners get one? And then where do they get guidance on what to do next?

The online Purdue Plant Doctor does just that, plus offers possible solutions.

Two Purdue Professors, entomologist Cliff Sadof and plant pathologist Janna Beckerman, collaborated on the recent enhancements to the online tool at PurduePlantDoctor.com. They also have some advice on how to sharpen our diagnostic skills and to learn how to do the homework it takes to plot a course of action when trouble strikes.

Read along as you listen to the April 10, 2023 edition of my public-radio show and podcast using the player below. You can subscribe to all future editions on Apple Podcasts (iTunes) or Spotify or Stitcher (and browse my archive of podcasts here).

the plant doctor, with cliff sadof and janna beckerman

Margaret Roach: Now, I’ll just say, even though the Plant Doctor database of plants and plant troubles is geared to the East and Midwest, the same logical, methodical process that we’re going to talk about, that you’re going to help us with today, of how to get an answer applies to gardeners in any region. Yes?

Cliff Sadof: Absolutely. It’s the same process. Jan and I taught a class together for many years, and we found that this was the process to help people come up with a diagnosis when they had to determine whether it was an insect or a disease or if it was some environmental problem that was causing it.

Margaret: First, we need to know what the plant is, right? [Laughter.]

Cliff: Oh yeah.

Margaret: How do we do that, you guys? How do you recommend? Because I get a lot of questions and they’re like, “Oh, my pine tree is sick,” and then I look at the picture and it’s a different conifer.

Janna Beckerman: We have a joke that when people ask us and tell us that they have a pine tree, we ask them if it has five needles, two to three needles, or one needle. Of course, the horticulturists among us know that the one needle “pines” are spruce or fir.

Margaret: Yes. I mean, not trying to disparage; it requires some homework. And even for those of us—I’m an intermediate-level person, not as expert as you two—but I have to look closely and I have to think. Let’s talk about are there some apps or whatever that we can get an ID, first of all, of what we’re looking at.

Cliff: Since we both were faced with the same problem, teaching undergraduates who don’t know their plants. Especially during COVID, we figured out that some of these online apps are pretty good at getting you in the ballpark. There’s something called iNaturalist, which was done in part with the National Geographic Society and some other places. That tends to be pretty good. You can upload a picture and it can tell you what it is. Also, Google Lens does also a halfway-decent job. The problem is that sometimes it’s wrong. [Laughter.]

Margaret: Of course. Of course. Of course.

Cliff: But most of the time it’s right. Nine out of 10 times it gets you right. That’s an A.

Margaret: Number 1, we get an ID. I don’t know, Janna, do you want to explain to us then… We know it’s a juniper. We come to PurduePlantDoctor.com and we type in juniper, Janna, and then what happens?

Janna: Well, actually I’d like to take a step back, because we actually recognized the problems that people had using our apps—when they were dedicated cellphone apps—that they didn’t know what the plant host was. One of the ways they can use the website is to click on what type of plant they have [below, screenshot from PurduePlantDoctor].

They can click on flowers for their annuals and perennials. They can click on evergreen conifers or woody plants, and get the identification that way. And then when they click on that big button there, it opens up and we have pictures of all the different hosts that we vetted. They can also start at the website by clicking to the right side of the screen and choose some of those.

Margaret: We can look at a photo array of little thumbnails, and we can pop those up, click on those, pop them up, look at the big version and go, “Oh, that’s what it is. It’s a juniper. O.K., great.”

Janna: I really hope people use those, because I’m a plant pathologist. Prior to putting that page together, all my pictures were of dead and dying plants. [Laughter.] I had to go out and take lots of pictures of healthy plants, which as a plant pathologist, they’re so boring, but I did it and hopefully it’s really good pictures for you.

Margaret: One way or the other, either maybe using an app or using this array of photos based on these broader categories of plants on the PurduePlantDoctor.com, we’re going to get an ID. We’re going to figure out what our plant is. And then what do we do next? We need to key out what’s wrong. Yes?

