Lettuce Disease Identification Guide for Greenhouse Growers

Disclaimer: This guide is intended for general educational purposes only. Disease identification and management recommendations vary by region, production system, and regulatory environment. Always consult a qualified agronomist or extension specialist for diagnosis and treatment decisions specific to your operation. Pesticide registrations vary by jurisdiction — verify local label requirements before application.

Overview

Lettuce is susceptible to a range of fungal, bacterial, and physiological disorders that can significantly impact yield and quality in greenhouse production. Early and accurate identification is the foundation of effective integrated pest management (IPM). This guide covers the most common diseases encountered in controlled environment lettuce production.

1. Tipburn (Physiological)

Type: Physiological disorder (not infectious)

Symptoms

• Brown, necrotic margins on young inner leaves and heart leaves
• Damage appears first on the youngest, most rapidly expanding tissue
• External leaves typically remain healthy
• In severe cases, necrosis spreads inward and creates entry points for secondary bacterial infection

Cause

Tipburn is caused by localized calcium deficiency in rapidly expanding leaf tissue — not a lack of calcium in the nutrient solution, but insufficient transport to the leaf margins due to low transpiration. Common triggers include high temperatures, low air movement, high humidity, and rapid growth rates.

Management

• Increase air circulation within the canopy (horizontal airflow fans)
• Avoid excessive nitrogen that drives overly rapid growth
• Select tipburn-resistant varieties — resistance ratings are listed on variety data sheets
• Manage temperature differentials between day and night
• In NFT/DWC systems, ensure adequate dissolved oxygen and solution flow rate

Further reading: UC Cooperative Extension | MSU Extension — Lettuce Tipburn

2. Downy Mildew (Bremia lactucae)

Type: Fungal (oomycete)

Symptoms

• Pale yellow to light green angular lesions on upper leaf surface, bounded by leaf veins
• White to grey-white sporulation (downy growth) on the underside of lesions
• Lesions may turn brown and necrotic as they age
• Severe infections cause widespread leaf collapse and unmarketable heads

Conditions

• Favored by cool temperatures (10–15°C / 50–59°F) and high relative humidity (>85%)
• Spores spread rapidly by air movement within the greenhouse
• New races of Bremia emerge frequently — variety resistance can be overcome

Management

• Select varieties with current downy mildew resistance ratings (check EU and US race designations on data sheets)
• Reduce humidity through ventilation and heating
• Avoid overhead irrigation; keep foliage dry
• Remove and destroy infected plant material promptly
• Fungicide programs (where registered) should rotate modes of action

Further reading: USDA IPM Centers | American Phytopathological Society (APS) | EU Bremia Race Tracking

3. Botrytis Gray Mold (Botrytis cinerea)

Type: Fungal

Symptoms

• Water-soaked lesions on leaves, stems, or crowns that rapidly turn brown and collapse
• Characteristic grey, dusty sporulation on affected tissue under humid conditions
• Crown rot causes sudden plant collapse; entire heads may be lost
• Often enters through wounds, dead tissue, or senescent outer leaves

Conditions

• Favored by high humidity (>90%), moderate temperatures (15–20°C / 59–68°F), and poor air circulation
• Dead or dying plant tissue is the primary infection site
• Spores are ubiquitous and spread readily by air movement

Management

• Remove dead and senescent leaves promptly — do not leave debris on the growing surface
• Improve air circulation and reduce humidity, especially at night
• Avoid wetting foliage during irrigation
• Maintain adequate plant spacing to reduce canopy density
• Biological controls (Trichoderma spp., Bacillus subtilis) can be integrated into IPM programs

Further reading: American Phytopathological Society (APS) | UC IPM Online

4. Bacterial Leaf Spot (Xanthomonas campestris pv. vitians)

Type: Bacterial

Symptoms

• Small, water-soaked spots on leaves that turn brown to black with irregular margins
• Lesions may be surrounded by a yellow halo
• Spots often coalesce under wet conditions, causing large necrotic areas
• Severe infections render heads unmarketable

