Top 5 Lettuce Diseases—and What to Do When They Show Up

Let me tell you about the time I lost three rows of butterhead lettuce in less than 48 hours.

It was early spring—cool nights, a damp breeze, and perfect conditions for downy mildew (bremia lactucae) to show up uninvited. I walked out one morning, and my lettuce crop looked like a salad gone bad: yellow leaf spots, fuzzy mold underneath, and that sinking feeling every gardener knows when a plant turns south overnight.

That was the year I learned how fast a lettuce disease can take root—and why prevention and early action matter more than any spray.

If you’re growing lettuce in your garden—whether in raised beds, potting soil, or a container setup—this guide will show you how to spot the top 5 diseases, stop them early, and keep your crop clean and productive. These are the same management techniques I use here on The Grounded Homestead, with no fluff and no guesswork—just practical solutions that work.

Whether you’re dealing with downy mildew, tip burn, or root rot, let’s walk through what’s going wrong—and how to make it right.

1. Downy Mildew in Lettuce: The Silent Spread That Destroys Your Crop

If you grow lettuce and ignore airflow, this fungus will take you by surprise. Downy mildew—caused by the oomycete Bremia lactucae—is one of the most common threats in salad gardens, especially when humidity spikes and soil moisture lingers overnight.

How to Identify Downy Mildew on Lettuce

• Pale yellow or light-green angular spots on the upper leaf surface

• Fuzzy white or gray mold underneath (often in early morning)

• Leaves quickly turn necrotic, collapse, or develop lesions

This plant pathogen thrives in cool, wet weather—think March mornings or shaded garden beds with poor airflow. Once the spores take hold, they spread fast across leaf tissue and nearby host plants.

What Causes It

• Poor spacing and overcrowded lettuce beds

• High humidity, low air movement, and wet leaf surfaces

• Watering late in the day or overhead irrigation

• Infected plant debris or soil hosting Bremia lactucae spores

How to Treat Downy Mildew in Lettuce

• Improve airflow by spacing lettuce heads 8–10 inches apart

• Water early using drip irrigation or a soaker hose to keep foliage dry

• Rotate crops and avoid planting in areas where lettuce or brassicas previously grew

• Apply copper fungicide or OMRI-listed alternatives as a preventive treatment (check the label and intervals for efficacy)

• I keep a bottle of organic-approved copper fungicide on hand each season—this one’s been reliable when I need to knock mildew back fast without wrecking my soil biology.

> > But it Now

Zone-Specific Risk

• USDA Zones 7–10: Higher risk in spring and fall with fluctuating temperatures

• Cool-season climates: Monitor early morning dew and protect lettuce with row covers or low tunnels

Downy mildew doesn’t just ruin your salad harvest—it sets you back weeks in the growing season. Catch it early, act fast, and keep your garden breathing.

For in-depth guidance on identifying and managing Bremia lactucae, see Penn State Extension’s excellent overview on downy mildew in lettuce, which covers symptom progression and control measures under cool, wet conditions.

2. Damping Off: When Your Lettuce Never Gets a Chance

You prep your soil, sow your seeds, and just when you expect germination—your lettuce seedlings keel over like they’ve been cut at the ankles. That’s damping off, a silent killer in early-stage vegetable crops.

This seedling disease is caused by soilborne pathogens like Pythium, Rhizoctonia solani, and Fusarium—and once it starts, there’s no reversing the damage.

How to Recognize Damping Off in Lettuce

• Seedlings wilt and collapse at the soil line

• Stem base turns dark, thin, or pinched

• Germinated seeds may rot without ever forming true leaves

• No visible pests—but total loss within 24–48 hours

If you're using compost-heavy potting soil, reusing trays without sanitation, or letting humidity linger around your germination station, you're inviting this plant disease right in.

Grandma’s Tip: “Don’t drown a baby seed. They need breath, not a bath.”

