Basil Downy Mildew Threatens Basil Plants A potentially fatal fungal disease called downy mildew has been attacking basil plants in New York, New Jersey, Ohio and Florida. Gardeners are worried that it could spread farther throughout the summer. Margaret McGrath, an associate professor of plant pathology at Cornell University, says that at first sign of the blight, make pesto and freeze it.

Basil Blight Threatens Pesto Lovers

  • Download
  • <iframe src="" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
  • Transcript


Bad news for all those gardeners who grow basil to sprinkle over their Italian food or their Thai dishes, or anyone who thinks pesto is just about the best food on Earth.

A potentially fatal fungal disease called downy mildew has been attacking basil plants in New York and Florida. It may spread further throughout the summer, threatening to turn your delicate green basil leaves into an ugly shade of brown, yellow or gray.

To find out what to do about the so-called basil blight, we've called on Margaret McGrath. She is an associate professor of plant pathology at Cornell University. Welcome to the program.

Professor MARGARET McGRATH (Professor of Plant Pathology, Cornell University): Thank you very much. I appreciate the opportunity to be here and chat about this important disease.

NORRIS: Well, important but also kind of scary. How bad is this?

Prof. McGRATH: It's bad because it just, it wipes the leaves right out, and that's what you want to use. And it can go pretty darn fast. And it's like a couple other new diseases or diseases that we've had that have changed, the late blight in tomatoes, downy mildew in cucurbits.

We kind of refer to them as community diseases because they move so easily. So if you have it in your garden, you can affect other gardeners and farmers because the pathogen can just jump on to their crops and plants.

NORRIS: How widespread is it? We know that it's in New York and Florida. Is that just where it's been spotted? Is it suspected to be in other states, as well?

Prof. McGRATH: It's been found in a couple other states already. Ohio is another one that it's been seen this year, New Jersey. What is happening is it was first found in the U.S. in 2007 in the fall in Florida.

As I mentioned before, the spores can move very easily in the wind. They can then move from Florida on up the coast and through to other places where basil is being grown.

NORRIS: Now, if people are listening in their cars, I have a feeling that the first thing they do when they go home is going to be go straight to the garden to check their basil plants. If people are listening at home, they're long gone. They're probably headed out there right now. What are the signs that a basil plant might be infected? What do you look for if you're worried about this?

Prof. McGRATH: On the topside of the leaf, you're going to see yellowing, and it may be very nondescript. To really know what you have, you have to flip the leaves over, and that's where you'll see the pathogen, is on the underside of the leaves.

NORRIS: What's it look like?

Prof. McGRATH: You'll see kind of a grayish, almost purplish dusty growth on the underside, sometimes turning it almost black. And that is all of the pathogen spores. And then the wind will pick those up and blow them off, and the disease just keeps multiplying like crazy.

NORRIS: So what do you do if you find this on one or two of your leaves? Do you when you find this, do you have to remove the leaf or remove the plant?

Prof. McGRATH: In this situation, you could just get rid of the leaves, but realize that as you're disturbing those leaves, the spores are getting knocked around, and they're probably going to end up on other leaves of the plants.

When I've had outbreaks, particularly last year, when I first saw it, which was the beginning of August, I looked at my husband, and I said: We're not waiting until September when we normally made pesto. Get the machine out. We're making pesto now. And we just harvested everything out of the garden and made pesto. And that was that.

NORRIS: If you have to harvest all of your basil all at once and I guess do what you had to do, whip up a whole bunch of pesto, how do you store it? Do you put it in little ice cube trays and throw it in the freezer?

Prof. McGRATH: We make pesto balls. So we put it into little sandwich bags and freezer away.

NORRIS: Good advice.

Prof. McGRATH: But it's not the same as a fresh leaf. It's not the same.

NORRIS: Nothing is like a fresh leaf.

Prof. McGRATH: Absolutely.

NORRIS: Well, Margaret McGrath, it's been a pleasure to speak with you. Thanks so much for your time.

Prof. McGRATH: You are welcome. Thank you very much.

NORRIS: Come back and join us again.

Prof. McGRATH: I'd love to.

NORRIS: Margaret McGrath is an associate professor of plant pathology at Cornell University.

Copyright © 2010 NPR. All rights reserved. Visit our website terms of use and permissions pages at for further information.

NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.