Basil Downy Mildew and Prospera^® DMR Basil

Organic Prospera basil from Goodness Gardens, packed for market.

Photo courtesy of Goodness Gardens

Just 20 years old in 1980, Brian started growing hydroponically in 5-gallon buckets in New York. "We were doing research and back then, it was really the Wild West when it comes to hydroponics."

Very few consumers knew about hydroponics, and marketing their tomatoes, lettuce, and cucumbers was hard going, so in the mid-80s they started adding fresh herbs to their line-up. "Back then, there were no fresh herbs in the supermarkets other than parsley. I had grown some basil hydroponically and brought it in to show the buyer, and they didn't even know what it was!

"From there, we picked up a lot of new business and kept growing and growing and growing. Today we do multimillions in herbs sales a year from here. Now we're building a 2½-acre, high-tech hydroponic greenhouse to grow a lot of the herbs in this fully automatic system, and this is going to be all organic. We also started a sister company in Miami. We have another greenhouse operation in Orlando, Florida and another sister farm out in San Antonio, Texas."

But the course hasn't been a swift or smooth one: Brian has had to adapt to technological and cultural shifts in the hydroponic industry, new variety introductions, and evolving pathogens. He's also been a customer of Johnny's since the early days, so we wanted to hear about his experience with basil downy mildew. We asked him when he first started having issues with it.

Basil downy mildew hits Goodness Gardens

Above and below: Prospera basil fields at Goodness Gardens, located in the Black Dirt region of Orange County, New York.

Photos courtesy of Goodness Gardens

"It was 2012 when it first hit around here. I had a farm out here where we were growing 5 acres of basil; just Genovese. And I remember saying to one of my farm hands, 'I think you need to fertilize the basil; it's turning yellow.' I kept thinking, 'We've got a fertilizer problem,' because I'd never had a problem with basil before. So he goes out and puts on more fertilizer, but during the next couple days, it got yellower. And I thought, 'What the heck is going on here?'

"I started to notice that there was mildew on the underside of the leaves and realized, 'This is something else. I've never seen anything like this before.' Once the plant is infected, it won't recover. Once that fungus turns the leaves even half yellow they don't go back to green, even if you kill the downy mildew. It's dead. That's it. It's done! That made it really tough.

"That was in 2012, when that first started happening. Then Cornell came out with a bulletin about it. They started getting some better ideas, including how to control it. It's been a struggle, because in the beginning the fungicides were not working well, and obviously, there were no downy mildew-resistant varieties yet. It took years for that to happen. But today, between the new varieties that are downy mildew-resistant and the new fungicides it has become… almost not a problem anymore."

In his region, Brian notes, "Downy mildew in the greenhouse here starts in July and ends right around mid-November. In the spring months, it's moving up from the South. That takes until about July 1st. Then once the frost hits — September and October — the spores are killed, and I don't have any more problems for another 6 months."

What about downy mildew-resistant basil varieties?

Manuel Quesada Garcia, Director of Engineering and Continuous Improvement at Goodness Gardens, holding a 'Prospera Compact DMR' plant from the field, with his hand up for scale.

Photo courtesy of Goodness Gardens

We asked Brian about his experience with resistant basil varieties. While in some of his earlier trials the first-generation varieties demonstrated resistance, he felt they fell short on other qualities like color, leaf shape, leaf size, and plant habit.

In comparing the more recently released Prospera to non-DMR-resistant varieties, he observes, "I noticed it might get some downy mildew but it doesn't spread. It's weird; you'll see it show up on a few leaves but it doesn't go any further. On the regular plants, overnight, you can lose the whole crop. Being able to identify what's going on and spray before it gets any further gives you a huge advantage. You may lose a few leaves here and there, but it's nothing. That's what I noticed that was really good about it.

"This year, in our outdoor fields, we had no outbreak of downy mildew. Now we were doing a maintenance spray every week. But last year, we lost 10 acres of basil. We weren't using Prospera. This year, we used only Prospera in the field and didn't even see it out in the field. So that is a big change."

Brian is guardedly hopeful that the disease can be brought under better control. "I hear they're coming out with new varieties, in the Prospera category, that are even better." But after 40 years of growing, he recognizes that even as new tools become available, the ongoing challenge with a lot of pathogens is that they continue to evolve.

Managing the thrust and parry between this pathogen and its host is a goal Brian shares with one of the leading experts in the U.S. in basil downy mildew.

Margaret T. McGrath, PhD, School of Integrative Plant Science, Cornell

Photo courtesy of Meg McGrath, PhD

Dr. Meg McGrath is a New York-based plant pathologist whose primary directive is conducting practical research on diseases of vegetable crops with a focus on management. When asked how she came to work on basil downy mildew, Meg explains, "Because many diversified vegetable growers in our region grow basil, it was logical, when downy mildew appeared, for me to do some research, learn more about the disease, and share that information.