What Causes Damping Off

• Overwatering or poor soil drainage

• Excess humidity around seedlings

• Unsterile trays, tools, or leftover organic matter

• Dense sowing with limited airflow between young shoots

How to Prevent Damping Off

• Use sterile potting mix for seed starting (not old garden soil or unaged compost)

• If you’re still using leftover soil to start seeds, switch to a sterile seed-starting mix like this one—it’s made a night-and-day difference in my germination success

• Water from the base—avoid soaking young seedlings

• Allow surface of the soil to dry between waterings

• Sanitize trays with diluted hydrogen peroxide or a light bleach solution

• Space seeds for airflow and don’t cover trays during daylight hours

Optional Additions for Disease Management

• Dust seeds with cinnamon (natural antifungal) before sowing

• Use a weak chamomile or hydrogen peroxide solution to mist emerging seedlings

Once damping off takes hold, there’s no saving infected seedlings—only prevention. This is one of those times where sanitation and seed-starting discipline pay off more than any pesticide ever could.

3. Tip Burn in Lettuce: The Hidden Imbalance That Ruins Perfect Leaves

Your lettuce looks lush and green… until one day, the outer leaves crisp up at the tips like they’ve been blowtorched. That’s tip burn, and while it’s not contagious like fungus or blight, it’ll wreck your salad harvest all the same.

Tip burn is a physiological disorder—not a plant pathogen—but it’s rooted in a calcium imbalance inside the leaf tissue, often made worse by inconsistent soil moisture and hot climate conditions.

How to Spot Tip Burn

• Brown or black necrosis at the edges of outer leaves

• Leaf tips curl, dry, or break off entirely

• Often appears on fast-growing cultivars like romaine or butterhead

• Inner leaves may look fine while outer ones deteriorate

What Causes Tip Burn

• Inconsistent watering leading to poor calcium uptake

• Sudden spikes in temperature or heat stress

• Soil too acidic (low soil pH) or lacking essential nutrients

• Excessive nitrogen fertilizers that trigger fast leaf growth before root absorption keeps up

How to Fix and Prevent Tip Burn

• Water deeply and consistently—lettuce needs even moisture to avoid stress

• Apply mulch to reduce evaporation and stabilize soil temperature

• Add crushed eggshells, oyster shell flour, or gypsum to support calcium availability

• Test and amend soil pH to keep it between 6.0–6.8 for proper nutrient uptake

• This simple 4-in-1 soil meter takes the guesswork out of watering and pH—especially if you're growing lettuce in containers or raised beds.

> > But it Now

• Reduce synthetic nitrogen-heavy fertilizers in favor of balanced organic options

Who’s Most at Risk

• Container gardens or shallow soil beds with fast evaporation

• Gardeners in Zones 8–10, especially during early summer

• Hydroponics and indoor growers with limited calcium access in nutrient solution

Tip burn won’t spread from leaf to leaf, but it’s still a disease of imbalance—where nutrition, irrigation, and climate don’t line up. Fix that triangle, and you’ll have crisp leaves without the crunch of damage.

4. Powdery Mildew on Lettuce: The Dust That Doesn’t Belong in Your Salad

Powdery mildew may sound harmless, but this fungal disease can quickly coat your crop in a white, chalky mess—ruining both the appearance and the vitality of your lettuce plants.

Unlike downy mildew, which needs moisture, powdery mildew thrives in dry air—especially where airflow is poor and humidity builds overnight.

How to Identify Powdery Mildew

• White or grayish “dust” coating older leaves

• Starts as small circular spots, then spreads outward

• May cause leaf curling, yellowing, and eventual wilting

• Often appears in late spring or early fall when days are warm and nights cool

This disease is typically caused by Erysiphe cichoracearum, a fungal pathogen that infects a wide range of vegetables in the Asteraceae family, including lactuca (lettuce).