Conditions

• Favored by warm temperatures (24–30°C / 75–86°F) and free moisture on leaf surfaces
• Spreads through splashing water, contaminated tools, and infected transplants
• Can persist in plant debris and on greenhouse surfaces

Management

• Avoid overhead irrigation; use drip or sub-irrigation systems
• Sanitize tools, trays, and growing surfaces between crops
• Source transplants from reputable, disease-free suppliers
• Remove and destroy infected plants promptly
• Copper-based bactericides may provide suppression (verify registration)

Further reading: UC IPM Online | American Phytopathological Society (APS)

5. Lettuce Big Vein (Mirafiori Lettuce Big Vein Virus / MLBVV)

Type: Viral (transmitted by Olpidium virulentus, a soil-borne organism)

Symptoms

• Pronounced clearing and yellowing of veins, giving leaves a banded or mosaic appearance
• Leaves may be puckered, distorted, or ruffled along the veins
• Heads are stunted and may fail to form properly
• Symptoms are most severe in cool, wet conditions

Conditions

• Transmitted by the soil-borne organism Olpidium virulentus, which persists in growing media and water
• Favored by cool root zone temperatures (below 15°C / 59°F) and wet conditions
• Common in recirculating hydroponic systems where the vector can persist

Management

• Maintain root zone temperatures above 15°C / 59°F where possible
• Use UV sterilization or slow sand filtration on recirculating nutrient solutions
• Select big vein tolerant varieties where available
• Sanitize systems thoroughly between crops; avoid reusing growing media

Further reading: American Phytopathological Society (APS) | UC Cooperative Extension

6. Corky Root (Rhizorhapis suberifaciens)

Type: Bacterial

Symptoms

• Roots develop a corky, brown, cracked appearance
• Affected plants are stunted with yellowing outer leaves
• Root systems are reduced and discolored; plants may wilt under heat stress
• Above-ground symptoms can resemble nutrient deficiency

Conditions

• Soil-borne pathogen that persists in growing media, debris, and recirculating systems
• Favored by cool temperatures and high moisture
• More common in soil-based and peat-based production than in rockwool or NFT systems

Management

• Thorough sanitation between crops — remove all root debris from channels, slabs, and trays
• Avoid reusing growing media
• UV sterilization of recirculating nutrient solution reduces pathogen load
• Select tolerant varieties where available

Further reading: UC IPM Online | American Phytopathological Society (APS)

Additional Resources

• American Phytopathological Society (APS) — peer-reviewed disease fact sheets and identification resources
• UC IPM Online — University of California integrated pest management guidelines for lettuce
• USDA IPM Centers — national IPM data and pest management resources
• Purdue Extension — vegetable disease identification and management guides
• OMAFRA (Ontario) — greenhouse vegetable production and disease management resources

Quick Reference: Disease Identification Summary

Disease	Key Symptom	Type	Primary Trigger
Tipburn	Brown inner leaf margins	Physiological	Low transpiration, rapid growth
Downy Mildew	Yellow lesions + white underside sporulation	Fungal	Cool temps, high humidity
Botrytis	Grey mold on collapsed tissue	Fungal	High humidity, dead tissue
Bacterial Leaf Spot	Dark water-soaked spots, yellow halo	Bacterial	Warm temps, wet foliage
Big Vein	Vein clearing, leaf distortion	Viral	Cool root zone, wet conditions
Corky Root	Corky brown roots, stunted plants	Bacterial	Cool temps, poor sanitation

Shop Lettuce Varieties at Paramount Seeds

Variety selection is one of the most effective tools for managing disease pressure. Browse our full range of greenhouse lettuce varieties, many with rated resistance to downy mildew, tipburn, and other key pathogens.

• All Lettuce Varieties
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• Romaine Lettuce Varieties
• Multileaf Lettuce Varieties
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• Red Lettuce Varieties