Black Root Rot (Thielaviopsis): Diagnosis & Management in Ornamentals

Thielaviopsis basicola is a soil-inhabiting fungus causing root rot and branch dieback on a wide range of woody and herbaceous plants: holly, begonia, geranium, poinsettia, cyclamen, gerbera, pansy, and more.

Conditions Favoring Disease

Soil temperatures of 55–65°F, especially with wet soil. Found in field soil and occasionally in commercial peat moss.

Symptoms

• Stunting of foliage and root systems
• Blackened areas on roots
• Yellowing of leaves between veins or along margins
• Branch dieback

Signs (Microscopic)

• Dark brown, multicelled spores (chlamydospores) in infected roots; cells snap apart
• Light-colored spores (endoconidia) formed in tapering cells, extruded in chains

Management

• Steam treatment: Heat field soil and peat moss to 180°F, hold 30 minutes
• Fungicides: Several effective products available. Consult Cooperative Extension or state Department of Agriculture for current recommendations

Holly Cultivar Susceptibility

Highly susceptible: Ilex crenata 'Helleri', 'Hoogendorn', 'Nigra', 'Green Cushion', 'Mobjack Supreme', 'Hetzzi'
Moderately resistant: Ilex vomitoria, Ilex opaca
Highly resistant: English holly (Ilex aquifolium), Chinese holly (Ilex cornuta)

Select resistant cultivars where black root rot history exists. Combine with sanitation and preventive fungicide programs for best results.