vegetable diseases: basil and cucurbit downy mildews, with cornell’s meg mcgrath

HERE’S HOW my head, which is always lost in the garden, I guess, works: The first thing I thought about when the pandemic started—when we got news of a new pathogen in the world—was how many plant pathogens I’ve written about in my career, and the trajectories of each of them.

Dr. Margaret McGrath is a longtime vegetable pathologist for Cornell University, whom I’ve turned to over and again to better understand many such plant diseases. Meg, in turn, turns to backyard gardeners, like us, to help her learn more about basil downy mildew (above, spores on the undersides of affected leaves) and late blight in tomatoes and, now, cucurbit downy mildew (below, in a cucumber vine), too.

It’s true, she wants to know what we’re seeing in our backyards. We can help her and vice versa.

Besides her role as a scientist at Cornell’s Long Island Horticultural Research and Extension Center in Riverhead, New York, Meg McGrath is a keen gardener. Her applied research aims to improve the management of important vegetable diseases, and develop effective components for integrated pest management programs (IPM).

Read along as you listen to the Aug. 30, 2021 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).

basil and cucurbit downy mildew, with meg mcgrath


Margaret Roach: Hi, Meg. How are you and how’s the garden?

Margaret McGrath: Good morning. It’s a pleasure to chat with you. The garden is doing O.K. I’ve got a few diseases, as usual, unfortunately, but I’m not going to admit that on air. Did I already do that [laughter]?

Margaret: I think you did that. The cat’s out of the bag. The pathogen is out of the bag.

Meg: Indeed.

Margaret: Yeah. I have to ask you, before we get started with the serious topics at hand—as in what’s in our gardens right now and so forth—I guess I have to ask, as a plant pathologist, do you think you had a different take on the news in early 2020 of the coronavirus? Because you’ve spent a career investigating pathogens, and even with my very intermediate-level layperson knowledge, all I kept thinking was, this is going to take a long time. This is a long haul ahead.

Meg: I had a couple thoughts. I wasn’t thinking so much long haul, but there was a couple things that went through my head. One was thank goodness the Covid virus does not move like some plant pathogens can. The cucurbit downy mildew one you mentioned, that we monitor, overwinters in Florida, and it moves through the entire East Coast every year. I mean, that’s a lot of movement. The fungal spores and oomycete spores, which is what that pathogen is, are dispersed by air, and they can move quite a distance. Thank goodness that doesn’t happen for us. We would have all been in trouble. It wouldn’t have mattered staying 6 feet away from other people.

Margaret: Right, 6 miles would have been more like it.

Meg: Even more. The other thought that went through my head is in the fall of 2019, we had a new virus affecting tomatoes, particularly in greenhouses is tomato brown rugose fruit virus. It had a lot of similarities. It was transmitted more easily than any previous virus we had dealt with. It’s an mechanically transmitted one, so workers are moving it on their hands. It is thought to have originated as a host jump. So you have so many interesting parallels.

And then the other odd thought that went through my head is: Thank goodness Covid doesn’t make us more beautiful, like the virus that affected tulips many years ago, the tulip breaking virus.

Margaret: Right.

Meg: Isn’t that an odd pathologist’s thought?

Margaret: Right. That’s where we see in those 300-year-old paintings, you see the “broken” tulips—as in the colors are mosaic, all irregular, and many colors in one flower and so forth.

Meg: And they were beautiful, and those bulbs were sold for many, many dollars. They were valued. They knew there was something about the bulb. Didn’t realize at that point that it was a virus.

Margaret: Right, right. I mean, it’s crazy. It’s funny how each of us how our take on, of course, everything; our perspective. I thought with plant diseases, you said you have some that we’re not going to confess to in your own garden [laughter]. But we have had, in many parts of the country, extreme weather of varying kinds and, of course, that can contribute.

What’s on the hit parade in the Northeast, in terms of plant diseases, because you’re located in Long Island, and a lot of your work is focused on our region. And anywhere else around the country, any headline items?

Meg: Well, the downy mildews. We are always monitoring those, and interested in knowing where they are, and being aware of how widespread they are. It adds to our knowledge base and helps us in giving advice to growers.

Particularly with the cucurbit downy mildew one, as I commented earlier, it’s moving up through the Eastern coast each year. It occurs at different times. This year, it was a little bit earlier up here in the Northeast. I saw it on Long Island a little bit earlier than I ever have before. We see it anywhere from mid-July to early September. It’s a big range.