Cliff: O.K. so this is a picture-driven process. Janna and I learned early on that our students could know the difference between leaves, flowers, stems, and roots. Basically what we do is you click on the plant, and then you’ll have a choice to look at images of different plant parts: flowers, branches, leaves, or what have you. And then these images come up and then you match the image that you’re seeing with the plant that’s in your hand.

Margaret: We’re going to follow this photo-driven trail of decision-making deeper, from identifying the plant and then the part of the plant, and then: oh wait, it looks like that kind of spot, or it looks like that kind of damage, or whatever. We’re going to go deeper, deeper, deeper until we get an idea of what may be wrong with our plant.

Janna: Exactly. I also want to clarify one of the things that Cliff said, so it doesn’t come across wrong, is when you’re looking at an iris, for example, and you’re looking at the rhizome, we think of that as the roots, but it’s really a modified stem.

Sometimes it’s confusing even which part of the plant you’re looking at because there’s just so much diversity in the plants. What Cliff and I kept trying to do is how can we make this simpler, but still effective and get the user to the right answer. That’s our overarching goal.

Margaret: Right. I put in petunia, and I got choices to look at petunias or Calibrachoa, which kind of looked like petunias. And then I said, “Oh, leaf problem,” let’s say. I didn’t have a Calibrachoa or a petunia that had a leaf problem, but I clicked on leaves, and then I saw an array of photos of different ornamental plants with different types of leaf issues.

Some were from nutrient deficiencies, and some were from whiteflies, and some were from powdery mildew. It’s like, oh, right, O.K., click, and I could go deeper. I love that you also have a clickable icon on the homepage that says “beneficials,” that takes me to a grid of images of everyone from ground beetles to millipedes and centipedes and spiders and so on.

I think that gardeners are too quick to assign blame to insects or other arthropods when they see someone living near or on their plant. I love that you had the beneficials category. Just tell me quickly about that.

Cliff: It started out when I first got to Purdue 30 years ago, the head landscaper said that he had this plant that the leaves were all curled and they were covered with black and orange insects. I couldn’t quite figure out what he was talking about. I went over to take a look at them and they were loaded with ladybug larvae [laughter]. This happens an awful lot.

The ladybug larvae [above; photo from PurduePlantDoctor] had just eaten all the aphids and proceeding to eat each other. We have the beneficials in there just so they don’t associate every single insect with necessarily being a problem. Janna and I both believe very firmly in this that what we want gardeners to do is to set the stage for success by using appropriate cultural controls.

It means good horticultural practices, planting things at the right spacing, using resistant varieties when possible, not overwatering, not underwatering, being careful with the fertilization. If that fails to keep the plants free from problems, then we suggest pesticides that can be used to correct the situation, but we start off with the least-toxic approach first.

Margaret: An Integrated Pest Management, an IPM, type of philosophy then.

Cliff: Absolutely.

Janna: Absolutely.

Cliff: Yes, yes. Again, I’m a layperson, but one thing that’s really helped me over the years in not jumping to the wrong conclusions is to grasp that not everything is a pest or a disease and to understand, one of you alluded to this earlier, there are the biotic and abiotic things—like road-salt damage isn’t a disease.

It might look like, “Oh, what’s wrong, my plant is sick,” but that’s not exactly the situation. I call the abiotic ones the mechanical failures [laughter]. I think that’s another thing. I noticed that you have those categories where appropriate in the selection. There’s things that are not diseases and not pests, right?

Janna: Well, we do. Sometimes we call them disorders, abiotic disorders, to distinguish between disease and disorder. Actually most plant problems if they aren’t directly caused by these abiotic disorders might have these abiotic disorders as an underlying problem. It’s really important to make sure you identify where those may be factoring in.

As a plant pathologist, one that pops into my head all the time is if you have a low spot or a soggy site, plants that are oftentimes flooded are predisposed to root rots like Phytophthora and Pythium. The diagnosis is a lot more complicated than just Pythium or Phytophthora as a root rot.