What Causes Powdery Mildew

• Overcrowded plants blocking air circulation

• Shaded areas with limited sunlight

• Excessive nitrogen fertilization that creates dense leaf tissue

• Poorly timed watering that raises overnight humidity

How to Treat and Manage Powdery Mildew

• Remove infected leaves immediately and discard away from garden beds

• Improve airflow through pruning or spacing out plants

• Avoid overhead irrigation—use drip systems or water at the base

• Spray organic treatments like:

  • Neem oil

  • Potassium bicarbonate

  • Milk spray (1 part milk to 9 parts water, misted early in the day)

Advanced Organic Management Options

• Use OMRI-listed fungicides containing sulfur or serenade

• Incorporate disease-resistant cultivars into your crop rotation plan

• Encourage beneficial microbes in the soil through compost and mulch

5. Bottom Rot in Lettuce: Trouble from Below That Ruins the Whole Head

You reach down to harvest a full, vibrant head of lettuce… and your fingers sink into a slimy, brown base. That’s bottom rot—caused by the soilborne fungus Rhizoctonia solani—and it’s one of the most destructive lettuce diseases in warm, wet conditions.

This fungal infection attacks the crown and root tissue, often after heavy rain or overwatering, and can wipe out an entire row if ignored.

How to Spot Bottom Rot in Lettuce

• Dark, mushy rot at the base of the plant, near the soil line

• Leaves yellow and wilt from the bottom up

• Affected plants may smell sour or collapse when pulled

• May appear with secondary pathogens like sclerotinia or bacterial soft rot

What Causes Bottom Rot

• Poor drainage or compacted garden beds

• Overhead watering late in the day

• Crop debris left from previous lettuce or brassica plantings

• High soil moisture and warm temperatures (especially in Zones 8–10)

How to Prevent and Treat Bottom Rot

• Rotate crops annually—don’t plant lettuce or leafy greens in the same bed twice in a row

• Elevate beds or amend heavy clay with sand and organic matter for better drainage

• Water in the morning and avoid oversaturating the roots

• Use straw mulch to reduce water splash and prevent spore contact on leaves

• Remove all infected plants and surrounding soil debris immediately

Integrated Pest Management Tip

• Avoid planting lettuce after carrots, cabbage, or other susceptible crops

• Solarize infected beds in summer by covering them with clear plastic for 4–6 weeks

• Test and balance soil pH to discourage pathogen spread

Bottom rot doesn’t just take a few leaves—it takes the whole plant down from the ground up. Good drainage, crop rotation, and sharp observation will keep your salad crops healthy and clean.

Catch It Early: Your Lettuce Is Talking—If You Know What to Look For

Every plant disease starts small. And lettuce? It’ll whisper before it screams.

The key to avoiding major loss is recognizing early stress signals in your crop. The right observation—paired with a little gardening intuition—can stop a full-blown infestation, fungal outbreak, or rot problem before it ruins your growing season.

Here’s your quick-look chart for spotting issues before they spread:

Know What’s Normal

• A few outer leaves yellowing? That can be age.

• Every seedling collapsing? That’s a red flag.

• One moldy plant in a raised bed? Pull it and monitor.

• Mold on multiple plants? Start your management program now.

The best plant pathology lesson is hands-on. Make it a habit to walk your garden daily and get eye-level with your lettuce. Look under the leaves. Feel the soil. Smell for rot. You’ll start catching problems before they get a foothold.

Washington State University’s Hortsense offers a comprehensive fact sheet on lettuce diseases—covering damping-off, bottom rot, anthracnose, and more—to guide your integrated pest management program.

How to Prevent Lettuce Diseases Before They Start

It’s easier to prevent a lettuce disease than to fix one mid-season. Whether you’re growing iceberg in a raised bed, romaine in containers, or leaf lettuce in a backyard kitchen garden, a little forethought beats fighting off fungus, rot, and wilt once they’ve taken hold.

Below are the same prevention steps I use on The Grounded Homestead to stop disease before it ever reaches my salad rows.

1. Practice Smart Crop Rotation

• Don’t plant lettuce or other Asteraceae crops in the same spot two years in a row

• Rotate with non-host crops like tomatoes, onions, or radish

• Helps reduce soil-borne pathogens like Rhizoctonia solani and Fusarium

2. Use Disease-Resistant Varieties

• Look for cultivars marked as resistant to:

  • Downy mildew (Bremia lactucae)

  • Lettuce mosaic virus

  • Sclerotinia minor and botrytis cinerea

• Good choices: ‘Muir’, ‘Sparx’, ‘Coastal Star’, and ‘Pelican’

  Cornell’s Vegetable Program maintains a current list of disease-resistant lettuce cultivars, including those with downy mildew, mosaic virus, and bottom rot resistance—perfect for planning your next crop with confidence.