That’s where it’s really valuable to know when gardeners are seeing it, as well as growers. To just have a feel for is it out there early this year? What crops are getting hit? The pathogen varies; it’s got different strains that effects the different cucurbit types. We see it first in cucumbers and cantaloupes. We may see in squash in the Northeast, and watermelons—or we may not, depending upon what strain makes it up here. The monitoring is really important.

Margaret: We’ll talk more about that and how we can all contribute. But first, I just want to back up to downy mildews. They’re not fungal, they’re fungus-like in a way.

Meg: Exactly. We used to think they were fungi. We know recognize they’re more closely related to some algae, and they’re called oomycetes.

Margaret: And so, what are the symptoms? The cucurbits, what do they look like? How would I know if I had that going on? [Above, cucurbit downy mildew in a pumpkin leaf.]

Meg: A typical symptom with them is an angular yellow leaf spot. It varies amongst the host how clear the angular look is, but what’s happening is the pathogen, as it’s growing in the leaf, it can’t get by major veins, so you end up with this angular look. And then on the underside of the leaf, you will see sporulation of the pathogen, and it’s typically a gray sporulation, particularly with basil and cucurbit downy mildew.

The pathogen doesn’t sporulate well on all of the cucurbit hosts. It sporulates beautifully on cucumber, well on basil—and those are different pathogens. That’s another thing to be aware of. The downy mildews are all very specialized, so they’re not going to multiple different hosts.

There’s a bunch of other downy mildews. Some of them produce white spores, like the one that goes to lettuce.

Margaret: Yes, yes, yes. How long have pathologists known about downy mildews? Is this something from… Because I don’t remember earlier in my years as a gardener hearing about them, as much as I have in the last decade or something. I guess it was because of the basil one that was so devastating. What year was that, that it knocked out huge amounts of basil crop?

Meg: It arrived in the U.S… Well, we were first aware of it in the US in fall of 2007.

Margaret: Right.

Meg: And then it was somewhat widespread the next year.

That’s an interesting odd one, in that you go back in the records, and it was found in Uganda back in the ’30s. And then nothing until it ended up in Europe. It is a pathogen that can be in seed, and that’s how it’s gotten spread widely around the world.

Margaret: Oh.

Meg: And then it produces these wind-dispersed spores. And that’s its next method of getting around.

Margaret: And so, is the future, if we’re going to solve this, or manage this—is it management? Or maybe it’s different for the cucurbit versus the basil etc., I don’t know. But is it resistant varieties being developed? What’s the race to sort of stay ahead of these things about? What are the components?

Meg: Both resistant varieties, and fungicides—and fungicides for commercial growers. My experience, at least with basil downy mildew, with the organic products that are out there, it’s too hard to reach the level of control that you need. I mean, as a home gardener, we can cut off the part of the leaf that’s affected. But to market it, the leaves have got to be perfect.

Margaret: Right.

Meg: So, it’s very hard. We’ve got some conventional fungicides that conventional growers can use that are extremely effective. Resistant varieties is always number one in managing diseases like this that can’t be avoided.

But the pathogen can evolve to overcome them. Cucurbit downy mildew has been a big issue since 2004, and that’s the year that we believe either the pathogen evolved, or we brought a new strain in from Europe that was able to overcome the resistance that was had been bred into cucumbers back in, I think, it was the ’30s, which was exquisitely good resistance. But the pathogen evolved to overcome it.

We believe the basil downy mildew pathogen now, in the U.S., has overcome some of our resistant varieties. I’ve been getting reports, and that’s scary.

Margaret: Oh, Meg? Come on. No! [Above: Yellowing of the upper surface of affected basil leaves often occurs in sections of the leaf delineated by veins, says Dr. McGrath, because the pathogen cannot grow past major veins in leaves.]

Meg: Yes.

Margaret: No!

Meg: What is on the horizon, and it gets back to the topic of new technologies, and people being nervous about new technologies, is genetically breeding resistance, because you can do it a lot faster.

Basil is particularly difficult to breed, because when you get crosses you end up with a sterile plant. ‘Amazel Basil,’ one of the resistant varieties that came out, it can only be produced by cuttings, because it doesn’t produce seed. It’s sterile because of the cross.

