See:
Greenhouse Plants, Ornamental - Black Root Rot
Cause Berkeleyomyces sp. (formerly Thielaviopsis basicola), a soilborne fungus that can survive long periods without a host plant. Cool soils (55°F to 70°F), high pH (at or above 5.5), and high soil moisture content (more than 70%) favor the disease. Plants are susceptible at all growth stages. Infection can occur anywhere on the root system.
Symptoms Black root rot causes lower leaves to yellow, wilt, and roll. Longitudinal black cracks may appear in stems. Infected plants are stunted and may defoliate and die. Root discoloration is brown initially, then a dark, flat, black as abundant chlamydospores form on and in the root. Infected roots are generally left intact by comparison with Pythium root rot.
Cultural control Focus on cultural control tactics as fungicides do not compensate for poor sanitation or environmental controls. Note that a pH low enough to suppress black root rot (5.2) is lower than the recommended pH range of 5.8 to 6.2.
- Plant pathogen-free cuttings.
- Use soilless, well-drained potting mix or steam-treated soil and rooting media. Steam 30 min. at 180°F.
- Use new or sterile pots. Avoid reusing pots but if reusing, thoroughly remove organic matter and old media, then wash with a disinfectant such as a 10% bleach solution.
- Grow plants at night temperatures of 70°F to 76°F.
- Carefully remove diseased plants during the growing cycle so that adjacent plants are not contaminated, and remove diseased plants from the plant production or adjacent areas.
- Control fungus gnats and shore flies, which spread viable spores on their bodies and in their feces. Consult the PNW Insect Management Handbook for current control tactics.
- After the production cycle, remove any remaining plants and disinfect the section.
Chemical control Integrate fungicides with cultural controls. This pathogen can be difficult to control, and often the higher rate is more effective.
- Affirm WDG at 0.5 lb/100 gal water. Group 19 fungicide. 4-hr reentry.
- Banrot 40 WP at 6 to 12 oz/100 gal water. Avoid using the granular formulation. Group 1 + 14 fungicide. 12-hr reentry.
- Medallion WDG at 1 to 2 oz/100 gal water. Using with oils or adjuvants may damage plant. Group 12 fungicide. 12-hr reentry.
- Terraguard SC at 2 to 8 fl oz/100 gal water as a soil drench. Group 3 fungicide. 12-hr reentry.
- Thiophanate-methyl is registered as a drench. Group 1 fungicides. 12-hr reentry.
- Cleary's 3336 EG at 8 to 16 oz/100 gal water.
- OHP 6672 4.5 F at 7.5 to 20 fl oz/100 gal/water.
- Systec 1998 at 20 fl oz/100 gal water. Washington Only.
Reference Keller, J.R. and Shanks, J.B. 1955. Poinsettia root rot. Phytopathology 45:552-558.