Bremia lactucae
Spots on leopards? Fabulous. Spots on dalmatians? Adorable. A begonia with spotted leaves? Irresistible.
But spots on lettuce leaves are bad news. If you notice light green or yellow angular lesions on the leaves, you might be looking at a downy mildew infection.
If so, let me be the first to offer my condolences. This disease sucks. But you don’t need commiseration. You need solutions, right?
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In lettuce, this disease is caused by Bremia lactucae and can spread quickly under the right conditions. It reduces yield, affects quality, and in severe cases can wipe out entire crops.
Our guide to growing lettuce has everything you need to cultivate this Lactuca sativa in your vegetable garden.
In this guide, we’ll cover what downy mildew is, how to identify it, and the cultural and chemical tools available to keep it under control.
Here’s what’s on the menu:
What You’ll Learn
What Is Downy Mildew?
Downy mildew in lettuce is caused by an oomycete known as Bremia lactucae.
Oomycetes are fungus-like organisms, often called water molds, though they don’t always require standing water to survive.
There are many different races of B. lactucae that infect different types of lettuce, and the pathogen can also infect artichokes (Cynara cardunculus var. scolymus), cornflowers (Centaurea cyanus), and strawflowers (Helichrysum bracteatum).
In North America, nine races have been identified, though only races five through nine are currently active. In Europe, 37 have been described, with races 16 through 37 active.
That’s the situation at the time of writing, but these oomycetes adapt quickly. New races appear, old ones can re-emerge, and resistance patterns shift.
Isn’t nature fun? You treat them all the same, so don’t worry about which is attacking your lettuce.
But we mention it because some races are more resistant to certain treatments than others. So if one method isn’t working well, you may need to try another one.
B. lactucae is an obligate biotroph, which is science speak for a pathogen that needs a living host. That matters because it means the pathogen doesn’t want to kill its host, it requires a live host.
Lettuce seedlings are especially vulnerable and can be killed outright. In leaf lettuce, the disease can destroy entire plants.
Head lettuce like ‘Ice Queen’ sometimes survives if caught early, but harvest quality is often reduced.
For commercial lettuce growers, the losses can be devastating. In some regions, the majority of fungicide applied to lettuce crops is aimed at controlling this single disease.
Identification
The first sign of downy mildew on lettuce is the appearance of yellow or light green patches on the leaves.
These patches are typically angular in shape because the discoloration is limited by the leaf veins.
On red-leafed varieties, the lesions may appear creamy gray instead of yellow. The affected tissue can look water-soaked, though not always.
Flip over the lettuce leaves and you may see gray or white, mold-like coating in the same areas.
Sporulation usually appears on the undersides but can also occur on the upper surface as well.
It takes at least a week after infection for the spores to develop, so by the time you see them, your plant has been infected for a while.
This sporulation can be mistaken for powdery mildew, but the two are different. Powdery mildew favors warm, dry conditions, while downy mildew requires cool, moist weather.
As the disease progresses, the entire leaf turns brown, papery, and shriveled before dropping from the plant.
Growth is stunted, and while mature heads may survive, many leaves are lost and infected plants are pretty unappealing.
In severe cases, systemic infection can cause dark discoloration throughout the lettuce plant.
Biology and Life Cycle
B. lactucae is an obligate parasite, which means it can only reproduce on living plants. A suitable host is essential to its survival.
This oomycete thrives in cool, moist conditions. Germination occurs between 40 and 68°F, with 50°F being optimal.
Unfortunately, lettuce grows best in the same temperature range, which is why downy mildew is such a common problem.
The disease is most prevalent in spring and fall, especially when there is lots of dew overnight and in the morning. In greenhouses, it can appear throughout the winter.
While outbreaks are uncommon in summer, they can occur during extended cool, damp periods.
In the Pacific Northwest where I live, we can have cool, wet periods during the summer. When that happens, B. lactucae can get a toehold even during the summer months.
Spores reproduce and spread rapidly when conditions remain favorable for 12 hours or more.
They are dispersed by wind and water droplets, and can survive on leaves, stems, and even seeds.
Once the spores find a suitable host, they only need to be there for about four hours before they start reproducing. The cooler it is, the slower the disease progresses.
Hot, dry, and sunny weather limits the spread and extended dry conditions, direct sun, or high temperatures can kill spores.
Although B. lactucae typically does not kill mature plants, seedlings are highly vulnerable and may collapse after infection.
When a host dies, the pathogen must spread to new living tissue to continue its life cycle.
Organic Control Methods
When you spot downy mildew, quick action is essential. The earlier you intervene, the more likely you’ll save your crop.