One of the interesting things about this: I’m not in the molecular world. I’m an old-fashioned plant pathologist. But I listen to talks that my fellow pathologists do, and what they’re finding, now that they can just look at the genes of these plants and pathogens so inexpensively now, a lot of things are getting looked at molecularly, is they’re discovering that plants that are susceptible to a pathogen have a susceptibility gene. That kind of makes sense. Basil is the only one that’s susceptible to downy mildew. Why are other plants not? Well, only basil has got the susceptibility gene.

So with genetic engineering you can… With the new technology that’s very specific, this CRISPR-Cas9 system, you can knock out that gene and produce a resistant variety. That work is being done. The issue is educating about new technology, and that gets us back to COVID-19 and what we have now for vaccines.

Margaret: Right.

Meg: I just wonder, back when we first started developing the vaccines, and thinking about it, if it was on the news that this is what the technology is, and this is why it’s going to come out faster than other vaccines. Similar to genetic engineering—if scientists had talked about these things way before they came out, so the public had time to think about it, would it make a difference in accepting some of these technologies?

Margaret: Right, right. Education, proactive education, and…

Meg: Exactly.

Margaret: Not like emergency, here’s the only thing we can do. It’s this or nothing. And then people resist, and understandably, because they’re unaware. It all sounds too startling and so forth.

Meg: Exactly.

Margaret: Yeah, yeah. Interesting, interesting parallel. With the basil, with the downy mildew, what does it look like? You said with the cucurbits that you have these angular yellow leaf spots, and then the sporulation. Is it fuzzy whitish or grayish underneath or something?

Meg: It’s fuzzy grayish. So very similar to cucurbit downy mildew. And same angular lesions, but with basil you get those long veins that run the width of the leaf, so it tends to be a yellow band, until the whole leaf is affected, then the whole leaf is going to be yellow.

Margaret: O.K.

Meg: With all of the downy mildews, the best time to get out there and look for the sporulation is in the morning, particularly if it was humid night or dewy night is the best. The spores get produced overnight and then, during the day, they get wind-dispersed away, so by the end of the day you’re not going to see as many.

Margaret: O.K.

Meg: The other thing a gardener can do, or a grower, if you think you’ve got downy mildew on a leaf, but you’re not seeing the sporulation, is you take the leaf and you put it upside-down on wet paper towel in a closed plastic bag, like a Ziploc bag. Sitting there overnight it should produce its spores, because you’ve given it a good, humid environment. I do that all the time.

Margaret: You would get a spore print, so to speak, like you could from a fern? You’d get spores on the towel, is that the idea?

Meg: No, you’ve got the leaf sitting upside-down.

Margaret: Upside-down, O.K., so they’d stay on the leaf.

Meg: Yes.

Margaret: O.K. Now, just in terms of possible tactical ways around some of these things, I’m always wondering if succession crops, or harvesting in a younger age would help. Is there an age with, say, the basil—does it tend to happen when the basil is getting older? Because basil can be, if you want to buy enough seeds, you could grow it and cut it younger and use it. Do you know what I mean, as opposed to letting the plant get big. Is there a correlation between the age of the plant and when this occurs, when it’s more susceptible?

Meg: Not, unfortunately, with downy mildew.

Margaret: Oh damn [laughter].

Meg: Yeah. I do see that with powdery mildew with cucurbits.

Margaret: O.K.

Meg: And that’s the stress of fruit production is when you’ll see powdery mildew. Basil downy mildew, often we don’t see it until plants are a little bit older, and it’s more just that it’s taken time for the pathogen to finally, by chance… I mean, these spores aren’t controlling where they’re going; it’s all the wind currents and when they, by chance, land on the leaves.

I have seen basil downy mildew on cotyledon stage of microgreens. And my colleague, who works on the disease in Florida, has seen it on plants as soon as they emerge [above].

Margaret: O.K. I was wondering if there’s something timing wise I can do, as an organic gardener. You know what I mean, to outsmart the pathogen, or the pest, or whatever.

Meg: The best way to outsmart it, as a gardener, is grow your basil in pots, and if you’ve got humid overnights, bring the pots inside.

Margaret: O.K.

Meg: Then they’re not exposed to the conditions that the pathogen needs in order to infect. The pathogen needs high humidity to produce the spores, and it also needs wetness or high humidity to infect.