First and foremost, we needed a monitoring program

In developing the reporting system at AgPest Monitor, Dr. McGrath has given both growers and scientists a powerful mapping tool for examining the pathogen's behavior.

Meg says she had her eyes opened to the disease with the first reports of its spread to the U.S. in October 2007. "In 2008 we first saw it here on Long Island. When it made a reappearance in 2009, the first thing she did was to set up a monitoring program so anyone (grower, extension specialist, gardener) could immediately report observations of downy mildew.

Digging deeper to develop a resistance program

Meg had previously worked on Phytophthora infestans, the notorious cousin that causes late blight in tomatoes and potatoes, and was working on cucurbit downy mildew, which had started to become more prevalent in 2004.

"I got into doing research in 2010, initially evaluating fungicides, work that was supported by the IR-4 program, which supports research to register products, both conventional and organic, for specialty crops. We were able to convince them that basil downy mildew was a topic that needed to be worked on. I got into evaluating resistant varieties in 2013 through USDA-funded projects."

Pathogens do their own thing

Map of the pathogen's incidence in the contiguous U.S. over the 12-year period between 2009 and 2021, showing number of years the pathogen has been reported by state.

Schematic courtesy of Meg McGrath, PhD / Long Island Horticultural Research & Extension Center

Examining new and established DMR varieties was again a topic of Meg's research last year, and here are some of her observations:

• "In 2021, as in 2020, I saw downy mildew more on the Rutgers DMR lines than in the past, but still much less than on the susceptible variety.
• "I've always seen some downy mildew on the Rutgers DMR lines but less than on earlier DMR varieties like 'Eleonora', whereas on the parent for their resistance, we did not see any symptoms when tested previously.
• "On Prospera, I've seen exceptional control of downy mildew, including this past year, when adjacent Rutgers DMR lines did not do as well in years past. That was a surprise.

But as an experienced pathologist, Meg cautions that "[P]athogens do their own thing. And they are going to evolve, and they may not evolve the way we think they are going to evolve.

"I've received a few reports of people seeing more downy mildew than in the past on Rutgers DMR lines, as well as a couple reports of people seeing downy mildew on the Prospera lines. So the pathogen may have made multiple kinds of changes to overcome resistance. We may be in a situation similar to spinach downy mildew, where breeders are going to have to work hard and continually find new resistance genes."

Verification & reporting matter

Close-up view of the purplish-gray spores on the underside of a basil seedling's cotyledons. Plants are susceptible to the pathogen from emergence.

Photo courtesy of Meg McGrath, PhD

Many plant diseases closely resemble each other or problems that are physiological, such as nutrient deficiency, rather than pathological. To avoid misdiagnosis and inappropriate treatment, Meg adds, "You have to verify the pathogen's present. My experience has been, if spores aren't visible on the underside of yellow basil leaves, put them in a moist plastic bag overnight, and if it's downy mildew you're going to see the spores within a day."

Through the AgPest Monitor program she initiated, Meg confirms they have learned a lot about how widespread it can be in the U.S. "I think one of the greatest values of the monitoring programs is in the mapping. It is so much easier to look at occurrences and see them on a map than to see them in a spreadsheet format. That has been really valuable in keeping people engaged.

"What I find really interesting is that most of the reporters have been gardeners. They are not treating their basil, so they are more likely to see downy mildew."

Hard to avoid, hard to manage

Sporulation in a section of the leaf underside.

Photo courtesy of Meg McGrath, PhD

"I think the bottom line for management is that growers have to use an integrated program," Meg says. Along with adopting an integrated approach, Meg suggests reckoning with the disease as something you cannot avoid. "You're dealing with a pathogen that is spread by wind; the pathogen produces spores that can move long distances. And, like other mildews, it's a disease you can't manage through crop rotations."

Meg also points out that although we may think of drought conditions as being less likely to promote downy mildews, "It's the nighttime when pathogens are most active. We've had summers with long rain-free periods — which is not unusual for Long Island — and we feel as though we're not really experiencing 'disease weather.' Then you look at the nighttime conditions and it's nearing 100% humidity, the temperatures come down from the day, and the pathogens are going, 'Hmm, this is great!' Especially downy, which is just looking for high humidity and even better, a good dewy night; I've seen my best symptoms after a heavy dewy night.

"Growers have to be on their toes and expect that this disease is possibly going to show up every year. The pathogen is likely going to keep evolving on us — or, maybe it won't — that was our expectation with the late blight pathogen. It was one of the first to develop resistance to fungicides, and we thought that would keep going, but it hasn't. We're not always good at predicting what's going to happen! But it is safe to assume the pathogen is going to keep changing.

