Botrytis (Gray Mold): The Ubiquitous Greenhouse Pathogen

Botrytis is found virtually everywhere plants are grown. Fast-growing, nutritionally versatile, greenhouse-adapted, and broadly pathogenic—it causes Botrytis blight or gray mold on countless crops.

The Fungus

Appears first as white growth, quickly darkening to gray. Smoky-gray "dusty" spores spread by air and water. Any greenhouse activity releases spores—even turning on trickle irrigation triggers spore discharge. Spores persist on plant surfaces and seed coats. Two resting structures: dark multicelled sclerotia and thick-walled single-celled chlamydospores.

Infection Requirements

Botrytis needs a nutrient base before invading healthy tissue. Leaking nutrients from wounds, fading flowers, or dying tissue provide this foothold. High humidity favors growth. A dark to light brown rot develops in diseased tissue.

Common Initial Infection Sites

• Wounded tissue (large stubs from cuttings)
• Fading flowers
• Leaves where infected petals have fallen
• Broken stems, injured leaves
• Fertilizer burn, spray damage, mechanical injury
• Cool, moist seedling conditions
• Cuttings from heavily infested stock plants

Management: Sanitation + Environment + Protection

1. Sanitation first: Remove dead/dying tissue from plants and soil surface. Remove refuse from greenhouse—never throw under benches or on walks. One fingernail-sized piece of tissue can produce 60,000+ spores; one spore can infect. Sanitation alone is insufficient.
2. Avoid injury: No large stubs on stock plants when taking cuttings.
3. Heat and vent to reduce humidity: Especially early morning when condensation occurs. Even slight humidity reduction significantly inhibits Botrytis. Outdoor: plan for good air circulation—the single most important inhibitor.
4. Fungicides/biologicals: Rotate modes of action (different FRAC groups). Benzimidazoles (FRAC 1): resistance common, not recommended. Dicarboximides (FRAC 2): resistance in ~50% of operations. Mix chemicals with different modes if label allows. Contact Extension for current recommendations.

Persistent, adaptable, and ever-present—Botrytis demands a systems approach, not a single tactic.