Downy Mildew: A Plant Pathogen Targeting Your Lettuce

The article below is based on a podcast interview with Dr. Kelsey Wood.

Have you ever noticed yellowish patches or fuzzy white growths on your lettuce? These could be signs of downy mildew, a plant disease that weakens lettuce and other leafy greens, making them vulnerable to other problems.

Downy mildew is a plant pathogen that infects vegetables like lettuce, spinach, and even your favorite herbs. It can appear in your garden and on your store-bought produce, and it’s a significant problem for farmers.

But what is downy mildew, how does it cause disease, what is the scope of its impact on agriculture, and how can you spot it in the grocery store? Keep reading to find out.

How downy mildew causes disease

Unlike some plant diseases, downy mildew doesn’t directly kill its host. Instead, this microbe acts as a parasite, stealing nutrients from the lettuce plant while keeping it alive for continued exploitation. This one-sided relationship weakens the plant, making it susceptible to other diseases (bacteria or fungi) that can deliver the final blow.

The impact of downy mildew can be devastating, especially for young lettuce seedlings. A full-blown infection at this early stage can wipe out an entire field. While conventional farming practices heavily rely on fungicides to control downy mildew, organic growers face a significant challenge. They lack highly effective organic-approved controls for downy mildew.

Downy mildew particularly is highly host specific. There’s a unique type of downy mildew for each plant species, so the lettuce downy mildew won’t bother your spinach, and vice versa. This specialization allows the pathogen to become highly adapted to its target plant, making it an efficient nutrient thief.

The real danger of downy mildew lies in its rapid reproductive cycle. In just a week, downy mildew can reproduce and release spores into the air, ready to infect new lettuce plants. This rapid spread, coupled with the weakened immune system left behind in the infected plant, and resulting secondary infections by bacteria and fungi is what leads to the great loss of crops.

Is downy mildew a fungus? Nope!

Downy mildew might be a common problem for lettuce growers, but there’s a surprising fact about this pathogen — it’s not actually a fungus! This misconception is understandable due to its name containing “mildew.”

So, what exactly is downy mildew? It belongs to a different group of organisms called oomycetes. This name adds to the confusion because it contains “mycete,” which means fungi. However, these microbes are more closely related to algae (kelp) than fungi. One key difference is that, unlike fungi, oomycetes have cell walls made of cellulose, similar to plants, while fungi have chitinous walls.

Even at the microscopic level, identification is challenging. Downy mildew can be easily confused with powdery mildew, another plant pathogen but a true fungus. Both appear fuzzy or powdery under magnification but with some key differences. Downy mildew, as the name suggests, has a more downy appearance, while powdery mildew is, well, more powdery.

Here’s a quick comparison:

• Downy Mildew:
  • Oomycete (not a fungus)
  • Cell walls made of cellulose
  • Downy appearance
• Powdery Mildew:
  • True fungus
  • Cell walls made of chitin
  • Powdery appearance

Controlling downy mildew

Though it is not a fungus, the current treatments for downy mildew are fungicides. They are effective against it because they disrupt some essential functions in oomycetes.

Though using fungicides is a necessary practice right now, the reliance on them in conventional farming comes with a price. The use of broad-spectrum chemicals can disrupt the delicate balance of the soil microbiome. This essential community of microbes plays a crucial role in plant health, and impairing them can have unintended consequences.

However, scientific research offers a glimmer of hope here. Understanding the differences between downy mildew and fungi will allow researchers to develop more targeted control methods.

How can you spot downy mildew at the grocery store?

When you’re looking at lettuce, downy mildew typically will show on the outer leaves. Thankfully, often these affected leaves are discarded during processing, so consumers rarely encounter them at the grocery store.

However, downy mildew can still impact the quality of mature lettuce even if the outer leaves are removed. It weakens the plant’s immune system, making it more susceptible to other pathogens. Studies even suggest that downy mildew infection can increase the presence of harmful bacteria like E. coli and Salmonella on lettuce.

