Downy mildew on lettuce — symptoms and treatment

Severity: high

What is Downy mildew

Lettuce downy mildew is caused by Bremia lactucae, and it is the most important disease of lettuce worldwide — no other pathogen costs the crop more. It attacks the leaves, which on lettuce is the entire harvest, so damage is never merely cosmetic. Severity is rated high: beyond the yellowing and browning it causes directly, the damaged tissue opens the door to the soft rots that finish a head off. Bremia lactucae exists as many races, which is why resistant varieties must be matched to what is actually circulating.

Symptoms

Look at the older leaves first — the disease starts on them and works inward. Pale angular yellow spots appear on the upper surface, and the word angular matters: the patches are bounded by the leaf veins, giving them straight edges and a blocky outline rather than the round spots most fungal diseases make. Turn the leaf over and under each yellow patch is a white sporulation, a downy bloom on the underside. That pairing — angular yellow above, white fuzz directly beneath — is the diagnosis. As it advances the yellow turns to brown necrotic patches, and heavily infected plants collapse.

Key signs:

• Early: angular yellow spots on the older, outer leaves
• Early: white sporulation on leaf undersides, matching the spots above
• Advanced: brown necrotic patches replacing the yellow
• Advanced: plant collapse

Do not confuse it with gray mold, a soft rot starting at soil level with a gray felt on it. Downy mildew is white, on the underside, and shaped by the veins.

Causes and conditions

Bremia lactucae needs free water on the leaf to infect, and its spores travel on air currents and splashing water. Cool, humid weather with long dew periods is what it wants — the mild, damp conditions lettuce itself is grown in. Anything that keeps leaves wet or the air still works in its favour: dense stands, overhead watering, poor drainage, and greenhouse air with nowhere to go. Infected debris and volunteer lettuce keep the pathogen present between crops. Because the pathogen is made up of distinct races, a variety that resisted it last season may not resist the population you have now.

Treatment

Resistance and airflow first. Fungicides are the backup, not the plan.

Resistant varieties and spacing — cultural

At planting. Plant resistant varieties (check for current race compatibility). Ensure good air circulation. Avoid overhead watering. Resistant varieties are the primary control — but check race compatibility, because resistance is race-specific and a variety listed as resistant may not hold in your area. Spacing and base watering remove the leaf wetness spores need to germinate.

Metalaxyl (Ridomil) — chemical

Preventive. Apply systemic fungicide preventively. Use resistant varieties as primary control. Preventive is the operative word: applied ahead of infection it protects, but sprayed onto leaves already carrying white sporulation it cannot undo the damage. Pre-harvest interval: 14 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Choose resistant varieties, and confirm they match the races circulating locally.
• Space plants so air moves through the crop and leaves dry after dew.
• Water at the base, never overhead, and early enough that foliage dries.
• Ventilate greenhouses and tunnels hard on still, humid days.
• Remove crop debris and volunteer lettuce, which carry the pathogen between crops.

Frequently asked questions

Is it contagious to my other lettuce? Yes, and quickly. The spores move on air currents and splashing water, so an infected plant seeds the rest of the bed. Volunteers keep it going too.

Can I eat lettuce with downy mildew? The disease is not a human health problem, and lightly marked outer leaves can be stripped off. Yellowed and browned tissue rots readily and is not worth eating. After Metalaxyl (Ridomil), respect the 14-day pre-harvest interval.

When should I spray? Preventively, when cool humid weather sets in — not after the spots appear. If your variety resists the local races and the crop is well spaced, you may not need to spray at all.