Agriculture: Floriculture and Ornamental Nurseries Pest Management Guidelines

Black Root Rot

  • Thielaviopsis basicola
  • Symptoms and Signs

    Black root rot is also called Thielaviopsis root rot. Plants are stunted and grow poorly. Infected roots may initially have small dark brown to black bands where infection has taken place. As the disease progresses, roots can become badly rotted. Stems below ground may enlarge and develop black, rough, longitudinal cracks. Characteristic dark brown to black, thick-walled, barrel-shaped chlamydospores form in infected tissues and may be visible under magnification.

    Comments on the Disease

    The fungus has a wide host range: 120 species in 15 families are known to be susceptible. Strains of the fungus are known that differ in pathogenicity and virulence. Important ornamental hosts include begonia, cyclamen, geranium, gerbera, kalanchoe, pansy, petunia, poinsettia, primula, snapdragon, sweet pea, verbena, and viola. The disease is favored by wet, cool soil and any condition that weakens plants; it is most severe from 55° to 61°F, while only a trace of disease develops at 86°F. Alkaline soil favors the disease, which can be prevented at pH 4.8 and greatly reduced at pH 5.5 or below. However, many plants do not grow well under such acidic conditions.

    The fungus is soilborne and capable of prolonged survival in the absence of susceptible plants. Two kinds of spores are formed:

    • barrel-shaped chlamydospores (resting spores) in short chains of three to seven and
    • rectangular-shaped endoconidia.

    The fungus can be spread in water, soil, by infected plants or vectored by fungus gnats and shore flies. Some sources of peat are known to harbor Thielaviopsis spores.

    Management

    Use appropriate sanitation measures to prevent spread of the pathogen via diseased plant material, contaminated soil mixes and containers, and contaminated water runoff. The use of pathogen-free plants, along with improved sanitation and cultural practices, has reduced the importance of this disease, which at one time was widespread, especially in poinsettias. The fungus can still be troublesome in field-grown flowers. The benzimidazole fungicides such as thiophanate-methyl are very active against the fungus and are used as soil treatments to control it.

    To treat container media, steam (at 140°F for 30 minutes), or solarize (double-tent at 160°F for 30 minutes or 140°F for 1 hour). For flower production in outdoor fields, solarization in warmer climates has been successful for control of Thielaviopsis in many crops. Solarization and steaming are acceptable for organic production. For more information, see MANAGEMENT OF SOILBORNE PATHOGENS.

    Common name Amount to use REI‡
    (Example trade name) (hours)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.
     
    A. THIOPHANATE-METHYL
      (Talaris 4.5 F) 20 fl oz/100 gal water 12
      MODE OF ACTION GROUP NAME (NUMBER1): Methyl benzimidazole (1)
      COMMENTS: Apply as a drench or heavy spray. Generally applied after sowing. Absorbed by plant parts exposed to the chemical. Roots may absorb the fungicide (or its breakdown product carbendazim), which moves in the xylem to transpiring leaves.
     
    B. TRIFLUMIZOLE
      (TerraGuard SC) 2–8 fl oz/100 gal water 12
      MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
      COMMENTS: Apply as a cutting soak or soil drench at 3 to 4 week intervals as needed on potted plants. A protectant fungicide; use is restricted to enclosed commercial structures such as greenhouses and shade houses.
     
    C. FLUDIOXONIL
      (Medallion WDG) Label rates 12
      MODE OF ACTION GROUP NAME (NUMBER1): Phenylpyrroles (12)
    1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action group number.
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing.
    Text Updated: 11/20
    Treatment Table Updated: 11/20