basil under pressure: the fight against devastating downy mildew

basil downy mildew by robert wickTHE BASIL LOOKED PEKID, the tops of its leaves marked with diffuse yellow, as if hungry for fertilizer, but then going brown. Turning them over revealed distinctive vein-bounded brown or fuzzy gray patches. The decline continued; leaves dropped.

If this sounds familiar, maybe you thought it was “your fault.” Chances are it was basil downy mildew, a devastating disease that in 2014 was reported to thwart backyard and commercial growers alike in at least 35 states plus the District of Columbia—up from 20 plus D.C. in 2013.

basil downy mildew 2 robert wickFor farmers, a once-easy, profitable cash crop is challenged; for gardeners, a beloved ingredient elusive. Is there a resistant variety, or a cultural trick to outsmarting the destructive pathogen?

University researchers are collaborating in a multistate effort on potential short-term and long-term solutions. They’re evaluating existing varieties for resistance; testing control with fungicides; exploring biological controls; and making new crosses aimed at creating varieties with excellent resistance, that also taste as good as longtime favorites. They met February 5, 2015 at the Vegetable Growers of New Jersey convention to share insights and hear presentations from Rutgers, Cornell and the University of Florida.

Quick fixes are proving elusive, as organic fungicides don’t provide control. Chemical fungicides combinations were most effective when applied preventively, weekly—which probably sounds unappealing on an edible, and is outside the gardener’s toolkit, anyhow.

I caught up with Dr. Robert Wick, Professor of Plant Pathology & Nematology at UMass Amherst, whose focus is on possible biological controls–specifically working with the genetic population structure and exploring a yeast that colonizes the downy mildew. He answered my questions and encouraged interjections from colleagues elsewhere, based on their latest findings.

why read 1,400-plus words about basil disease?

OLYMPUS DIGITAL CAMERATHE Q&A BELOW explains why (despite promising advancements) a solution will take time, and what you can try meantime in 2015. There’s no magic solution at the bottom of the page; sorry. But I hope you will read on for the glimpse into what plant pathologists and other university researchers, often unheralded, are doing nonstop in our behalf as consumers of basil—and much bigger-picture, on so many other favorite ornamentals and essential food crops. (Above: Tree-like sporangiophores and sporangia, the spores that causes infection in basil downy mildew.)

first, a few fast facts:

  • Discovered in Uganda in 1930, basil downy mildew (cause by the pathogen Peronospora belbahrii) didn’t get much notice until the early 2000s in Europe, and then a 2007 occurrence in Florida. Things have moved fast since.
  • It is called “downy mildew,” but it’s not the same pathogen as Impatiens downy mildew that devastated the popular shade annual recently, or as the one in cucurbits.
  • This is a fierce opponent, producing wind-dispersed spores that can potentially travel long distances.
  • The pathogen needs high humidity (at least 85%) or wet leaves to infect a plant, and favors moderate temperature, not high heat (which may in fact help limit its spread).

downy mildew q&a with dr. robert wick and colleagues:

the organism, and the 2014 season

Q. Dr. Wick, can you explain the downy mildew organism–which most gardeners probably think is a fungus?

A. Downy mildew is an oomycete; they evolved from brown algae and are not related to fungi at all. However, they look and behave like fungi, so they were classified as fungi for many years.

Incidentally, Phytophthora infestans, the cause of late blight of potato and tomato, is also an oomycete, and naturally related to the downy mildews.

Q. The words “downy mildew” seem to be more common lately–impatiens, coleus, lettuce, cucurbits. Are there more such issues, or simply better identification by researchers?

A. Impatiens and coleus, as well as a few other ornamental plants, have had “new” downy mildew emergences. These probably jumped from some native species into the horticulture trade. Lettuce and cucurbits have long been known to have downy mildew diseases.

Q. How would you characterize 2014? North Carolina State Extension said their outbreak was the “worst one yet.” Pathologist Dr. Margaret McGrath at Cornell counts 35 affected states plus D.C. last year, up from 20 plus D.C. in 2013.