So, how can you spot downy mildew at the grocery store? While the early stages might be invisible, keep an eye out for these signs:

• Yellowing: Downy mildew can cause yellowing of the leaves, a telltale sign that something is amiss.
• Angular Lesions: Look for geometric shapes on the lettuce leaves. Downy mildew tends to favor rectangular or diamond-shaped regions where the veins of the leaf intersect.
• Fluffy Structures: In the later stages of infection, you might see the namesake downy mildew itself. These fuzzy white structures, resembling microscopic white trees, erupt from the surface of the leaves.

It’s important to note that the appearance of downy mildew can vary depending on the plant species it infects. For example, spinach downy mildew has spores with a grayish pigment, while lettuce downy mildew has the characteristic white fuzz.

If you notice any signs of this plant pathogen on your produce, it’s best to avoid it. If you happen to take home a head of lettuce with downy mildew, you likely won’t get sick from consuming it, nor does it produce toxins. However, as mentioned above, downy mildew can be associated with a slightly higher chance of encountering E. coli and Salmonella on the infected plant. When in doubt, err on the side of caution and choose a fresher head of lettuce.

How can downy mildew spread?

Downy mildew isn’t a stationary microbe. It produces special spores called oospores that allow it to spread from plant to plant and ensure its survival. These oospores have thick walls, making them highly resistant to harsh conditions and allowing them to persist in the soil for extended periods.

When favorable conditions return, these resilient spores can germinate and infect new lettuce crops. The formation of oospores also allows for genetic mixing between different downy mildew strains, potentially leading to new variants that can overcome resistance in lettuce varieties.

Here are some key ways downy mildew travels:

• Wind: The primary mode of dispersal for downy mildew is through spores carried by the wind. These microscopic structures can travel long distances, infecting new lettuce plants far from the initial source.
• Wild Reservoirs: Wild lettuce species act as a natural reservoir for downy mildew. These wild plants can harbor the pathogen and contribute to its spread through spores or by infecting cultivated lettuce growing nearby.

Promising discoveries in downy mildew research

The world of downy mildew research is brimming with fascinating discoveries. Here are some promising areas of research:

• Genomic research: As more downy mildew species are sequenced, scientists are uncovering unexpected details. One such surprise is the discovery of horizontal gene transfer in a downy mildew species, where it acquired a gene from a fungus, potentially adding a new tool to its arsenal. Studying such genetic exchanges can provide insights into how downy mildew evolves and adapts.
• Plant-pathogen arms race: The interaction between plants and downy mildew is like a microscopic war. Downy mildew tries to suppress the plant’s defense system while the plant attempts to recognize and resist the pathogen. This constant back-and-forth drives both sides to evolve rapidly. Understanding this dynamic can aid in developing new disease control strategies.
• Resistant lettuce: Dr. Kelsey Wood, who I spoke with on the podcast, identified a lettuce variety with exceptional resistance to downy mildew. The key lies in its ability to recognize multiple proteins from the pathogen, making it a tougher target. This discovery holds promise for breeding disease-resistant lettuce varieties that could reduce reliance on pesticides. However, there are hurdles to overcome. The super resistant lettuce currently has a smaller size, and breeders need to refine it to achieve commercially viable yields. The development of disease-resistant crops could be a game-changer for farmers, potentially reducing pesticide use and creating a more sustainable food production system.

Summary

• Downy mildew is a plant pathogen that can infect vegetables like lettuce and spinach.
• It weakens the lettuce plant without directly killing it, making it susceptible to other diseases.
• There’s a unique type of downy mildew for each plant species.
• Downy mildew is not a fungus, it belongs to a different group of organisms called oomycetes.
• Oomycetes have cell walls made of cellulose, similar to plants, while fungi have chitinous walls.
• Downy mildew infection can increase the presence of harmful bacteria like E. coli and Salmonella on lettuce.
• Further reading on food safety: Understanding Food Safety with Chicken: From Farm to Fork