THE PURSUIT of perfection in tomatoes can be an elusive goal: The foliage gets spotty or yellow and may even start dropping; fruits may be reluctant to set, or slow to ripen, or disfigured in some frustrating way. But on we soldier against the odds because the juicy payoff is an essential rite of summer.
Are you facing any tomato troubles about now? Well, there is help to be had, in this updated interview from Dr. Meg McGrath, a longtime vegetable pathologist for Cornell University. Though people often lump everything into “late blight,” it often isn’t actually that disease at all, and it’s critical to know just what you are up against.
Meg knows from tomato diseases—both as a passionate backyard gardener and as a scientist, headquartered at Cornell’s Long Island Horticultural Research & Extension Center in Riverhead, New York.
Data and photos from Dr. Meg McGrath’s laboratory research, along with findings from expert colleagues around the nation including University of Minnesota, Purdue, UMass-Amherst and more, made their way in 2015 into an app called Tomato MD that can help gardeners narrow down what’s going wrong, and learn what to do.
Read along as you listen to the reprised August 3, 2015 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).
my q&a on tomato disease, with dr. meg mcgrath
Q. Welcome, Meg.
A. It sounded like you were describing my garden there at the start. [Laughter.]
Q. Oh, I could make an even longer list—I think I’ve seen it all over the years, of what I call “tomato troubles.”
I get questions from the beginning of the season and then especially now from readers and listeners, who all say, “I have late blight.” But of course it’s not all one big bucket; every tomato trouble isn’t late blight. Shall we start with an overview of the kinds of issues that can befall tomato plants?
A. I think sometimes people use “late blight” as a general term that plants are dying, but it is a specific disease and one that can be confused with other things.
So the kind of causes of diseases are what we refer to as “biotic” ones—the common things we think of as causing disease (in people or animals or plants), like bacteria and viruses and fungi. And then we have diseases that are more disorders—and we refer to those as having an “abiotic” cause, not a biological cause.
Probably the most common one is blossom end rot in tomatoes.
A. That’s a sure sign that you’re not applying enough water, or applying way too much water. What’s happening with blossom end rot is the plant is not taking up enough calcium. It’s a calcium deficiency, but it’s because the plant is not taking up enough calcium.
Q. People should know if they’ve seen it that the blossom end of the fruit is kind of discolored and shrunken?
A. Kind of leathery and black, yes.
Q. I think of the abiotic causes as like mechanical failures, so to speak—like something went wrong that isn’t a disease from bacteria or virus or fungi.
A. That’s a good description—nutrient deficiencies can fit there; sometimes you see blotchiness to lower leaves, kind of a discoloration in between the veins. That can be deficiencies happening in the lower leaves as the plant moves nutrients to the higher-up parts.
Air pollution also fits in there—we sometimes see that in the Northeast, and get some spotting on the leaves. And you can get spotting from insects. We tried to bring some of that into the tomato app to help with distinguishing diseases from spotting because of insects—mites are a primary one, where you get a little white flecking on the leaves.
Q. So we can have these biotic causes, or these disorders (abiotic causes), and as you pointed out earlier, late blight is a specific biotic problem, a specific disease.
A. Some people refer to it as a fungus, but it is caused by what we what we now realize is a fungus-like organism called an oomycete. It’s a whole other class of organism that can cause disease. It’s like downy mildew. [Late blight on a tomato leaf, from Meg McGrath, photo below.]
A. They’re cousins.
Q. Of course I never knew how to pronounce oomycete.
Q. You have the funniest words in science; they challenge us. [Laughter.]
A. We do, and this one sounds like you have to stutter.
Q. Are we having a “late blight year”? I know you track this; are there a lot of reports?
A. There are always some reports, and it depends where you are. Here on Long Island, we haven’t had a report yet [recorded July 23, 2015; as of mid-July, 2020, only North Carolina had so far formally reported confirmed outbreaks]. We’re now into some really dry weather. It all depends on where conditions are favorable, and where the pathogen is.
I think that’s a basic pathology lesson for us to keep in mind: We’ve got our tomato plants, and they’re susceptible to some of these diseases. But you’ve got to have the pathogen present, and the conditions have to be favorable for the disease to occur. If you have one or the other, you won’t have the disease. Or on the other hand, the conditions may be prime, but the pathogen isn’t present.
Q. So it may exist, but can’t flourish in the conditions available.
A. The best site for monitoring the disease is USABlight.org—which comes out of North Carolina, and is a national one.
[Above, map example on Aug. 1, 2015 from USABlight.org, with maroon being reports in previous 7 days, dark blue earlier than that.]
Q. Another crazy place for me to spend my internet browsing hours! [Laughter.] Is late blight something that always exists somewhere in the United States, but has to have the right conditions to move up the country seasonally? Is it endemic somewhere, persisting somewhere over the winter?
A. It’s going to be in Florida over the winter, because that’s their production season. So they typically see it sometime in December, or into early spring, and then it can move up the coast.
But the other and probably the more important source here in the North is having potato tubers that are harboring the pathogen surviving over winter. Those could be potatoes that get left in the ground and not harvested—either in a garden or a commercial field.
The other things gardeners want to be aware of who are growing potatoes, is that you don’t want to just take a potato that is being sold in a grocery store and plant it. There is a higher chance that it has the late-blight pathogen than if you buy tubers being sold as seed.
Q. Is that where you read in a potato catalog that they sell “certified” seed potatoes? Are they certified to be disease-free?
A. Not exactly disease-free, but below a tolerance level.
Q. So they are inspected or evaluated. Interesting.
So we have to be careful if we have late blight in the past, even in the North where it [the pathogen] is not hardy, about potatoes left in the ground that could transmit it. But what about volunteer tomato seedlings—they can’t carry over late blight, I believe, but can they carry other diseases?
A. They definitely can carry other diseases. The most common would be septoria leaf spot. There are some bacterial diseases that can get into seed, and early blight pathogen can get into seed. [Above, photo from Meg McGrath of early blight on foliage; note target-like concentric rings.]
Q. Do you rogue them out? I have always removed every volunteer tomato that sprouts. [Below photo.]
A. I absolutely do the same thing. A good way to keep the numbers down: I have the philosophy when I harvest tomatoes of, “No tomato left behind in my garden.” I go out with an extra bucket to get any that look like they’re starting to rot, or ones that dropped on the ground. Minimize the number of volunteers you have the next year.
The other is if you have some disease like anthracnose [below photo] that’s on the fruit, and you just leave it on the plants or on the ground, the next rainstorm it will get splashed around to other fruits.
Q. So good sanitation is a really key thing. I think this is true with growing anything—but with something like tomatoes, good garden sanitation is the gardener’s responsibility and can give you a better crop this year and in subsequent years.
A. Absolutely. That’s one of the big advantages we have as gardeners, is that we can take the time to remove diseased tissue. A commercial grower can’t haul the plant vines out of their fields at the end of the season, but we have fewer plants—but we also don’t have the tools that farmers have to better control these things. So a give and a take.
Q. Was it 2009 that was a bad late-blight year?
A. That was our first bad one, yes.
Q. Everyone was talking about it; it was in the headlines. We’re not having a year like that so far [in 2015]?
A. Depending on where you are; some people in Upstate New York [in 2015] would say they are.
Q. Are there other things in the headlines on the plant-pathology hotline [laughter]? Things that are troublesome at the moment.
A. Not that I am seeing. Septoria leaf spot [above photo] tends to be a very common one in gardens. Some people get into issues with powdery mildew. In Florida, they have more problems with bacterial spot than we do up in the North.
A number of these diseases it depends upon sources of pathogen nearby, and then other diseases can move longer distances, like late blight. That can move up to about 30 miles in a jump, whereas some of our bacterial diseases are going to move as far as splashing water can move.
Q. So a very different range—from 30 miles, to just the distance that water splashes.
Do some pathogens live in the soil, so that we really have to manage around them as gardeners—because you can’t really “get rid of them.”
A. The ones that truly live in the soil are things like root-rotting pathogens, and they tend to be more localized. You can get into issues with some other ones that cause a vascular dieback and a wilt, fusarium and verticillium.
In my own garden, I have not seen issues with those with tomatoes, but I have often seen verticillium affecting eggplant. That’s where you get one side of the plant kind of yellowing, or one side of the leaves, and that’s a pathogen that can just survive for the longest time.
I think I’m seeing it because years and years back, where my house is used to be a farm, and it may be that the pathogen has just hung out there.
A. We may start seeing some of these problems in tomatoes, because we are growing more and more heirloom types [above] that don’t have the resistance that some of the hybrids being sold do. In tomatoes, there is a lot of resistance to fusarium ad verticillium in hybrids that are being sold.
Q. When I became a gardener, the conventional wisdom was that you looked for a tomato that after its variety name it said, “VFN,” as in verticillium, fusarium and nematode resistant, right? That was like a seal of approval.
With the heirlooms, that may not be the case—but they are still some great tomatoes.
A. Absolutely, and what I will do in my garden is have a row or two that are resistant to late blight, and then I will grow a diversity of other things, because I just love the diversity of tomatoes, like other gardeners.
Q. Let’s talk about the app [update: Tomato MD and Turf MD are the two apps]. It’s a fusion of data from a number of experts, put together into this app created by the American Phytopathological Society.
A. They funded the project. I was amazed at how expensive apps are to create. I thought it might be a couple of thousand dollars—but that wasn’t so, and I was glad that we had their funding behind us.