Margaret: O.K., good. If we go then to another one that I know you probably hear people worrying about in advance, or maybe even sometimes generically terming “My tomatoes have blight,” even though it’s not technically blight, and especially it’s not late blight. Tell us a little bit about what’s going on in the tomato world right now? What are you hearing about?

Meg: Well, there are a number of diseases that infect tomatoes.

Margaret: Oh, yes. [Laughter.]

Meg: You’re admitting you have diseases in your tomatoes, too?

Margaret: I think we had 13 inches of rain in the first half of July, so I think that it was pretty interesting what’s going on outside.

Meg: A lot of pathogens needs leaf wetness in order to infect, and these are your fungus… Plants are susceptible to a number of fungal diseases.

We people don’t have too many fungi that affect us. That’s probably the number one with plants. And they need leaves to be wet or high humidity in order to infect. And some of them need splashing water in order to be dispersed, and septoria leaf spot is one of those. And that’s one of the main ones I see in my garden. And this has been a bad year for septoria leaf spot. Conventional growers are seeing more of it than usual. And that’s one that will start on the lower leaves.

Early blight will start on the lower leaves, as well, and you get some yellowing. You’ll also get some yellowing just because you’ve got tomatoes for a good long period of time, and they are going to start to senesce their lower leaves. So that can happen, as well.

Late blight is an interesting one from the perspective that it’s kind of disappeared a little bit in the Northeast, knock on wood. We’ve had a few cases, but nothing like back starting in 2009, where it was very widespread, and in a rainy year like this, it’s just the perfect conditions for it.

And it shows us that we’ve gotten a handle on managing it, and I think that reflects gardeners, as well as growers, are doing a better job of managing the sources. And a primary source is potato tubers that are infected. And the growers of potatoes—seed potatoes and potatoes for sale for our consumption—have been doing a better job of controlling late blight, so it’s disappeared for a while.

Margaret: Leaving our potatoes, either accidentally or on purpose, could potentially be a vector—I  don’t know if it’s called a vector, but could be a source.

Meg: A source, yeah. If they’ve got late blight.

Margaret: O.K., yes.

Meg: You’ve got to have the pathogen present.

Margaret: Right, right. But that’s a reason, since we, as backyard gardeners, wouldn’t be able to necessarily know that, it’s better to be scrupulously clean, perform good hygiene in the fall garden, in the vegetable garden, than to, potentially, leave a pathogen over winter. Yes?

Meg: Correct. And if you’re going to grow potatoes, buy seed potatoes.

Margaret: Certified, especially that have been…

Meg: Yes. Because there is a lower tolerance for late blight to be in those potatoes than the ones you buy in the grocery store to eat.

Margaret: O.K., good. That’s a very good point. I mean, I think that’s very, very important is that we don’t want to just start with any old potato [laughter]. Good plan. Tomato-wise, that’s interesting—you said that the growers, both backyard and commercial growers, that they’ve learned to manage a little bit. It’s that sanitation, but is it anything else that we do to avoid it? Is there anything else?

Meg: Well, the main one is if you can prevent the source. If you don’t have a pathogen present, conditions can be very favorable and you’re not going to have disease.

Margaret: Right, right.

Meg: Bottom line, there are highly effective fungicides for conventional growers, and late blight, that pathogen, has not shown ability to develop the resistance to fungicides that we thought we might see.

Margaret: I see.

Meg: And that’s been an issue. But when commercial growers are able to control a disease, we gardeners, who are not using these fungicides, are less likely to see the disease. They’re reducing the amount of inoculum in the world.

Margaret: Right, right. I know how you can help us [laughter], because I’m always pestering you for questions over the years. But how can we help you and other scientists, like yourself, researchers, who need more data, so to speak? Tell us a little bit about how we communicate to you, and what info you’re looking at for from us.

Meg: Well, the major diseases that I’m involved in monitoring are these two downy mildews we’ve been talking about. The one in basil, and the one that affects cucurbits. And there are monitoring programs for those, so a gardener could check out those websites. The one for cucurbit downy mildew is cdm.ipmpipe, and I think just Googling that, you should pick it right up.

Margaret: And I’ll give all the links.

Meg: And we’ve added a page there that is specifically for gardeners.