Margaret: How did we get here, right?

Janna: Exactly.

Margaret: Right, because I don’t want the plant 10 feet away or 2 feet away to succumb to, or the one in the bed across the garden, because I have the same conditions. Uh-oh, I’m making everybody vulnerable. I can learn about prevention at a much earlier stage from my…

Janna: Exactly. That most spot might be better suited for let’s not put azalea or a rhododendron there. Let’s put maybe a sweet shrub or Clethra or Itea there, which really the does so much better in a soggier sight.

Margaret: Right. I mean, I don’t know if you do, but you made this database and this tool online and you probably see the traffic reports and so forth. I just wondered, what are some of the early season troubles that people are clicking on or looking for information about? I’m in an area where, as I believe you are, where it snows in the winter usually.

The snow melts finally. And then I usually hear about everybody saying, “Oh, Margaret, my boxwoods or my junipers are discolored. They’re brown. They’re gold. They’re tan,” those types of things. Is that the kind of stuff at this time of year?

Cliff: Absolutely, yeah. We’re getting a lot of questions now of… We work closely with the Plant Pest Diagnostic Lab, and they tell us what’s going on as well. A lot of problems with boxwoods are happening right now between the boxwood leaf miner [above; photo from PurduePlantDoctor], and there’s just lots of other issues associated with that. I don’t know about where you are, but it was pretty warm in March and then it got pretty cold. Freeze injury.

Margaret: Yes. You mean plants don’t like it when one Saturday it’s minus 14 and the next Saturday it’s 62? I mean, that’s a problem? [Laughter.]

Janna: Exactly. We had a huge issue here where we dropped over 50 degrees in a single day. Generally, anyone who had peaches probably lost their peaches and lots of other stone fruit, and also your flowering cherries and things like that aren’t going to perform very well.

Right now though, the big one is definitely the freeze/winter injury starting to show up. As we get farther along, we’ll start to see your normal spring foliar disease problems coming in, like apple scab, the juniper rusts, and some powdery mildews, perhaps.

Margaret: Speaking of those starting to show up and surface, a lot of different things that can happen to plants, if we were just taking a quick look and saying what we saw, I would say, “Oh, there’s a brown spot on my such and such.” Well, that’s not really enough, is it? I mean, we really need as gardeners to slow down and observe.

I mean, I have a hand lens. I’m no scientist, but some stuff I want to look at more closely. Because if I want to reach out for help to my extension service, for instance, I want to be able to say what I’m seeing a little better than, “I’ve got a brown spot.” Do you know what I mean? Do you encourage people to look closely?

Cliff: Let me go first. What we have in there is once you get led to a diagnosis, we have a section called lookalikes. You can compare those things. Part of what we’re trying to do is give you a vocabulary to look at things. I don’t know, when I first started looking at insects and I guess I got a Borror and White’s guide to insects, all the insects looked alike. I didn’t know how to look at them.

This is the same thing that happens with gardeners and trying to diagnose problems. They just say, “Wow, it’s just brown leaves or crinkled leaves or there’s missing parts.” But as you get more and more familiar with it and you look at these pictures more closely, you get a better discerning eye.

As a plant pathologist, Janna, you have some really strong feelings about the limitations to picture diagnosis, right?

Janna: It is a huge issue, but I want to first commend Margaret: Getting a hand lens is so important. I think anyone who’s serious about their gardening or their plant healthcare really has to start at a hand lens, in order to start seeing these things and learning all of these differences.

And then to tag off of what Cliff says, that it is overwhelming at first, and to recognize that whole beginner mind and just taking the deep breath and reminding yourself that with practice you’ll get better. It really is just a matter of practice. Cliff has been practicing for over 30 years. I’ve been practicing for over 25 years.

We’ve had a lot of practice. In that process, what I’ve learned is as great of a tool as a hand lens is, most of my pathogens, unfortunately, are microscopic and that might not be sufficient. That’s when the extension service comes in and the plant and pest diagnostic labs in your respective states to help you out.