A. It was a bad year for the Northeast, because downy mildew showed up on basil plants in big-box stores. I believe this means it was widely planted into residential areas early in the season and then spread to farms.

[Dr. McGrath concurs, saying that 2014 weather patterns and the shipment of plants carrying the pathogen combined to favor downy mildew:

“More reports of downy mildew in 2014 than previous years was associated with affected plants for sale at garden centers in several states and cooler temperatures in the North Carolina to Maryland area,” she reports, “where typical high summer temperatures might have limited downy mildew development in other years.”]

how downy mildew gets around

Q. Does basil downy mildew overwinter in the U.S.?

A. It readily survives the entire season in the South, because basil can be grown all year around there, so Florida has a problem.

We are not aware that it can overwinter in the Northern states because no one has seen the survival structures, called oospores, in the U.S. It probably needs two mating types (sort of like a man and woman) to produce those oospores, and there may just be one mating type in the U.S.

Oospores have been reported in Israel, but I am skeptical until I see convincing pictures.

Q. Is the trend toward more widespread greenhouse culture in areas where plants wouldn’t otherwise grow out of season giving downy mildew a headstart each year?

A. First of all, downy mildews are very host-specific. The one on basil only occurs on basil, or perhaps some other closely related plant.

For downy mildews that don’t survive in the North, such as tobacco blue mold, cucurbit downy mildew and basil downy mildew, greenhouse culture on related ornamental plants (like Nicotiana or hydroponic lettuce or basil) allows the downy mildew to be right there at the beginning of the growing season, for example in a place like Canada.

With respect to cucurbit downy mildew and tobacco blue mold, they only survive in the Deep South and have to come up, cucurbit field by cucurbit field, as the growing season moves up farther north. Typically, we don’t see cucurbit downy mildew in Massachusetts until mid-August.

But if someone is growing hydroponic cucumbers in Massachusetts, they could have downy mildew all winter and release it early in the growing season.

For the Northern states, every year is a new year. We may see very little downy mildew this year, for example.

better varieties, or cultural tactics

Q. After investigations into basil downy mildew began, a 2009 round of Rutgers field trials noted some lemon and spice types (Ocimum x citriodorum and O. americanum cultivars) were less affected than common sweet basils (O. basilicum). Is this still the thinking?

A. Yes, there is resistance in some basil types, and the New Jersey researchers are trying to bring it into Genovese basil.

[Robert Pyne, a PhD candidate involved in that breeding program at Rutgers, adds that resistance is still observed in the O. americanum ‘Spice’ and ‘Blue Spice’ varieties. In lemon varieties trialed, researchers also observe reduced sporulation, but more severe yellowing. In their breeding, Rutgers has been able to introduce very high degrees of tolerance into a number of sweet basil breeding lines. Despite such progress, the team’s “best-guess estimate” is that it will be another three years before several such new varieties will reach the marketplace, with more to follow.]

Q. Some seed sellers tout ‘Eleonora,’ a new variety from Vitalis offering intermediate resistance. Any thoughts?

A. [From combined responses: “Varieties with a good degree of suppression are not available commercially yet,” says McGrath’s report from Cornell, released at the February 5 meeting. In Cornell’s trials ‘Eleonora’ showed some resistance, but not as much as some of the new Rutgers breeding developments still in research stages, she says.  ‘Eleonora’ has not been tested yet at Amherst or Rutgers.]

Q. The UMass-Amherst web page on basil downy mildew says: “Although the downy mildew pathogen has been detected in basil seed; seed transmission is probably a rare event. Air-borne dissemination from infected plants is more likely.”

For a gardener, is growing from seed safer than bringing home seedlings?

A. Growing from seed or purchasing transplants from the garden center would not make much of a difference. However, I would not recommend buying transplants that were grown in the South. Buy local transplants—and this is true for all vegetable transplants as well for the same reason; several important plant pathogens that cannot survive in the Northern states survive in the Southern states.”


