Identify mold vs. root hairs and know common mold species

Before treating mold, confirm it is actually mold. The most common false alarm in microgreen growing is confusing healthy root hairs with mold. Knowing which mold species you are dealing with also helps determine the right response.

Root Hairs vs. Mold -- The Water Test

Spray or gently mist the white fuzzy area with water: - Root hairs: Collapse and become invisible when wet. Reappear when dry. - Mold: Remains visible when wet. Does not retract or disappear.

Root Hair Characteristics

Fine white filaments radiating evenly outward from the stem base and roots
Uniform appearance across most seedlings in the tray
Grow in the same direction (outward from the root)
Not slimy to the touch
Most visible on radish, broccoli, and other brassica microgreens
Completely normal and healthy -- they are the plant seeking water and nutrients

Mold Characteristics

Cobwebby, spiderweb-like, or cotton candy texture
Appears in random patches, often on the soil surface or on dead/ungerminated seeds
Slimy to the touch
May be white, gray, green, blue, or black
Often has a musty or earthy smell
Does not disappear when misted

Common Mold Species on Microgreens

White mold (early stage of most species) - Most mold starts white before developing color - Fuzzy white patches on soil or between seedlings - Fast-spreading, especially in warm humid conditions

Rhizopus and Mucor (Black Pin Mold) - Starts as fuzzy white/gray mold, then develops tiny black pin-like dots (sporangia) - Extremely fast -- can colonize an entire tray within 24-48 hours - Common on bread and fruit; thrives in microgreen conditions - Requires immediate action: remove affected area, increase airflow, reduce moisture

Aspergillus species - Common indoor mold, can appear green, black, or yellow - Aspergillus niger produces black spores - Aspergillus oryzae is common on grain-based substrates

Penicillium - Blue-green colored mold - Slower growing than Rhizopus/Mucor - Common in cool, damp conditions

Botrytis (Gray Mold) - Fuzzy gray appearance - Common in greenhouses with poor air circulation - Particularly problematic in cool, humid conditions

Pythium, Rhizoctonia, Fusarium (Damping-Off Pathogens) - These are technically oomycetes and fungi that cause seedling collapse at the soil line - Seedlings topple over with pinched, water-soaked stems at the base - Different from surface mold -- these attack from within the growing medium - University research confirms these are the primary disease agents in commercial microgreen production - Pythium species have developed resistance to synthetic fungicides like mefenoxam

Tips

Do the water test before discarding any tray -- root hairs are extremely common and look alarming to new growers
If mold appears only on dead or ungerminated seeds, remove those seeds and the surrounding greens are usually fine
Black or green mold is more concerning than white surface mold
If you see seedlings collapsing at the base (damping-off), the problem is in the medium, not on the surface -- address with biological fungicides and medium sterilization
Take photos of recurring mold to track patterns and identify species