Basil Downy Mildew and Prospera® DMR Basil
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Breakthrough Breeding Innovation Provides Solutions
Buyers want basil that looks perfect when it reaches the shelf, so growers need the strongest, most disease-resistant varieties they can find. Choosing those varieties is part of how we support the people who grow from our seed. The Hensley's team runs basil trials every season, and we also learn a great deal by listening to people who work with this crop and its diseases every day.
For this piece the Hensley's team talked with three people who deal closely with basil downy mildew, now the most widespread and destructive disease of the crop: the breeder behind the Prospera® DMR (downy mildew-resistant) line, a commercial herb grower with four decades in the business, and a plant pathologist at Cornell. All three are cautiously hopeful. Their shared message is that useful tools exist now and more are coming, but growers still have to stay alert to what the pathogen does next.
Breeding Downy Mildew-Resistant Basil Varieties: Arnon Brand, PhD, Genesis Seeds
The breeding program we spoke with is run at an organic seed company— a family-owned organic seed production and research business in the Negev desert of Israel, founded in 1994. Its breeding work covers tomato-family and squash-family crops, but the breeder told us basil comes first.
He explained that the basil downy mildew project began around 2016, not long after he joined the company. The founder asked him to set his tomato work aside and take charge of basil, and the assignment had one clear goal: find an answer to downy mildew, the crop's most serious problem.
Hand-to-the-glove with academia
His first plans, he admits, probably would not have worked. Then he contacted a plant pathology lab at Bar-Ilan University, which had just found a way to cross sweet basil with an African basil species, two plants that normally do not produce fertile offspring together. The timing was a lucky coincidence, and the two groups began working together within days.
He described the lab's lead professor as a practical thinker who was looking for a small commercial partner to carry the discovery forward, so the fit between the university and the seed company came naturally.
That partnership became a lasting one. The university's plant pathologists screened hundreds of wild basil species collected around the world looking for resistance, and eventually found it in an African species. Crossing that source with a sweet Italian basil produced a handful of hybrids carrying the resistance gene. Working with the seed company, the team then built the organic Prospera DMR line while holding it to strict market standards for flavor, aroma and appearance.
We know the work is never finished
Moving resistance from a wild plant that looks and smells nothing like sweet Genovese basil into a commercially useful variety is a real achievement, and it happened faster than breeding projects usually do. The breeder credits steady work and close cooperation with the university team.
He also stressed that basil breeding is never finished. Resistance has already broken down slightly in a few places, a sign of how quickly the downy mildew pathogen changes. Like everyone we talked to, his team keeps working, and newer varieties with stronger resistance, including the Active series, are already on the market, with more on the way.
Looking back, he noted that young breeders often expect to change the world and instead end up refining familiar varieties. Building something truly new in a few years is rare, and being able to bring traits in from wild species was, in his words, a breeder's dream.
He also contrasted large and small companies. At a big firm, one person is a small part of a very large machine; at a small one, individual decisions carry real weight. A poor decision can hurt the business, but a good idea, even an unusual one, can be approved and put into action right away.
A long, steady path also describes the herb grower we interviewed, who runs a New York herb farm . He started with a few hydroponic buckets and, over about 40 years, built a large herb business. Along the way his farm was among the first in New York to face basil downy mildew.
Grower Success: Brian Murphy of Goodness Gardens
He began growing hydroponically in New York in 1980 as a young man, using 5-gallon buckets at a time when hydroponic growing was still largely uncharted.
Few shoppers knew what hydroponic produce was, and selling tomatoes, lettuce and cucumbers was difficult. In the mid-1980s the farm added fresh herbs. At that time supermarkets carried almost no fresh herbs besides parsley, and when he first showed basil to a produce buyer, the buyer did not recognize it.
Herbs turned the business around, and the farm kept expanding. Today it sells herbs in large volume, is building a highly automated organic hydroponic greenhouse of about 2½ acres, and has related operations in Florida and Texas.
Growth has not been simple. Over the years he has had to keep up with changes in hydroponic technology, shifts in the market, new varieties and new diseases. Because he has bought basil seed for many years, we asked when downy mildew first became a problem for him.
Basil downy mildew hits Goodness Gardens
The disease reached his area in 2012, when he was growing about 5 acres of Genovese basil. When the plants started yellowing, he assumed they needed fertilizer. More fertilizer went on, but over the next few days the leaves only got worse.
Then he spotted mildew on the leaf undersides and realized he was dealing with something new. He learned quickly that infected leaves do not recover: once they have yellowed, they stay that way even if the pathogen is controlled, which made the first seasons very hard.
Cornell soon released information on the disease and on ways to manage it. Early on, fungicides gave poor control and no resistant varieties existed; those took years to arrive. In his view, resistant varieties combined with newer fungicides have now made downy mildew close to a manageable problem on his farm.
In his part of New York, he said, greenhouse downy mildew typically appears in July and lasts until about mid-November. The pathogen moves north from southern states during spring and early summer, and once fall frosts arrive the spores die off, giving him several disease-free months.
What about downy mildew-resistant basil varieties?
When we asked about resistant varieties, he said the first ones he trialed did hold off the disease but were weaker in color, leaf shape, leaf size and plant habit.
With Prospera, he has noticed that a few leaves may show downy mildew, but the infection tends not to move further. On susceptible plants, by contrast, a crop can be lost almost overnight. Catching symptoms early and spraying before they spread gives a grower a big advantage, and losing a few leaves is a minor cost.
