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. jeremy sager says:

    Margaret–I have heard that spraying your Basil (weekly) with a solution of (2-3%) Hydrogen Peroxide may combat the Downy Mildew issues. Has anyone else heard of this and is there any downside to lightly spraying the plants with this? I was told it evaporates quite quickly and would not be a negative for the plant or for home consumption…

    1. margaret says:

      Hi, Jeremy. I know that some herb growers use a peroxide solution for powdery mildew (a true fungus) but don’t know anything about its effectiveness on downy mildew (an oomycete, not a true fungus). I don’t think there is any danger to you; just don’t know if it works! :)

  2. Kim says:

    Is the soil where infected basil plants are growing also infected with the disease? If I pulled the infected plants and replanted with healthy ones (if there are any where I live in Arkansas), are the new ones doomed to suffer the same fate?

    1. margaret says:

      Hi, Kim. If there is an infection anywhere within distance that the spores can blow in from, it can spread potentially a long distance. Do since you already have it, I’d suspect any new plants would be at risk, yes.

      1. Kim says:

        Thank you, Margaret. That’s what I was afraid of. I guess I’ll just wait for those resistant plants to come to market. Sigh.

  3. shiner says:

    Since I live in the mountains of NC, zone 7, (altitude 3000) I am unclear if the spores would overwinter here.
    The references to the “south” are a bit vague when referring to where I live. If they do overwinter, from what I read above I could plant squash in that bed since they seem to be different types of mildew.

  4. Cat says:

    I’ve noticed that when I buy basil in the store for cooking it frequently has downy mildew on it. Is it safe to consume plants that have downy mildew?

    My aerogarden basil contracted downy mildew, after which I got rid of all the basil & rinsed out the aerogarden. My next batch also contracted downy mildew- would washing the basin in a bleach solution kill the pathogen?

    I know it is airborne- but how long will it hang in the air? If I have it in my kitchen and I shut the door to the room with basil, how long should I quarantine the airspace? Could it settle on non-plant surfaces and survive to be transferred to the basil when I go in there (such as if it settled on scissors and I used them on the basil the next day)?

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