[Cornell’s McGrath suggests that home gardeners try growing their basil in pots. She had success managing around downy mildew by bringing her potted plants indoors when it is really humid outdoors–meaning overnight, and on rainy days. Details of her home experiment are at this link. Photo above: One of McGrath’s outdoor plants, left, beside one she protected most nights and from rain.]

(Photos from Dr. Robert Wick, except basils in pots and leaves on blue background from Dr. Margaret McGrath.)

  1. Sara says:

    Thank you Margaret,
    I bought some ‘Eleaonora’ seeds because I had read about it as well.
    I hope the Basil downy mildew will give us a break this year!
    Yours at the end of a snow shovel,

  2. Jan Smith says:

    I live on the coast of British Columbia (zone 7) and I lost most of my basil to downy mildew last year. Genovese, Thai, and spicy globe. I have been looking for maps to show the spread in the Pacific Northwest and can’t find any, but it is here. My plants came from an excellent local grower. The plants were beautiful when I first got them. We were able to use new growth that sprouted on still, hot, dry days, but as soon as there was rain, the remainder of the plant would be affected. I diligently picked off all affected leaves and cleaned around the plants, but the mildew would return as soon as moisture appeared. I guess I will be babysitting this year’s plants the way Dr. McGrath does…because who can live without basil?

  3. Caroline says:

    Very very interesting… Thanks for sharing.
    Here in Madrid, where is very very dry, I grew 5 different varieties on soil, outdoor, with shade and it went well, we are actually working on adapting these plants to our climate condition, and that the least water possible.
    But I know of other growers that also grew different varieties last year, in greenhouse and aroung august the lost all their basil due to some mildew…
    So its very interesting to see if these stories are in fact related.
    Regards from Madrid.

    1. margaret says:

      Thanks, Caroline. Of course the greenhouse conditions can be really tricky (humidity!). I don’t know about the presence of the pathogen in Spain (have seen reports about past years in France, Italy, Belgium, Switzerland…) but it sounds as if it’s worth inquiring about.

  4. Mary Sue says:

    I’m always so envious reading about your garden, Margaret. I live at 7300 ft. elevation in Colorado. It’s difficult to grow a lot of things but I can grow BASIL!!! It probably because of low humidity and cool nights. I still have a plant left from last summer in a window. Thank you for all your great knowledge.

    1. margaret says:

      Lucky you, Mary Sue, on this subject, but I know you have your own issues in the intermountain region, of course. Nice to hear from you, and thanks for the kind words.

  5. Beth Urie says:

    Hmmm, this is sad. Dr. McGrath’s growing method sounds most hopeful for the present, but not one a commercial organic grower could use (too cumbersome). I only hope a solution can be found that does not involve genetic engineering – or petrochemical brew. When it comes time, it would be nice to run something on alternatives to basil in cooking. For pesto, for example, we’ve enjoyed both young carrot leaves, and garlic scapes as the green ingredient. But that’s just one use of basil, and I can’t think of an herb or green as distinctive as sweet basil.

  6. Michelle, Zone 5b in WI says:

    I was planning on purchasing Eleonora but didn’t move fast enough on my order and it was quickly out of stock. I guess that is more evidence of how prevalent it was last year. We definitely saw it here. It was in my own garden and our school garden a few blocks away. My friend a few blocks in the other direction did not get it and had beautiful and abundant basil plants. Not sure what made the difference as we used her seeds in the school garden so it wasn’t the variety.
    Good luck in 2015 basil-lovers!

    1. margaret says:

      Nice to hear from you, Michelle. Sounds like the wind was blowing in your direction and not your neighbor’s. :) (I am semi-kidding, of course–probably the “answer” is more complicated than that.)