Let’s start with a few cultural methods:
Cultural
You’ve probably heard it before and you’ll definitely hear it again, but crop rotation is your friend! Not just with lettuce, but with pretty much every single annual in your garden.
Rotating to unrelated crops helps break the disease cycle and makes it harder for pathogens to build up in the soil.
So skip a year, or better yet, two or three years, between planting lettuce in one spot in your garden.
There are lots of resistant cultivars available, like ‘Pirat’ and many others. While no variety is completely immune, many are less prone to infection or show only mild symptoms.
Resistance is often labeled on seed packets as “DM” or “resistant to DM.” Keep in mind that resistance is race-specific.
A cultivar resistant to one race of B. lactucae may still be vulnerable to others. If symptoms appear on a “resistant” variety, try another cultivar suited to your region.
Beyond crop rotation and planting resistant cultivars, you should remove and destroy any debris left behind in your garden at season’s end.
Even small pieces of infected tissue can harbor the pathogen and provide a bridge to the next season.
Finally, water at the soil level and not on the foliage. The pathogen can travel in water, so if water splashes on the leaves, you’re making it easy for them to land on their preferred host.
Then, be sure to space your plants appropriately to facilitate air circulation.
When you crowd plants, it increases humidity and it makes it easier for the spores to travel from one plant to another.
In greenhouses, fans can further lower humidity and prevent prolonged leaf wetness.
Physical
If the disease is caught early and limited to the outer leaves, removing the symptomatic tissue can slow its spread. In some cases, this may be enough to salvage the crop.
However, leaf removal alone is not a cure. Combine it with preventative measures or organic treatments to reduce the chance of reinfection.
Biological
Treatment works best when you use it preventatively or on the first sign. Once the disease advances, your options are limited.
The following products are highly effective as preventatives, but won’t work so well if the disease is already established.
Always follow label directions, as manufacturers provide specific protocols for targeting this disease.
Streptomyces lydicus WYEC 108 is a naturally occurring bacterium that suppresses fungi and oomycetes.
It’s the active ingredient in products like Actinovate AG that can help stop downy mildew in its tracks.
You can find 18-ounce bags of Actinovate AG at Arbico Organics.
Another option is Bacillus subtilis strain QST 713. This beneficial bacterium colonizes plant surfaces and outcompetes pathogens without harming the host.
You can find it in products like Serenade ASO.
Serenade ASO is available at Arbico Organics.
Organic Fungicides
Like biological treatments, organic fungicides work best as preventatives. Once downy mildew is present, it’s much harder to stop or control, but that doesn’t mean you shouldn’t try.
If you see the disease on one plant, treat all of your lettuces, regardless of whether they’re symptomatic or not.
You can also alternate an organic herbicide with a biofungicide so long as you follow the manufacturer’s directions carefully.
Copper is a well-known fungicide that has been used for decades to suppress fungal and oomycete diseases.
It damages cell walls and causes the contents to leak out, killing the pathogen.
Monterey Liquid Copper Fungicide
You can find Monterey’s Liquid Copper available at Arbico Organics.
Sulfur is another tried-and-true option. Applied every two weeks, it can help suppress outbreaks alongside copper sprays.
You can find Bonide Sulfur Fungicide available from Arbico Organics.
Thyme oil contains thymol, which attacks the cell walls of pathogens, killing them. It’s effective at treating downy mildew if used at the first sign of trouble.
You can find Guarda, which harnesses thymol, available at Arbico Organics in two-and-a-half-gallon containers.
Potassium bicarbonate prevents and suppresses disease by disrupting fungal cell walls. It’s effective against downy mildew as well as powdery mildew, leaf spots, and botrytis.
Milstop SP is made using potassium bicarbonate and is available at Arbico Organics.
Citric acid acts both systemically and on plant surfaces. It can be applied repeatedly and provides protection against multiple diseases, including downy mildew.
Plus, you can spray it as often as needed. Look for products like Procidic, which contains citric acid.
Procidic is available at Arbico Organics in 32-ounce ready-to-use bottles, or 16-ounce and gallon concentrates.
In addition to these products, hydrogen dioxide is also an effective treatment option, often used in rotation with other products.
Chemical Control
For home gardeners, there are no chemical fungicides that consistently outperform the organic options listed above.
Stick with those recommended above and be sure to rotate through a few different ones to prevent the oomycetes from developing resistance.
Out, Damn Spot!
Downy mildew is distressingly common and frustratingly resilient. I hope you never have to deal with it, but don’t despair if you do.
Even the most experienced growers encounter it from time to time.
What kind of symptoms are you seeing? Are you having trouble identifying what you’re dealing with? Hit us up in the comments section below and we’ll see how we can help.
And for more information about common lettuce problems, have a read of these guides next:
