Gray mold (Botrytis) on lettuce — symptoms and treatment

Severity: high

What is Gray mold (Botrytis)

Gray mold is caused by Botrytis cinerea, a fungus that rots soft tissue and finishes fast. On lettuce it produces a soft rot that can take a plant from the first water-soaked lesion to total collapse, and it is especially problematic in greenhouse production, where humid, sheltered air is exactly what the fungus wants. Severity is rated high: a lettuce head is nearly all soft, close-packed leaf, so there is little the fungus cannot rot, and a collapsed plant is not salvageable.

Symptoms

It starts at the bottom. Water-soaked lesions appear on the leaves near the soil — the oldest, dampest tissue, in contact with the ground and slowest to dry. The lesions look greasy and darkened rather than spotted, and they spread. On the affected tissue the fungus then produces its signature: a gray fuzzy mold, a dense felt of spores that gives the disease its name and confirms it on sight. Behind that front the tissue turns to soft rot, and in wet conditions the whole plant collapses at the base, so the head slumps onto the soil.

Key signs:

• Early: water-soaked lesions on lower leaves near the soil
• Early: greasy, spreading margins rather than defined spots
• Advanced: gray fuzzy mold on affected tissue
• Advanced: soft rot and plant collapse in wet conditions

The gray fuzz settles it. Downy mildew produces white sporulation on leaf undersides with angular yellow spots above; gray mold is a rot with a gray felt on it, starting at soil level.

Causes and conditions

Botrytis cinerea is everywhere, surviving on dead and dying plant debris, and it releases spores that travel on air currents and in splashing water. It is an opportunist: it colonises damaged, senescing or wounded tissue most readily — a bruised leaf, a harvest cut, a leaf pressed against wet soil — and moves from there into healthy tissue. Free moisture on the leaf is what it needs to germinate, which is why humid conditions, poor air movement, dense stands and overhead watering all drive it. Crop debris left in the bed keeps a spore source in place.

Treatment

Lead with the growing conditions. Sprays cannot dry out a crowded, wet, badly ventilated crop.

Spacing and ventilation — cultural

Throughout season. Increase plant spacing. Improve greenhouse ventilation. Avoid wetting foliage. Remove debris promptly. Every one removes the leaf wetness the fungus depends on, and clearing debris takes away the tissue it lives on between crops. In a greenhouse, ventilation is the strongest lever you have.

Iprodione — chemical

Preventive in humid weather. Apply fungicide preventively in humid conditions. Rotate fungicide groups to prevent resistance. Preventive means before the gray fuzz shows: a fungicide protects healthy tissue but cannot rescue a leaf already turned to soft rot. Botrytis readily develops fungicide resistance, so rotating groups is not optional advice. Pre-harvest interval: 14 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Space plants generously so air moves through the stand and leaves dry.
• Ventilate greenhouses aggressively, especially on humid, still days.
• Water at the base, never over the foliage, early enough that leaves dry.
• Handle plants gently — bruises and wounds are the fungus's way in.
• Clear crop debris and rotting leaves promptly rather than leaving them in the bed.

Frequently asked questions

Is it contagious to my other plants? Yes, and widely — Botrytis cinerea attacks a broad range of soft-tissued plants, and its spores move on air currents and splashing water. A rotting plant left in the bed is a spore factory for everything around it.

Can I eat lettuce after spraying? Not immediately. After Iprodione, respect the 14-day pre-harvest interval. Rotted tissue itself should be discarded rather than trimmed and eaten.

When should I treat? Preventively, when humid conditions set in — not once the gray mold is visible. By the time you see the fuzz the tissue under it is already lost, and a spray protects only what is still healthy.