  7. Cindy says:

    We have have the basil problem for a few years but managed to get enough for dried, frozen and fresh use before the plants succumbed, though we don’t put up pesto. We used some sort of organic solution last year for mildews that comes in a small packet and thought it slowed emergence but now that seems as though it was merely chance. As for finding culinary replacements for favorite garden produce, we are flummoxed about tomatoes as well. We can’t grow any of those any more either due to late blight. And the ones we buy from our CSA farms are not very tasty any more between diseases and the weather we have been having in recent growing seasons. How to replace tomatoes in so many dishes and in the winter pantry? It is sad.

  8. Vickie says:

    I have noticed this and was surprised since basil is so easy to grow as long as watered properly. Thanks for the valuable info.

  9. Kit Duffield says:

    I grew Basil for Farmers Market booth last year, mine didn’t last the season, I credited to over picking and noted that this year I must plant it twice. I don’t lke to hear about the researchers changing the seed to grow through the disease.

  10. Susan Meeker-Lowry says:

    I live in Fryeburg, Maine. Every year brings new challenges mostly due to the changes in weather, especially more rainy days (and downpours rather than a nice gentle rain – the past 5 years especially and now it appears to be the norm here), and less sun along with plenty of humidity, sun or not. I’ve noticed some of my basil leaves look like the top picture but not the black on the underleaf. I start from seed and keep light fabric row covers held up with hoops in early summer to protect from cold nights and as much of the heavy rain as I can. Then the plants get too tall for the covers and they are on their own. I pick constantly, to discourage flowering as long as I can. I always have enough basil and the plants hang around until the colder nights, though a couple of years ago I did cut them in late August as they had had it. Every year I hope for a less destructive rainy summer, and I dream of the summers we used to have where the sun was out, certainly not every day, but more often than not. Now we’re lucky if 50% of the days have some sun and no rain.

    Tomatoes are always a challenge here. My biggest problem is early blight and leaf spot. No matter what I do, where I plant, how I mulch or what seeds I start, by the time the plants are fruiting the blight/spot begins. They say not to water from above which is a joke because I can’t stop the rain! They say to rotate but my garden is too small and the available sunny beds in short supply. I’ve started using those big fabric pots and move them from place to place. Last year I mulched with garden paper and hay on top of that. No way did any soil splash onto the plants but the blight/spot came as usual. I’ve tried spraying various forms of liquid copper but since it has to be applied after every rain, it’s just no longer practical. Plus I’m concerned about the disappearing bees. I feel their loss deeply and would rather lose some vegetables than inadvertently any that manage to make it to my garden. Last year there were virtually no bees until almost mid-summer. It was devastating!

    I usually get enough tomatoes to make one, two if I’m lucky, batches of my “famous” spaghetti sauce with lots of garlic and fresh herbs. But that’s only 14 pints and or quarts and I usually have between 12 and 18 plants of various types. Of course I don’t can the sungold cherries and they seem to be the most prolific and resistant. My favorite paste is Opalka. I love how sweet they are! But they do succumb to the blight and each year I get fewer and fewer. A couple of years ago I tried a variety supposed to be resistant to late blight (which isn’t a problem in my garden yet) and early blight though not as much as late. They didn’t get the blight until much later than all the other tomatoes but they didn’t taste that great either, took forever to ripen, and the fruits weren’t uniform. It actually looked like there were different types of tomatoes on a single plant. Perhaps the “parents” revealing themselves as happens with squash volunteers from hybrid seeds. I ended up giving them away and won’t try again. Catalogs describe them as juicy with good tomato taste but that was not my experience. Sorry to get off the topic of basil!

    1. Jo says:

      We had many of the same problems here in central Vermont, even though our summer was apparently sunnier than yours. Very interesting to hear of your various efforts. And I understand the limits of applying those efforts, too (eg, apply after every rain). I feel your pain. I am finally moving towards trying more hybrid, resistant seed this year. Good luck.

