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Rhizoctonia solani

Rhizoctonia solani, a form of rot which attacks many plants, can spread rapidly at temperatures below 20ºC (68ºF).

It is found in the upper layers of the soil, where it can remain for years on end. It attacks the plant at collar level, progressing to the tuber and the root system.

Peat suits this fungus: any peat-based compost or growing medium, if not disinfected, should be considered potentially contaminated.

Chemical controls are effective.

> Rhizoctonia solani : rot

Fungi of the genus Rhizoctonia belong to the order Agonomycetales in the class Adelomycetes. These fungi never have a stage of sexual reproduction, whatever conditions they develop under; nor do they produce conidia (spores) by asexual reproduction. They are fairly well adapted to life in the soil, even in the absence of living plants: in such conditions they live as saprophytes, off dead organic matter.

Rhizoctonia solani has a very general range of victims, including the cyclamen.

> Features of the fungus and its life cycle

Rhizoctonia solani is in fact the imperfect stage (asexual) of Thanatephorus cucumeris (syn. Corticium vagum var. solani, Corticium solani, Pelicularia filamentosa) which belongs to the Corticiaceae family, in the Homenomycetales order of the Basidiomycetes class.

Rhizoctonia solani is a damp-loving, warmth-loving fungus. Below 9ºC (48ºF) there can be no infection; but at 20ºC (68ºF) the incubation period is only 3 days, and infection spreads with great rapidity. This is a disease of summer (June-October), or of hot spells.


For preference, Rhizoctonia solani lives in the upper layers of the soil, the top 2cm. Here, even if conditions are unfavourable or no plants are available it can survive for a number of years in the form of resting bodies (sclerotia), lumps of intertwined hyphae, hard and dark in colour.

> Symptoms

The fungus attacks seedlings above all, and young plants that have just been pricked out (this fungus is one of those that cause ‘damping-off disease’). The site is at the level of the collar and the disease spreads from patch to patch.

On the roots and those leafstalks that are in contact with the soil, small red/brown patches of dead cells appear. The brown colour spreads, and the whole plant will fall over and rot.

The infection may then spread to the tuber: a shiny grey-chestnut mycelium can be seen at the sites of attack.

After transplanting the root system may also turn brown. Upper parts, by contrast, behave as if chlorophyll-deficient.

Older plants also may be attacked, and other fungal infections often supervene, such as Nectria radicicola and Botrytis cinerea.

These fungi produce various enzymes which enable them to break down the cellulose and lignin of the plant tissues, and this is how the rot develops.

> How Rhizoctonia solani spreads

Since there is no production of asexual spores, there is no danger of spreading in water, on breezes, or by way of insects.

Plants are in fact the main source of contamination.

The pathogen is present on a great number of host species, cyclamen inside the glasshouse but also other plants outside.

All soil not disinfected is considered a potential source of infection. The sclerotia can continue for years on end as saprophytes, feeding on plant debris. Once environmental conditions are suitable, root exsudates from a plant will be enough to stimulate them into activity.

With its great ability to get nourishment from dead matter, the mycelium grows rapidly in the soil until it reaches the plant which it then infects.

Suitable conditions are:

  • soil or compost pH above 5.8
  • soil temperature of the order of 15ºC (59ºF) to 35ºC (95ºF); dry soil

> Management measures

  • use disinfected growing medium, containers for sowing and pricking out, and new or disinfected pots
  • regulate irrigation and temperature carefully
  • the use of organic compost restricts attacks, since it contains antifungal bacteria
  • peat favours the growth of this fungus.

> Chemical control

The constant development of the regulations and homologations of phytosanitary treatment products, and the differences in regulations according to each country make it impossible for us to include updated information on homologations. Each producer will have to contact his local plant protection bureau to obtain the latest updates concerning the regulations and use of phytosanitary products. We strongly advise testing beforehand on a plant sample in order to measure the chemical’s activity (establishing the dose) and any effect on the plant (plant poisoning).


This advice sheet is based on the methods used at the SCEA at Montourey (Fréjus, France). These procedures may need some modification to adapt them to other climatic situations. Before starting to grow cyclamen there needs to be a review of precautions against pests and diseases.   We must point out that our advice and suggestions are offered for information purposes and therefore cannot include any guarantee of specific results; it is a good idea to carry out trials beforehand.


Fungi :

S.A.S Morel Diffusion

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83600 Fréjus - France

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