Spotting Summer Sickness In The Garden

It’s summertime but the living isn’t easy for everything. Plants are under attack by blight, wilt, rust and newcomer downy mildew, which kills basil. Plant pathologist Margaret McGrath runs through symptoms of plant sickness and shares tips for preserving pesto prospects.

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IRA FLATOW, host:

This is SCIENCE FRIDAY from NPR. I'm Ira Flatow.

You may know that we're having a real big heat wave here in the East. Maybe you are also. And I was sure that I went outside yesterday and put a lot of water on my pot of pesto that's growing on my deck. Of course, it's not pesto yet. It's just a nice basil plant. But I've got big plans for it and I don't want the summer heat to do it in because while it is summertime and the living is easy, it's not easy for everything or everybody, especially plants.

Not only does heat stress the plants, but they're under a lot of attack. Think about it. There's wilt. There's rot, light, rust, and now, the newcomer that worries me the most, the basil downy mildew. Oh, my plant is in trouble.

This is a fungus that was first spotted way back in 2007 in the United States, and it's been spreading across the country ever since. So how do you identify the disease and what can you do to preserve your pesto prospects? And what about the other sicknesses that are going around your garden?

My next guest is going to run through some of the signs and symptoms of plant diseases you might encounter this season. Margaret McGrath is a plant pathologist at Cornell University. She's based at the Long Island Horticultural Research and Extension Center in Riverhead, New York.

Welcome to SCIENCE FRIDAY.

Professor MARGARET McGRATH (Plant Pathology, Cornell University): Thank you very much.

FLATOW: What does basil downy mildew look like? What should I watch out for?

Prof. McGRATH: It's a rather non-descript yellowing of the top side of the leaves is the first symptom. If you're lucky, you might see it in bands of yellowing. But not much - not a distinctive spot like you get with some other diseases. And then, really, the way to know is to turn the leaf over and that's where you will see the fungus producing its spores and getting ready to multiply and head off to another garden.

FLATOW: Mm-hmm. And so, is it very widespread around the country at this point?

Prof. McGRATH: We're starting to get a number of reports in. So it is starting to appear in a lot of different areas.

FLATOW: And so what can we basil lovers do to protect our plants?

Prof. McGRATH: There's not a lot you as a gardener can do. There's - in fact, even for commercial growers, there's few fungicides at this point that are registered for this problem. I think one of the best bets is just to be alert and know where the disease is occurring. I've got a monitoring spreadsheet up on the Web now and just be aware of when conditions could be favorable in your garden.

FLATOW: Mm-hmm.

Prof. McGRATH: Monitor if you can spot it early...

FLATOW: Right.

Prof. McGRATH: ...harvest it.

FLATOW: Yeah. Turn it into pesto. I imagine that this must be scaring a lot of commercial basil farmers.

Prof. McGRATH: It absolutely is. That's a very good point. Basil growers had not many diseases they've had to deal with. They haven't had a need to apply many, if any, fungicides, so this very much changes production for them, greatly increases their costs. Some growers have had complete losses. I know I talked to one greenhouse grower who'd lost two crops last year valued at $250,000 apiece.

FLATOW: Wow.

Prof. McGRATH: That's a lot of money. Yeah.

FLATOW: Wow.

Prof. McGRATH: Yeah.

FLATOW: 1-800-989-8255. Talking with Meg McGrath. If you have a question about gardening and seeing some diseases in your garden this summer and you want to ask her about it, we - the folks over there at the Cornell Extension all know everything about gardening. So I've been dealing with them for years.

When was it first sighted? I said 2007, but it goes way back further than that, doesn't it?

Prof. McGRATH: It goes back but it was just a non-known disease. So there's a report from 1933 in Uganda, and then nothing until we get into 2000s and a lot of reports out of Europe. To me, as a scientist, it's fascinating. What happened? Did the pathogen somehow find a way to get out of Africa or did it evolve and it's now a more aggressive critter out there? A lot of interesting questions.

FLATOW: Mm-hmm. Well, let's move on to another fear in your garden and that is late blight. Tell us what that is.

Prof. McGRATH: Late blight, that's - that pathogen, that's the one that's responsible for the disease that lead to the Irish potato famine.

