What’s lurking in your lettuce?


Everyone knows that eating salads is good for your health. As it turns out, microbes like to eat lettuce too, with serious consequences for farmers and consumer health.


Salinas Valley is located a few miles inland of the Monterey Bay.

In California, lettuce brought in $2.2 billion dollars in 2015, which was half a billion more than tomatoes and almost double the amount brought in by oranges and avocados combined [1]. Most of this lettuce is grown in Salinas Valley, also known as the “Salad Bowl of the World”, which can provide over 80% of lettuce for the whole US [2].

The climate and soil in the Salinas Valley make it perfect for growing lettuce, unfortunately the cool, damp coastal areas also provide prime conditions for lettuce downy mildew, the most damaging disease of lettuce leaves. When the weather is right and downy mildew strikes, losses can be up to 100% for an individual lettuce field.

The downy mildew parasite resembles a fungus, but is actually part of a group of organisms known as the oomycetes (this group includes the famous late blight pathogen that caused the Irish potato famine). Downy mildew by itself does not actually kill the lettuce plant, but it does ruin leaves and weakens the plant immune system, allowing bacterial infection and other diseases to proliferate.

Video showing what downy mildew looks like in the field:

To help fight downy mildew, lettuce breeders take resistance genes from wild lettuce varieties and cross them into commercial varieties; this is known as genetic resistance. Most of the time, neither the breeders nor plant scientists know exactly what genes are being introduced, only that they give downy mildew resistance in selection tests. One problem with this approach is that there are multiple strains of downy mildew present in California and an individual resistance gene won’t provide resistance to all pathogen strains. Also, the pathogen is able to rapidly evolve to overcome the resistance genes faster than breeders can come up with new varieties.

When downy mildew overcomes genetic resistance, the next approach used by farmers is application of pesticides. Over 1/2 of fungicide use on lettuce is specifically targeted to control downy mildew [3]. Some of these pesticides are being phased out due to concerns about human health. One can go organic to avoid the pesticides, but will pay a premium: organic lettuce faces the greatest losses from downy mildew, as there are currently no organic pesticides that are effective at controlling the pathogen.

Scientists in California and elsewhere are studying both the lettuce and the downy mildew pathogen to better understand what biological weapons are used by each side during infection. When a new strain of downy mildew is discovered that can attack lettuce that was previously resistant, we can sequence its genes and find out which ones have changed. We hope to get a better idea of the existing downy mildew strain variation in California to better predict which varieties will be resistant in the future. More resistant lettuce will mean less pesticide use, meaning cheaper and healthier lettuce for use in your salads!

[1] http://www.nass.usda.gov/Quick_Stats/Ag_Overview/stateOverview.php?state=CALIFORNIA

[2] http://californiacountry.org/features/article.aspx?arID=563

[3] http://cesantabarbara.ucanr.edu/files/75296.pdf

Note: this blog post was originally written as part of the excellent SciFund Challenge outreach class

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