Margaret: And I can just say I have it, and here’s where I’m located. Is that basically it? Like the way I report my birds on Cornell’s ebird.org, right? Is that how it is?

Meg: Exactly. And they’ll ask you to submit a photo. From the photo, we can diagnose it, hopefully. Or someone will reach out for more information, and same thing with basil. The basil one is at an Ag Test Monitor page. There’s information at that page, as well, about how to manage basil downy mildew for gardeners, as well as growers. Both websites have a map that show where downy mildew has been found already this year, which can be very helpful.

Margaret: And those maps are updated in real time, that we can see how it’s happening and spreading, or not?

Meg: Absolutely. As soon as a report is confirmed, it gets posted. [Above, the 8/27/21 map of cucurbit downy mildew from cdm.ipmpipe.org.]

Margaret: O.K. How does that help you to know that I have it, or someone in Speonk has it, or someone in Douglaston has it? How does that help you as a scientist?

Meg: It helps me, as a scientist, to realize how widespread it is.

I’ve also gotten a lot of comments from gardeners about conditions when they’ve seen it—and if they see it every year, or if “this is the first time I’ve seen it.” That’s valuable to know, just how commonly and how widespread.

I had someone from Houston, two years ago, whose balcony planting—a couple pots of basil—and had downy mildew. So to realize where this pathogen can occur, the disease, is very helpful to know. The information is also used to let growers know where these diseases are present and, therefore, when they need to be thinking about using fungicides.

So it helps them reduce their fungicide use. They can wait until it’s been reported and there’s a risk for them. The cucurbit downy mildew program is not just monitoring, it’s also forecasting where the pathogen is likely to move and infect next.

Margaret: I see. Meg McGrath from Cornell, I am so interested, and I always learn so much from you. I hope that you have a productive harvest in the garden, even though there’s those unmentionable things happening [laughter]. I hope I’ll talk to you again soon. Thank you for making time today.

Meg: This has been fun.

(All photos from Cornell Vegetables website.)

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  1. Rhona Bork says:

    After removing affected leaves, I spray cucumbers with this mixture. 5 tbs baking soda, 5 tbs 3% hydrogen peroxide, one gallon of water. ,

  2. This helps me understand why my years of cucumber crop success with wonderful yeilds came to such a sudden end on my Central Florida farm. Fall 2011 is the last year I recorded normal harvests. I had a similar problem with certain winter squash also. I loved the basil tip on spore checking by placing leaves in a bag.

  3. Brenna says:

    So far, (the last two years), I have had success with the Prospera variety of basil. It’s available from several organic seed sites. (I’m in Dutchess County, NY)

  4. Carol Kelly says:

    Well, darn. This wonderfully informative podcast/article was too late for me to report BOTH the downy mildew that recently killed my 2 Genovese basil plants within a couple of days, and also that which murdered my 4 (2 varieties) of cucumber plants. I also removed all the cucumber vines a few days ago since, in addition to the mildew, the cucumber beetles did a fine job of spreading bacterial wilt as well. For a while, I was cutting back the worst-affected leaves and watching the vines struggle along in an attempt to extend my cucumber harvest as long as possible., but finally gave up.
    I have saved this page as well as the links to Cornell that Meg supplied, and will report things next year. also will be purchasing a resistant variety of my beloved basil for next year!
    Incidentally, it was only my Genovese basil that was affected; I have numerous African Blue Basil in several places around the yard since it is beautiful and a myriad of pollinators adore it. I also have 3-4 lemon basil plants as well as a number of holy basils and a Greek columnar basil. None of these varieties were affected.

  5. I’ve noticed, this summer, so many yellow leaves on so many of my plants. I guessed it had to do with the abundance of rain but I did not guess mildew. Thanks for the article.

  6. Martha says:

    At the URI Master Gardener demonstration garden we’re growing Obsession basil and the leaves are pretty clean. Some yellowed leaves, but the plants seem to be faring well so far. Last year Obsession was pristine all season until frost. According to trials (and commenter Brenna below), as your link said, Prospera is another good variety, and Thunderstruck is supposed to be excellent. The Greek basils in our garden look awful. The Thai is unaffected.

  7. barb f says:

    Very Interesting thank you both. Question – Is it ok/safe to eat the infected basil leaves? I’m making pesto and a lot of the leaves are afflicted. Will I invite unwanted pathogens to my internal systems if I eat them? thanks barb

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