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

Let me tell you about the time I lost a whole bed of butterhead lettuce in less than 48 hours.

It was early spring. Cool mornings, warm days, and a stretch of rain. I thought everything looked perfect. Then I walked out one morning, and the leaves were spotted, wilted, and gray with fuzz. Just like that, the row was gone.

That was the year I learned what downy mildew looks like—and how fast a disease can take hold if you’re not watching closely.

Lettuce is fast, tender, and delicious. But it’s also fragile. This guide breaks down five of the most common lettuce diseases, how to catch them early, and what to do when they show up in your patch. I’ve dealt with every one of these myself, and I’ll walk you through the solutions that work.

1. Downy Mildew: The Silent Spread

What It Looks Like:

• Yellow or pale green spots on upper leaf surfaces

• White or gray fuzzy growth underneath

• Starts slow, then moves fast

What Causes It:

Cool, wet nights and poor airflow are the perfect breeding ground for this fungal-like pathogen. Overcrowded beds and morning dew help it move plant to plant before you even notice.

What To Do:

• Give lettuce space — 8–10 inches apart, especially in spring

• Water at the base, early in the day

• Harvest outer leaves regularly to keep airflow up

• Use copper fungicide if you're in a high-risk zone (Zone 7–10)

🧓 Grandma’s Tip: “Lettuce likes a breeze more than gossip does.”

2. Damping Off: Death Before Growth

What It Looks Like:

• Healthy seedlings suddenly collapse at the soil line

• Stems may look pinched or darkened

• Seedlings rot and die before true leaves emerge

What Causes It:

This one hits during seed starting. Overwatering and contaminated trays let fungal pathogens thrive, especially in poorly ventilated indoor setups.

What To Do:

• Use sterile seed-starting mix

• Water from below, not overhead

• Let soil dry slightly between waterings

• Sanitize trays with vinegar before reusing

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

3. Tip Burn: The Hidden Imbalance

What It Looks Like:

• Brown, dry, or scorched edges on outer leaves

• Can show up even in fast-growing, otherwise healthy plants

What Causes It:

Tip burn comes from calcium deficiency paired with erratic moisture. If you water inconsistently—too dry, then soaked—your plants can’t absorb nutrients properly.

What To Do:

• Keep moisture steady with deep watering and mulch

• Avoid over-fertilizing, which can worsen the imbalance

• Add calcium-rich amendments like crushed eggshells or oyster shell flour

🧓 Grandma’s Tip: “The edges go first when a plant is mad.”

4. Powdery Mildew: The Dust You Don’t Want

What It Looks Like:

• White, powdery blotches on the tops of older leaves

• May spread in patches and cause leaf curling or yellowing

What Causes It:

This one thrives when days are warm and nights are cool and humid. Poor airflow, overhead watering, and dense planting make it worse.

What To Do:

• Prune inner leaves to boost airflow

• Avoid overhead watering — water at the base instead

• Apply neem oil or potassium bicarbonate at first signs

🧓 Grandma’s Tip: “Sunlight, space, and early action. That’s your cure.”

5. Bottom Rot: Trouble from Below

What It Looks Like:

• Slimy, discolored base of lettuce head

• May smell sour and feel mushy

• Starts at soil level and moves upward

What Causes It:

Soil-borne fungi like Rhizoctonia thrive in damp, compacted soil. Overwatering and poor drainage speed it up.

What To Do:

• Rotate crops yearly — don’t plant lettuce in the same spot again next year

• Amend soil with compost, sand, or perlite for better drainage

• Water early in the day, and let soil dry between waterings

• Raise your beds if your soil stays soggy

🧓 Grandma’s Tip: “Don’t grow lettuce where water naps.”

Catch It Early: Your Lettuce Is Talking

Before any of these diseases fully take hold, your plants will give you clues. Here’s what to watch for:

Early SignPossible IssueSeedlings falling overDamping OffYellow patches + white fuzzDowny MildewCrispy edges on outer leavesTip BurnWhite spots on old leavesPowdery MildewBase of plant feels mushyBottom Rot

Trust me—learning to catch these signs early will save your salad patch.

Preventive Practices That Pay Off

The best disease fix? Don’t get it in the first place. Here’s what helps long-term:

• Rotate your lettuce bed each year

• Choose disease-resistant varieties like ‘Muir’, ‘Sparx’, or ‘Pelican’

• Sanitize trays, tools, and seed-starting gear

• Grow in raised beds or well-drained rows

• Space plants for airflow and mulch lightly

A little planning and observation go a long way in avoiding outbreaks altogether.

✅ Download This: Lettuce Disease Quick-ID Chart

Need to keep these disease signs close at hand?

Print out our Quick-ID Chart — complete with symptom snapshots, what causes them, and what to do. It’s a one-page reference you’ll actually use. Stick it in your seed box, tape it to the greenhouse wall, or slide it in your garden journal.

👉 [Download the Chart Now]

Keep Growing

You’re not failing if you run into disease—you’re learning what your garden’s telling you.

Every patch teaches you something. The trick is paying attention, adjusting early, and not giving up.

Next up? Learn how to harvest lettuce the right way without pulling the whole head:
→ Read: “Cut-and-Come-Again Harvesting: How to Make One Row Feed You All Season”

“Build houses and settle in them; plant gardens and eat what they produce.” – Jeremiah 29:5