"It's valuable for growers to realize that downy mildew is a hard disease to manage adequately, especially in an herb crop. I have found through my research that if I started spraying after seeing symptoms in my planting, then I wasn't going to get good control. Now I start before we see symptoms — I think that is really critical. With a fresh herb, there is no tolerance for symptoms on the leaves. That's what you're marketing. That makes the situation different from something like cucurbit downy mildew, where yes, with poor control you're going to have a reduction in yield and fruit quality, but the pathogen isn't directly affecting what you're marketing so total crop loss won't occur as it can with basil. I think that, too, makes basil downy mildew a tougher disease to control — it's attacking what you're trying to market, and it's a fresh green."

The importance of both formal and informal reporting structures

Side-by-side comparison of 4 varieties in Johnny's basil downy mildew resistance trials.

Left foreground: 'Prospera Compact DMR'. Left background: 'Prospera Italian Large Leaf DMR'. Right foreground: 'Genovese'. Right background: 'Red Rubin'.

"The other thing I would point out," says Meg, "as we're talking about pathogens possibly evolving and changing, is for growers to realize that does happen and that they need to be on their toes — but they also need to share when they see something unusual.

"The best person for them to share it with would be their local extension specialist. Be it that they have used a resistant variety that they used in the past and saw good efficacy with it, and now, all of a sudden, they're seeing a lot of symptoms — then report it. Or, maybe they have a great fungicide program, that worked great last year, but this year it's not working so well — that, too, is something to report.

"Communicating with extension personnel makes it possible for applied pathologists to look at the grower's pathogen and see if it has changed in some way, or to just try to assess what's happened." It also gives the extension agent an opportunity to pass information along to other researchers and plant breeders who are working on these crops.

"And, same with seeing a new disease. Not that many years ago, basil downy mildew was brand new. We could have another brand new pathogen in the U.S., at any time.

"If as a grower you don't know who is in tune with a new disease, your local extension specialist probably does. Or, they can do the footwork and find out, who is the person in the U.S. to contact, if you have something new happening."

The newest Prospera® Active series offers even more protection against downy mildew. Pictured is 'Mia Prospera Active DMR'—a multi-use basil with dense, slow-bolting plants

Not too long ago, there were no varieties that showed true resistance to basil downy mildew, but now Johnny's research team trials new "resistant" varieties every year. This past season at Johnny's we had our highest downy mildew pressure to date, which gave us an opportunity to see stark differences in the varieties. We like the Prospera varieties not only because they have shown the highest level of resistance to downy mildew, but also for characteristics like aroma, plant habit, leaf shape, and color — everything that makes basil the beautiful, aromatic, delicious, high-demand, high-value crop it has become over the past 40 years. This season really proved what the Prospera varieties can do.

In deciding which basil varieties to plant, it's useful to hear from those whose knowledge runs deep into downy mildew strains, monitoring, management, and breeding for resistance. And with such a delicate herb, it's encouraging to find that plant breeding for disease resistance can be successful, especially so in the organic setting. While breeding for resistance against a rapidly evolving pathogen is a developmental process that never really ends, the value of this work extends into the future. "The idea here is to be ready with new resistance when there starts to be actual pressure on the Prospera," says Arnon. "I, as a breeder, need to be ready."

Learn More

Johnny's basil trial technician Rachel Katz and product manager Hillary Alger present a synopsis of their extensive trialing research on Prospera basil.

Resources from Johnny's

• Prospera®️ DMR Basil Varieties Comparison Chart
• Visit our Basil Grower's Library…
• Browse Prospera Basil Seeds from Johnny's …

Since publication of this article, Genesis has developed the first red basil with downy mildew resistance! In this video Hillary introduces 'Prospera Red DMR' basil, now available from Johnny's.

References & Further Reading

Ben-Naim, Y. & M. Weitman. 2021. Joint action of Pb1 and Pb2 provide dominant complementary resistance against new races of Peronospora belbahrii (basil downy mildew). Phytopathology. URL: https://doi.org/10.1094/PHYTO-02-21-0065-R (accessed 01.30.2022; online ahead of print).

Ben-Naim, Y., et al. 2018. Transfer of downy mildew resistance from wild basil (Ocimum americanum) to sweet basil (O. basilicum). Phytopathology, 108 (1), 114–123. URL: https://doi.org/10.1094/PHYTO-06-17-0207-R (accessed 12.30.2021).

Ben-Naim, Y., et al. 2015. Resistance against basil downy mildew in Ocimum species. Phytopathology, 105 (6), 778–85. URL: https://doi.org/10.1094/PHYTO-11-14-0295-R (accessed 12.30.2021).

McGrath, M.T. 2021. Basil downy mildew. Cornell University. URL: https://www.vegetables.cornell.edu/pest-management/disease-factsheets/basil-downy-mildew/ (accessed 12.30.2021).

Patel, J.S., et al. 2021. Effective downy mildew management in basil using resistant varieties, environment modifications, and fungicides. Plant Health Progress, 22 (3), 226–234. URL: https://doi.org/10.1094/PHP-02-21-0041-FI (accessed 01.30.2022).