FLATOW: Right.

Prof. McGRATH: Very deadly disease. I think it is probably, arguably our most nastiest plant disease in how quickly it can kill plants. I think a lot of gardeners experienced that last summer.

FLATOW: Yeah, we talked about that one last summer. Yeah.

Prof. McGRATH: Another situation that's changed. The pathogen has changed. The situation...

FLATOW: Oh, really?

Prof. McGRATH: ...in the U.S. - yes. We now believe that we have strains of the pathogen that can tolerate warmer temperature.

FLATOW: Oh, no.

Prof. McGRATH: Yes. So we're seeing it later into the spring in Florida than we have in the past. We're starting to see it in the fall, up in the north more frequently than we have. I've been here working as a vegetable pathologist since 1988 here on Long Island

First time I saw late blight was in 2002, and I've now seen it - last year was the fifth - well, I've seen it this year. I take that back. So I've now seen it six times on Long Island.

FLATOW: Wow.

Prof. McGRATH: Things have changed.

FLATOW: They outwit us, don't they?

Prof. McGRATH: Yeah, they do. The other is the interconnection, which I think is very interesting, with some of these diseases, between farmers and gardeners. A lot of the diseases we deal with don't move very far. But the late blight pathogen, the downy mildew in basil, downy mildew in cucurbit is another big disease that's increased recently. The pathogens produce spores. It could be moved by air - distances.

And now, we have this interconnection. If a gardener doesn't control one of those diseases, they can impact the farmer and vice versa. When farmers are having trouble controlling these diseases...

FLATOW: Mm-hmm.

Prof. McGRATH: ...gardeners are going to be impacted.

FLATOW: Well, the fact...

Prof. McGRATH: We're all interconnected.

FLATOW: Yeah. I know the fact that you're having the same weather we're having, the fact that so far this summer has been very hot and very dry.

Prof. McGRATH: Is fabulous.

(Soundbite of laughter)

Prof. McGRATH: Particularly the sunny days.

FLATOW: Yeah.

Prof. McGRATH: Because any spores that get taken up into the air, the UV radiation is going to kill them.

FLATOW: Mm-hmm. Let's see if we can get some calls in here. Let's go to Francis(ph) in Mount Shasta, California. Hi, Francis.

FRANCIS (Caller): Good evening, gentlemen. Ira, good morning.

FLATOW: Well, good afternoon to you. Are you there?

FRANCIS: Yes. What I do to take care of the basil problem, is I have a lot of trouble with basil myself, so I grow a German or French tarragon, which is a superhot basil-like flavor, and that's the way I get around the problem with the basil.

FLATOW: Wow. I'll have to try that. Thanks for calling.

Prof. McGRATH: Another way is the spice type basils. They are definitely less susceptible. We started doing some variety evaluations, and they've turned out as being much less susceptible, not the same flavor of the basil.

FLATOW: Mm-hmm.

Prof. McGRATH: What we're hoping to able to do is secure some funding that we can figure out why those basils are less susceptible and create a true basil that's resistant.

FLATOW: What would be a couple of names?

Prof. McGRATH: Oh, there is all kinds. There's a lemons, the limes, the Thais, some of the red basils.

FLATOW: Mm-hmm.

Prof. McGRATH: Many, many, many.

FLATOW: Mm-hmm. Let's get some more free advice. Tracy(ph) in Fair Oaks, California. Hi, Tracy. Tracy, are you there? Last chance, Tracy.

TRACY (Caller): Oh, hi. Sorry, I had a CHP officer behind me. I had to put my earphone on.

(Soundbite of laughter)

FLATOW: Oh, we don't want to do that.

TRACY: No, not again. I have some wine grape vines out in the backyard. They're in their third year. And I just noticed this morning and haven't had a chance to look it up that some of the vines - the leaves are showing some dimpled marks, like little wrinkles. Those (unintelligible) that is, if that's a pathogen.

FLATOW: Tracy, again. Are you a wine expert, right?

TRACY: I am a sommelier, yes. Now, I'm getting...

FLATOW: Yeah.

TRACY: ...into the oenology and viticulture part of it.

