Pest & disease factsheet - Black root rot

This disease caused by soil-dwelling fungus can impact a wide range of ornamental plant species.

Black root rot - affected and unaffected - image: Dove Associates
Black root rot - affected and unaffected - image: Dove Associates

Cyclamen, poinsettia, primula, impatiens, begonia, nicotiana, geranium and sweet peas are among 120 species in 15 families known to be susceptible to black root rot (Thielaviopsis basicola). But the disease is at its most troublesome on bedding and pot plant nurseries, with pansy and viola crops among the most commonly affected.

Accurate identification is important because although black root rot symptoms appear similar to those of damping off (caused by Pythium species), the fungi belong to two different groups.

Fungicides used to control oomycete fungi such as Pythium and Phytophthora have little or no effect on Thielaviopsis.

Because of the limited range of fungicides available, routine hygiene measures are particularly important to reduce the risk of black root rot becoming established on a nursery. Disinfectants can play a crucial role but remember that the activity of most disinfectants is dramatically reduced in the presence of organic material, so surfaces, equipment and plastic trays or containers should be thoroughly washed before being treated with disinfectants.

In the past, AHDB-funded research generated much of the currently accepted epidemiological information about Thielaviopsis basicola, including the discovery of the role of sciarid flies and similar pests in its transmission.

How to recognise it

Symptoms during the early stages of infection are very similar to those of damping-off caused by Pythium species, or may be confused with nutrient deficiency or other physical damage to the roots.

Thielaviopsis infection can be distinguished by pulling up a plant showing symptoms and washing the growing media from the roots. Black or grey rot in distinct patches or segments on the roots indicates Thielaviopsis — more general brown or grey rotting indicates Pythium. The characteristic symptoms are easier to distinguish using a good-quality hand lens or a microscope if available.


Black root rot is caused by the soil-dwelling fungus Thielaviopsis basicola. It is widespread and common, and is pathogenic on a wide range of ornamental plant species.

The fungus infects the cortical and epidermal tissues of the root system. Once established, it produces two types of spores — tough, black, thick-walled chlamydospores, which give the infected roots their black appearance; and short-lived conidiospores.

The conidiospores are responsible for rapid local spread of the fungus from plant to plant. Chlamydospores are long-term survival spores that can persist for months or even years on surfaces, in pots and containers, infected plant material or growing media dust and debris.
Spores can be spread by water splash and by insects such as shore and sciarid flies. The fungus can survive and grow in a wide range of conditions but infections are most severe when the root zone is waterlogged or poorly aerated, with root zone temperatures between 15°C and 20°C, and a pH of between 5.7 and 5.9.


Seedlings or young plants develop yellowed leaves and show patches of uneven growth in a tray. Older leaves can develop purpled edges due to root damage.

As the disease develops, plants become stunted or may rot off completely at the base of the stem. Roots rot, developing black patches or segments — stems below ground may swell and develop black, crack-like lesions.

On poinsettia, the fungus may infect the lower stem, causing cracks blackened by the presence of chlamydospores.

Treatment: biological control

Prestop (EAMU 2012-0564), Serenade ASO (EAMUs required) or Trianum P will help enhance a plant’s natural protection against a range of fungal pathogens.

Treatment: cultural control

• Inspect all incoming consignments of young plants and regularly monitor crops.
• Avoid spreading spores when disposing of infected plants.
• Control sciarid flies and other insects that may spread spores and control root zone pests.
• Use disinfectants between crops and in the end-of-year clean-up
to control spores surviving on surfaces or in growing media dust on tray filling machines and potting equpiment.
• Sterilise pots and trays or use new ones for each crop.
• Do not allow plant debris or growing media dust to build up over time.
• Use well-aerated growing media and avoid high water content.
• Spores can be dispersed by water splash, so you should consider sub-irrigation.
• Cover stored growing media to reduce entry of wind-blown or insect-carried spores.
• Avoid root damage when handling and transplanting.
• Maintain the root zone pH below 5.5 if possible — this can reduce disease severity.

Treatment: chemical control

Active ingredient Bacillus subtilis
strain QST713
Formulation Serenade ASO* (Fargro)
Action(s) Protectant biopesticide compatible with some biological controls.

Active ingredients Boscalid + pyraclostrobin
FRAC code 7 + 11
Formulation Signum* (BASF)
Action(s) Systemic, protectant and curative fungicide, compatible with some biological controls.

Active ingredient Gliocladium catenulatum strain J1446
Formulation Prestop* (Fargro)
Action(s) Biopesticide able to deal with a range of basal and root rot pathogens.

Active ingredient Iprodione
FRAC code 2
Formulation Rovral WG* (BASF)
Action(s) A protectant fungicide with some eradicant activity, compatible with biological control.

Active ingredient Prochloraz
FRAC code 3
Formulation Octave (Everris)
Action(s) Broad-spectrum protectant and eradicant fungicide, compatible with some biological controls.

Active ingredient Thiophanate-methyl
FRAC code 1
Formulation Cercobin WG* (Certis)
Action(s) Systemic, protectant and curative fungicide with application restrictions on protected ornamentals.

Fully updated by Dove Associates

Use plant protection products safely. Always read the label and product information before use.

* Extension of Authorisation for Minor Use (EAMU) required for use in ornamental plant production outdoors and/or under protection.

Dove Associates shall in no event be liable for the loss or damage to any crops or biological control agents caused by the use of products mentioned.

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