Q. I see that they also have a turfgrass troubles app [called Turfgrass MD]—issues with turfgrass. And the tomato app is inexpensive. What does the tomato app do?
A. Basically it has pictures to help you with diagnosing what you’ve got. You can go in and pick whether you’re seeing symptoms on leaves, stems, or fruit, and go through the possibilities. So for instance if you go to leaves it will say, “small spots,” or “large spots.” You flip through and look at the possibilities.
Q. And it keeps narrowing your results—so it’s leaves, it’s big spots…
A. Exactly. The other thing we tried to put in there: what are other possibilities—or late-blight imitators. Just to help you go, “Oh, OK, I think I have late blight, but what other things can look like late blight?” so you can compare and decide if you really have it, or if it’s leaf mold or drought stress.
One case a couple of years back, visiting some gardeners, and they were so sure they’d had late blight. They tore out their plants and had one left, and I looked at it and said, “That’s drought stress.” I felt terrible. [More on late blight from Meg’s lab.]
Q. We can’t just rush to judgment—we have to look at so many factors, as you said at the beginning of our conversation. There are many biotic and abiotic causes for issues with tomatoes, or any plant.
You have to be a bit of a sleuth, or have the app, or be a plant pathologist [laughter] to really step back and look carefully and closely for all the symptoms. I’ve always used the Vegetable MD Online from Cornell. It shows you a silhouette of the plant, and if it’s a problem with fruit—like blossom end rot—you click on the fruit, and it shows you pictures of fruit issues, and you find which one looks most like it.
The app is the next version of that, sort of.
A. And with a lot more pictures, plus we’ve included a longer description—more about the disease, where and when you might see it, and what you might do to manage it as gardeners.
Q. So I want to ask you how to grow a tomato. What do you think about preventing, and how do you give your plants the best chance? What’s the Meg McGrath plant pathologist/home gardener method?
A. I think a major thing is to minimize leaf wetness. I trellis them; I trellis them high, and I water at the base of the plant. If I was really sophisticated, I’d have them set up on drip irrigation, but I go out with a hose—and I’ll confess, it’s usually my husband who goes out with a hose and waters at the base of plant. We’re not wetting the leaves.
Being out there regularly to see what diseases are developing is also important. The one I will intervene on is late blight, because I know that will be so destructive and I will lose all the plants if I don’t do something. Plus if I don’t do something, I’m producing a lot of spores of the pathogen that can get onto a farm nearby or another garden.
Q. By “do something,” you mean removing and destroying the plants?
A. Yes, or applying fungicide. I don’t like to in the home garden, but if I am going to try to rescue anything it will be tomatoes.
Q. Do you grow with a mulch layer, or some barrier to spores splashing up onto the plants?
A. Absolutely. My mulch is much more for weed control, but certainly helps a lot with that. And for keeping things dry. If you’ve got a wet soil surface, you have higher humidity than if you have grass clippings, for instance, which is what I usually use since I have a big lawn.
Q. So the McGrath mulch of choice is grass clippings?
A. It’s actually straw mulch from our ornamental grasses—we chip those up in spring.
Q. That’s a great idea. How stupid of me not to think of that all these years. So you can put those in a chipper?
A. Yes, and then grass clippings between the rows.
Q. As Martha Stewart used to say: That’s a good thing—and actually that’s an excellent thing. [Laughter.] Do you rotate your tomatoes in the home garden?
A. I do. It’s not a major rotation. I have a long, thin garden, and I do peas on one side and tomatoes on the other. When the peas are done, the eggplants, peppers go there—and I swap the two sides, year to year.
Q. Feeding? Everyone says tomatoes are “heavy feeders,” but what do you think about that?
A. I put compost—our own homemade compost and a little bit of fertilizer into the planting hole.
Q. Some people say, also to prevent spores from moving up the plant, that they remove the lower leaves from the plants, to prevent splashing upward from the soil.
A. That would probably be helpful, but I don’t. I always feel bad about pruning things back. [Laughter.] I will take leaves off as they yellow and die.
A. They are doing great, but they’re slow to ripen because we’re in to lower temperatures than we usually have. We’ve just had some hot days now, but the nights are still cool.
Q. Where I am, a couple of hours north of New York City, we’ve been having low 50s even at night. Can temperature affect fruit set?
A. Heat can prevent fruit set. Cool can affect ripening, and slow it down. Are you seeing that in your garden?
Q. The fruit is hanging, but not coloring up. Some years we have long hot spells at night, and then you don’t get a lot of fruit set.
A. It affects bee activity, or sometimes flowers will just abort.
Q. Thank you, Meg.
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MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its 11th year in March 2020. In 2016, the show won three silver medals for excellence from the Garden Writers Association. 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 August 3, 2015 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).
(Tomato doodle top of page by Andre Jordan.)