FLATOW: Ah. Well, you may know more than Meg does about grapes. There's only a little bit of grape growing out on North Fork there, right, Meg?

Prof. McGRATH: We've got a fair amount.

FLATOW: Yeah?

Prof. McGRATH: Actually, it's completely out of my area of expertise, I'm afraid.

FLATOW: Yeah, you're right in between the forks there. So, how about...

TRACY: Well, maybe I'll just pick the leaves off and make some Greek dolmades out of them.

(Soundbite of laughter)

FLATOW: There you go. Make lemonade.

TRACY: I'm a chef too.

FLATOW: Good luck to you.

TRACY: I can find something to do with them. Thank you.

FLATOW: Thanks. 1-800-989-8255. Is fungus really, sort of, the biggest thing of all kinds during the summer, it's hot and humid, that sort of weather?

Prof. McGRATH: Most of our plant diseases are caused by fungi.

FLATOW: Mm-hmm.

Prof. McGRATH: There are a number of bacteria that also cause diseases. They're going to be much more of an issue in the rainy time period.

FLATOW: Mm-hmm.

Prof. McGRATH: And that's because the bacteria - most of them are moved by splashing water.

FLATOW: Mm-hmm. Let's go to...

Prof. McGRATH: Some are moved by insects.

FLATOW: Anne(ph) in Avon, New York. Hi, Anne.

ANNE (Caller): Hi. I live in western New York State. We love Cornell University all over the place up here. And I have a question for Meg, regarding transmission of the blight. I happened to plant a couple of basil plants that look healthy so far and I'm watching them carefully next to, because it was just a good place when the dirt was dug up, a new baby rose bush. And I'm just wondering if the basil starts to show evidence of illness, is the rose bush in jeopardy at all?

FLATOW: Because they like - they get that mildew too, don't they?

ANNE: Well, they can, but I guess that's a separate thing. But what should I watch for?

Prof. McGRATH: They are all very different pathogens, different diseases. Number of different downy mildews, but they're pretty much all caused by different pathogens. In fact, there is one now on coleus, very similar but enough difference that the pathogen can't go between the two.

TRACY: So if the basil started to look...

Prof. McGRATH: So, you don't need to worry about the downy mildew on basil going to anything else. It's a very specific pathogen.

FLATOW: There you go.

TRACY: Okay. Great.

FLATOW: All right. Good luck to you. Are we seeing more and more of these for -or this - for any special reasons, seasonal, climatic or is it just - this is how things work?

Prof. McGRATH: I think we're just in a phase where pathogens have been doing some evolving on us. And we've moved them. Clearly, the basil downy mildew pathogen snuck in through our guard and got in to the U.S. We've got a fabulous system to try and prevent new pests from getting into the U.S., but it does happen, and here is a case where it snuck in - probably on seed, contaminated seed.

FLATOW: Hmm. Interesting. Let's go to the phones. Matt(ph) in Buffalo, New York. Hi, Matt.

MATT (Caller): Hi, how are you?

FLATOW: Hi there.

MATT: Good, thank you. I'm just calling as a comment. I do a lot of restoration work, and I'm a student of ecology. And a lot of what I'm hearing is kind of bringing about a point I like to drive home to a lot of people, is using native plants, especially native plants that can provide some sort of edible value as a good way to keep from introducing new pathogens, new blights that tend to come in with the ornamental trade that are...

FLATOW: Hmm.

MATT: ...having, you know, bad affects on our environment, kind of like the Chinese chestnut bringing in chestnut blight.

FLATOW: Hmm. Good point, Meg?

Prof. McGRATH: Yes.

FLATOW: So we should try to stick to native plants, and they've been around for a while.

Prof. McGRATH: Unfortunately, we make a lot of those imports.

(Soundbite of laughter)

FLATOW: I know what you mean. Meg, I want to thank you for taking time to be with us today.

Prof. McGRATH: You are welcome.

FLATOW: Meg McGrath is a plant pathologist at Cornell University, based at the Long Island Horticultural Research and Extension Center out there in Riverhead, New York.

I'm Ira Flatow. This is SCIENCE FRIDAY from NPR.

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