Causal agent(s) and transmission
Violet root rot is caused by a fungus, Rhizoctonia crocorum, and is responsible for rare but spectacular damage to potato tubers.
This fungus is a saprophyte and lives in the soil in the form of sclerotia that can survive for several years. Its development is favoured by humidity and relatively high temperatures. It is capable of developing on a large number of crops (alfalfa, beetroot, carrot, chicory etc.).
The occurrence of violet root rot is rare and sporadic. As its incidence on yield is usually limited, no specific control measures are generally required.
Symptoms on foliage
There is no specific foliage symptom but in the most serious cases, there may be yellowing and then wilting of the plants.
Symptoms on tubers
At harvest, infected tubers are collected from localised areas (the foci of soil infection), randomly dispersed in the field.
The infected roots and tubers become covered with red-violet mycelium, which is highly characteristic (photos 1 to 4) sometimes accompanied by small black sclerotia (photo 5).
The mycelium then penetrates the tuber. The skin cracks, the flesh changes into a greyish dry rot and finally the tuber disintegrates completely (photos 6 and 7).
A light and moist soil and relatively high temperatures (20°C) are the conditionsthat favour the development of the fungus. Repeated use of crops that are prone to this disease increases its development.
The fungus may remain for several years in a latent state in the soil in the form of sclerotia or mycelium on infected tubers and plant debris.
- Avoid susceptible crops such as beetroot or potato in an infected field for several years.
- Use healthy certified seed tubers.
- Eliminate groundkeepers and infected tubers.
- Apply adequate drainage to avoid excess of humidity in the soil.
- Dry the tubers thoroughly before storage.