  11. Louise says:

    Does anyone know if it is safe to eat pesto if it was made from basil leaves that may have had downy mildew on them? I did have downy mildew on my genovese basil last year, and while I did not use leaves that were obviously diseased, some leaves with a small amount of mildew may have been included. I still have the pesto in my freezer.

    1. margaret says:

      I have not seen any warnings about eating infected leaves–and of course as you say you’d cull the worst ones, anyhow (since they’re discolored/dried up!).

  12. Jo says:

    We had many of the same problems here in central Vermont, even though our summer was apparently sunnier than yours. Very interesting to hear of your various efforts. And I understand the limits of applying those efforts, too (eg, apply after every rain). I feel your pain. I am finally moving towards trying more hybrid, resistant seed this year. Good luck.

  13. I managed to save almost all my basil plants last year by nipping all the affected leaves and it worked. I also learned that the fungal affected leaves do no harm to humans, so I froze them for using in cooked recipes and that worked fine too.

    I managed to save all my basil. Definitely potted. My garden is like a sump so I have to watch all my plants for rot. Upstate NY lots of rain last summer.

  14. Beverly, zone 6, eastern PA says:

    I had at least twelve basil plants distributed throughout large mixed garden beds, each one partnered with a caged tomato plant. By August of 2014, every basil was affected with the discolored foliage and I thought it was mites, having never experienced this problem before. I was confused how the mites got to every single basil plant which were spaced many yards apart, but seemed to be nowhere else. Now I see it was an airborne affliction. (And I should wear my bifocals more often outside, apparently…)

    My pesto’s last summer were limited to Garlic Scape Pesto, Dried Tomato Pesto and Asparagus Pesto. Basil lost out.

    Hoping for a better crop this year, but wondering if I should toss my own saved seeds as they were collected and bagged with some of the outer pod coverings of the seed capsules. Hhhmmmmmm.

    1. margaret says:

      For the small price of fresh seed, Beverly, why not eliminate that factor from the equation and buy some? I don’t know the answer with certainty, but I am all for giving myself the very best chance I can, and the saved seed from infected plants seems like a wild card you don’t need.

  15. Kathy says:

    Thank you for confirming what I thought was going on with my basil plants. I lost three different plantings then gave up. No one I asked about mildew on basil seemed to know what I was talking about here in North Carolina. It was like a cover-up was going on!I talked to extension agents, gardencenters and several Master gardeners to no avail. I hope the scientists will find a solution because Basil holds a prominent place in my herb garden as well as decorative in my containers.

  16. Alan Grossberg says:

    My common basil (five plants) were trash by mid-late July, but the only thing which stopped my lemon basil and Thai basil was the first frost.

    1. margaret says:

      Good to hear, Alan, and seems to parallel the kinds of observations from the field trials (you’re growing your own mini-experimental station!). :) Thanks for saying hello.

  17. Beth Urie says:

    Another from me on this one … thinking about the plants seen at the grocery store being sold as fresh basil. These are very young plants. I wonder about planting basil as a fast growing crop intended to be clean-cut when young. Under protection, with drip line for watering, I will try seeding directly once soil is warm, thinning to 3-4″ and cutting the whole crop at once
    when plants reach around 10″. Hopefully, several plantings can be achieved over the season here in central Vermont. We’ll see…

    1. margaret says:

      It can strike at any time in the age of the plants, I think, Beth (even when quite young), but the type of experiment you’re pondering is definitely worth a try.

    2. Louise says:

      I’ve seen basil at the grocery store with downy mildew on their leaves. It was in an early stage, but since I’d seen it in my garden (northwest NJ), I recognized it on the plants in the store.

      1. margaret says:

        Hi, Amanda. Having the basil around and growing for a shorter time limits the potential for exposure, if it is around or “blows in,” it can even affect little seedlings.