Margaret: That’s why I was going to bring that up because, for instance, I had this small tree, this Korean maple, Acer pseudosieboldianum, that I had for many years and I just loved this tree. It was right at the entrance of the garden. Fall color [above] and much, much hardier than a lot of the so-called “Japanese maples.”

Janna: What a great tree.

Margaret: Yeah. The poor thing, it was fabulous for years and years and years and years. And then maybe three or four winters ago, the winter ended and not a leaf opened on the tree. It had not looked sickly. There had been nothing. It had gorgeous fall color. It had done all its usual things, and it was just dead. It was just totally dead.

And that was one where I turned to the extension service for help and asked, “Should I look around in the root system? Should I send a sample? What should I do?” Not that I could revive it, I don’t mean, but just because I wanted to know. Do you know what I mean? It stumped me.

Janna: It’s so important. Well, it’s so important. Were you punning “stump” there?

Margaret: Was I? I’m sorry.

Janna: Punning the word stump.

Margaret: Oh, Janna, I do that all the time. I say things like dig in, we got to dig into… I mean, I am awful. I’m sorry.

Janna: [Laughter.] I love it.

Margaret: It’s subconscious. It’s totally subconscious.

Janna: I love your plant choice because I tell people over and over to get the Korean maple because it’s so much hardier. Plus, I really like saying pseudosieboldianum.

Margaret: Yes, yes, yes.

Janna: And now I’ve completely forgotten where I was going. I’m so sorry.

Margaret: Well, no, I said I was trying to figure out should I have gone to the extension service with something just to get the forensics, not to fix anything?

Janna: Well, it’s important to know what went wrong so you can make sure that… Let’s say for example, I don’t think this is likely, that it was something like Verticillium. You wouldn’t want to come back and plant another maple in that spot.

Margaret: That was what my first guess was when I talked to a nurseryman friend and he said, “Oh, I wonder.” He said, “Let’s look around.” Exactly. I haven’t planted another one nearby because that was a warning, but I could have sent a sample, I suppose then, to the extension. Yes?

Janna: For situations like that when plants die suddenly, those are probably some of the worst to diagnose, unless there are obvious symptoms of herbicide injury.

Margaret: No.

Janna: And especially with these unusual plants or the collector plants like green maple because we really don’t have the widespread planting like we do with the freemanii maples. We don’t have the data to know all of the different things that can go wrong. One of the worst feelings I get, and I know Cliff feels the same way, is when our best answer for you is, “I don’t know.”

It’s an honest answer, it’s a horrible answer, but sometimes that’s the best any of us can do. We just don’t have enough data. I would rather tell somebody, honestly, I don’t know, than give them an answer with a certain degree of certitude that has a greater likelihood of being wrong.

Cliff: As professionals, we wanted people to understand that we do not know everything and we’re giving it our best shot with the best scientific tools that we have available. And not knowing is O.K.

Margaret: Along the way through the season as it progresses, are there other greatest hits or surges in searches and traffic? I would think roses must be one where a lot of people look for help.

Janna: Well, it’s interesting because as more and more people start growing the Knockout roses were just so insanely popular and remain so popular, I’m really sad because I don’t see black spot as a sample anymore like we used to.

Margaret: That’s so funny, a pathologist who misses black spot [above; photo from PurduePlantDoctor].

Janna: I do. Well, I miss it for two reasons. One, it’s one of those problems that it’s pretty easy to diagnose, and it’s also pretty straightforward to manage, so I’m right there. But for me, one of the things that I really miss is when people are growing those roses, and there’s a lot to be said for them, but very few of them actually have fragrance.

To my mind, a rose without fragrance is just… It’s missing something. For a little bit of effort, growing a David Austin Rose, the Meidiland or any of the floribundas or hybrid tea, there’s an experience that you just don’t get. I miss it.