3. Sanitize Tools and Start Clean

• Wipe pruners, seed trays, and transplanting gear with vinegar or hydrogen peroxide

• Start seeds in sterile potting soil or organic starter mix

• Remove weeds and leftover plant debris, which often harbor spores, mildews, and insects

4. Master Watering and Drainage

• Use drip irrigation or soaker hoses to keep leaf surfaces dry

• Water early in the morning so plants dry before nightfall

• Amend soil with sand or compost to prevent standing water

• Avoid watering intervals that cause stress, rot, or root suffocation

5. Design Your Garden for Airflow

• Space lettuce plants 8–10 inches apart (or more for large-head varieties)

• Prune low-hanging leaves to boost ventilation

• Avoid planting near structures that block wind and trap humidity

This is the kind of garden work that pays off every time. No chemical spray can match what good drainage, clean tools, smart rotation, and healthy soil can do for disease resistance.

Disease Isn’t Defeat—It’s a Lesson in Listening

Lettuce may be a fast crop, but that speed cuts both ways—it grows quickly, but it fails fast if you miss the warning signs.

Every fungus, every rot, every curling leaf is a message from your soil, your climate, or your watering habits. Your job isn’t to panic—it’s to pay attention. These five diseases are common because most gardeners miss the signals until it’s too late.

Now you don’t have to.

Here on The Grounded Homestead, I’ve learned (usually the hard way) that plant pathology isn’t just for scientists—it’s for salad growers who want to eat what they’ve sown. Whether you're growing in raised beds, hydroponic trays, or clay-heavy soil, these tools work. They’re not theory. They’re practice.

So keep a close eye on your lettuce patch. Space it right. Water it early. Trust what you see—and act before the mildew moves in.

Frequently Asked Questions About Lettuce Diseases

❓ What’s the difference between downy mildew and powdery mildew on lettuce?

Downy mildew (caused by Bremia lactucae) typically shows up on the underside of the leaves and thrives in cool, wet conditions. It causes yellow angular spots on the top side and fuzzy white growth underneath.

Powdery mildew (caused by Erysiphe cichoracearum) forms a white, dusty coating on the upper surface, prefers dry air with high humidity, and often spreads in low-light or shady gardens.

❓ Can I eat lettuce with tip burn or leaf spotting?

Yes—tip burn is safe to eat. It’s a sign of nutrient or moisture imbalance, not infection. Just trim off the damaged leaf edges. If the lettuce shows signs of mold, rot, or sliminess (especially at the base), it’s best to compost it.

❓ Does crop rotation really matter for lettuce?

Absolutely. Many pathogens like Rhizoctonia solani and Fusarium can overwinter in soil or garden debris. By rotating out of lettuce, brassicas, or leafy greens, you break the host-pathogen cycle and reduce the risk of future infestations.

❓ Are there organic fungicides that work for lettuce diseases?

Yes. Neem oil, potassium bicarbonate, and copper-based sprays are commonly used in organic production. For seedling protection, chamomile tea, hydrogen peroxide, or cinnamon powder are gentle, natural alternatives. Always follow label directions and observe reentry intervals.

❓ How do I stop lettuce from wilting in heat without causing rot?

• Water early in the morning

• Use mulch to retain moisture and shield roots

• Provide afternoon shade with row covers or interplanting with taller crops

• Avoid overhead watering late in the day to reduce humidity buildup and fungal pressure

❓ What’s the best soil pH for lettuce disease prevention?

Keep soil pH between 6.0 and 6.8. This range helps maximize nutrient uptake and limits pathogen spread. Highly acidic or alkaline soils can lead to nutrient lockout, calcium deficiencies, and stunted growth.

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