  18. Patricia says:

    Thank you for culling important information. The Q&A format gets right to the most important points, which is appreciated. I can’t help but think that part of the plant problems we gardeners are having is due to the sale of plants at “big box stores.” I am thinking of the tomato problem and now this. And, it seems that one infected plant in a neighborhood could easily be carried by the wind to neighbors’ plants. I am guilty of purchasing flowers and occassionally herbs from them. This year, I will not. I will try to grow from seed or buy from the farmer’s market.

  19. june veloce says:

    Wow. The year before I had TONS of basil to share
    and last year barely enough for my pasta sauce. Now I know why!
    Thanks for all the info.

  20. Tony says:

    Thanks for covering this topic with an excellent article. (Seems to me it is not being well addressed in other gardening information outlets.)

    At 5400′ on the Colorado Front Range, lots of infection late summer 2014. “Serrata”, while infected, seemed to hold-on with some marginal productivity longer than the smooth leaf Italian types.

    1. margaret says:

      You’re welcome, Tony. I know it’s all dense and heavy going, but I felt like I wanted to get all the details down to educate myself as we head into another season. ‘Serrata’ is a beautiful basil; glad to hear you got to harvest some!

  21. Frankie says:

    Soooo glad to hear this problem is being looked into in such a through manner, even if there is no clear answer to overall prevention. As with one of the prior comments, in 2013 I had it seemed baskets of basil to eat, freeze, share, all from pots on my patio. 2014 was almost a write-off except for a somewhat leftover plant that I stuck in a hanging basket, on the full sun side. It was under a roof overhang and received moisture via a drip system. Conditions were just right for this little orphan and it was profuse in providing enough basil for numerous meals. With the research update though will for sure seed my own plants this year.

    1. margaret says:

      Interesting about your solitary hanging-basket plant making it–sort of like Cornell’s Meg McGrath’s “experiment” in her yard. Nice to see you; thanks for saying hello.

  22. Caroline Roi says:

    The leaves looked “peaked,” actually. Pronounced as you wrote the word. My mother and aunts would say this if someone was looking tired.

    1. margaret says:

      Hi, Caroline. That word is easier to say when it’s spelled in the “alternate” way, pekid. When I see it spelled the preferred was, peaked, I get confused. :)

  23. Erica Renaud says:

    I want to clarify some of the information communicated in this article and following communications regarding the resistance and availability of ‘Eleonora’ basil. ‘Eleonora’ basil is the first and only intermediate resistant basil available commercially in the US and Canada. As Dr. McGrath communicated in her presentation,’Eleonora’ did eventually get DM, however, 10-12 days later than other commercial material. The data on the Rutgers material performing better than ‘Eleonora’ was not actually presented formally at the meeting, nor is the material commercially available. ‘Eleonora’ is commercially available in limited quantities through Johnny’s Selected Seeds and will have a wider distribution in 2016. The limitations in the biological and organic control methods research presented at the meeting was that the applications were every 7 days (set up along the same application rates as the conventional controls). Considering organic growers use soil soaks and application rates of 3-5 times per week, the data presented was not representative of how organic growers actually manage their crops.

    1. margaret says:

      Thanks, Erica, for the further detail on ‘Eleonora,’ which I described as “offering intermediate resistance,” as you say. The article also does state that best-case scenario, the Rutgers material would not be out for 3-ish years. We gardeners all look forward to the progress Vitalis and also university researchers are making and will continue to make in behalf of us and the organic farming community.

  24. maryann says:

    You are a more patient (and tactful) person than I….how any of your readers have the nerve (rudeness) to question your punctuation or spelling (or ANY language skills). I guess these people belong to that special group of humans who never make a mistake. Also,when I saw your spelling of pekid, I didn’t assume you didn’t know the correct spelling. I figured that you knew something that I didn’t and was going to look the word up (not so I could “school” you but to learn something new). You are an impressive writer. The kind readers who have informed you of any lapses (real or imagined) probably think they are “doing you a favor ” by “helping ” you with your writing.
    They are welcome to correct my writing since I am sure it has many errors to tempt them with.

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