Cliff: On the insects side, there’s two things that come to my mind, and it all has to do with climate change. What we’re finding is that the weather becomes more chaotic. We have wetter springs and drier summers. The wetter springs cause this flush of growth that make it great for sucking insects and also for a lot of Janna’s foliar diseases. I’m sure some others too.

But then when it gets hot and dry, we start getting problems with spider mites, and then we start getting problems with borers, insects that bore beneath the trunk of trees and the like. I see that pattern is something that is recurring. And then of course, we’ve got the lovely invasives like spotted lantern fly, which is another nightmare.

Margaret: Oh my goodness, yes. It’s funny, you just did one like I did. You said the lovely invasives. They are lovely. You meant it sarcastically, but that happens to be a gorgeous animal, but oops, hideous, right?

Cliff: Well, the ash borer was another pretty one.

Margaret: It was beautiful. It is beautiful, yes. Hideous, right. No, no, I know. Nature is fascinating.

In the last few minutes, in terms of using this tool and in terms of slowing down and looking more carefully, first, ID’ing the plant and then figuring out what the troubles are: any other things that you really want to highlight about being a better gardener that way?

Because I think learning these lessons and then preventing, like the example you gave, Janna, that I probably shouldn’t plant another maple where I might have had Verticillium, a soil-borne disease like that. I think that’s super important. Any other things that you want to really stress about or things that people come to you for?

Cliff: It’s hard to do something about too much rain. I mean, within limits, you can improve drainage. But in terms of drought, you can install an irrigation system to irrigate the plant appropriately. I say that so you don’t want to be wetting the leaves every week if you don’t have to.

The right kind of irrigation would help prevent a tree from succumbing to borers. There’s a whole other area of things, scale insects and the like, that also become problematic with intensive drought as well.

Margaret: They prey upon the stressed-out, the vulnerable individual plants, right?

Cliff: Correct.

Margaret: That’s a good point is the more that we support the plants, the more that we help prevent susceptibility to some of these pests and diseases. Yes.

Cliff: Water is something that you use without…

Janna: No. I was going to go back and actually use Margaret’s example with her maple and Verticillium. One of the first things I would do assuming you haven’t put anything back in that site, is planting some eggplant there just for a year [laughter]. Nothing gets Verticillium like an eggplant.

Margaret: That’s hilarious. I had no idea.

Janna: If Verticillium is there, your eggplant should go down fairly quickly and then you’ll at least know what’s going on. Maybe you want to try again and put in another Korean maple.

But the other thing you can do is ,if it does turn out that your eggplant succumbed to Verticillium, is on the website, there’s a button you can press for “host plants attacked,” and we’d have a list of all the other plants that are susceptible.

That way you could use it to avoid making a mistake and putting something in that would also end up getting Verticillium wilt, like say a redbud, for example, since we know that’s a possible host.

I guess the last thing I would say is have fun with your gardening.

Plants die. Anybody who’s come into my yard, I mean, most of the pictures of the plant diseases, I’m not joking when I say a lot of them come from my backyard.

Margaret: Yes, no, of course. And that’s why we have compost heaps, right? [Laughter.]

Janna: Exactly. Exactly. Sometimes my garden looks like a compost heap. Have fun with it.

Margaret: We’ve run out of time, but we’ve been talking to two Purdue professors, Cliff Sadof and Janna Beckerman, about the PurduePlantDoctor diagnostic tool. I appreciate your taking the time today. I look forward to speaking to you again as the season heats up and we see what wants to drop dead over here next time [laughter]. Thanks a lot to both of you.

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MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its 14th year in March 2023. It’s produced at Robin Hood Radio, the smallest NPR station in the nation. Listen locally in the Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) Mondays at 8:30 AM Eastern, rerun at 8:30 Saturdays. Or play the April 10, 2023 show using the player near the top of this transcript. You can subscribe to all future editions on iTunes/Apple Podcasts or Spotify or Stitcher (and browse my archive of podcasts here).