In one recent season he reported no downy mildew outbreak in his outdoor fields after planting only Prospera there, while also applying a routine weekly protective spray. The year before, without Prospera, he had lost a large basil planting to the disease.
He is hopeful but careful. He has heard that even stronger Prospera varieties are coming, yet after four decades of farming he knows that many pathogens keep adapting even as new tools appear.
That back-and-forth between basil and the pathogen is something he has in common with a leading U.S. researcher on the disease.
Monitoring, Reporting & Management: Margaret McGrath, PhD, School of Integrative Plant Science, Cornell
The plant pathologist we spoke with is based in New York and focuses on practical, management-oriented research on vegetable diseases. Since many diversified vegetable farms in her region grow basil, it made sense for her to study downy mildew as soon as it arrived and share what she learned with growers.
First and foremost, we needed a monitoring program
Basil downy mildew was first reported in the U.S. in October 2007 and showed up on Long Island in 2008. When it returned in 2009, she set up a monitoring program that lets growers, extension staff and gardeners report sightings right away.
Digging deeper to develop a resistance program
Before this, she had worked on Phytophthora infestans, the related pathogen behind late blight of tomato and potato, and on cucurbit downy mildew, which had become more common after 2004.
Her basil research began around 2010 with fungicide evaluations, funded through the IR-4 program, which helps register both conventional and organic products for specialty crops. Starting in 2013, USDA-funded projects allowed her to begin testing resistant varieties as well.
Pathogens do their own thing
Her recent work again compared newer and older DMR varieties. Here is what she observed:
- Over two recent seasons she saw more downy mildew on the Rutgers DMR lines than before, though still far less than on a susceptible variety.
- She has always found some disease on the Rutgers lines, though less than on earlier DMR types such as 'Eleonora', while the wild parent used as their resistance source showed no symptoms in earlier tests.
- Prospera, in her trials, gave very strong control of downy mildew, including in a season when nearby Rutgers DMR lines performed worse than usual, which surprised her.
Even so, she warns that
She has had a few reports of heavier downy mildew on Rutgers DMR lines and a small number of reports on Prospera. The pathogen may be changing in more than one way, and basil could end up like spinach, where breeders must keep finding new resistance genes.
Verification & reporting matter
Many diseases look alike, and some look like nutrient problems rather than infections. To avoid treating the wrong problem, confirm the pathogen first. If yellow leaves show no spores underneath, seal them in a damp plastic bag overnight; with downy mildew, spores usually appear within a day.
Through AgPest Monitor reports, researchers have learned how widespread the disease is across the country. She sees the maps as the program's biggest strength, since a map makes the spread far easier to grasp than a spreadsheet and keeps people involved.
Interestingly, most reports come from home gardeners. Because they usually do not spray their basil, they are more likely to notice the disease.
Hard to avoid, hard to manage
Her main management advice is to use an integrated program and to plan as if the disease will arrive. The pathogen spreads on the wind, and its spores can travel long distances. As with other mildews, crop rotation does not control it.
She also notes that dry summers do not mean low risk. Pathogens are most active at night, and even during long rainless spells, humidity can approach 100% after dark as temperatures fall. Downy mildew thrives under those conditions, and she has seen the worst symptoms after heavy dew.
Growers should stay alert and assume the disease may well appear every season. The pathogen will probably keep changing, though it might not ; researchers expected the late blight pathogen to keep developing fungicide resistance after it became one of the first to do so, and that did not happen. Predictions are often wrong, but it is reasonable to expect continued change.
She points out that downy mildew is difficult to control well, particularly in an herb. In her research, sprays started after symptoms appeared did not give good control, so she now begins protective applications before any symptoms show and considers that timing essential. Buyers of fresh herbs will not accept spotted leaves. The leaf is the product. With cucurbit downy mildew, poor control lowers yield and fruit quality, but the harvested fruit is not directly attacked, so a total loss is less likely. Basil downy mildew damages the very leaves being sold, which makes it harder to manage.
The importance of both formal and informal reporting structures
She also urges growers to remember that pathogens do change and to watch closely — and to report anything unusual they notice.
The best contact is usually the local extension office. Report it if a resistant variety that worked well before suddenly shows heavy symptoms, or if a fungicide program that worked last season is failing this year.
Those reports let applied plant pathologists check whether the pathogen has changed and figure out what happened. Extension staff can also pass the information on to other researchers and to breeders working on the crop.
The same goes for a disease nobody has seen before. Basil downy mildew itself was new to the U.S. not long ago, and another new pathogen could appear at any time.
If you are not sure who studies a new problem, your extension specialist probably knows, or can find the right person to contact.
What's Next: Resistance in an Ever-Changing Environment
Not long ago, no basil showed true resistance to downy mildew. Now the Hensley's team trials new resistant varieties every season. One recent season brought very heavy disease pressure, which made differences between varieties easy to see. The Prospera varieties stood out for resistance and also for aroma, habit, leaf shape and color, the qualities that have made basil such a valued crop.
When choosing basil varieties, it helps to learn from people who study downy mildew strains, track the disease and breed for resistance. It is encouraging that resistance breeding is working in such a delicate herb, including in organic systems. Breeding against a fast-changing pathogen never really ends, and the breeder told us his goal is to have new resistance ready before pressure on Prospera builds.
Learn More
Resources
- Prospera®️ DMR Basil Varieties Comparison Chart
- See our Basil Grower's Library…
- Shop Prospera Basil Seeds …
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).
