Agrobacterium radiobacter biovar 1 ¹ (« root mat »)

The mat root is a relatively old disease since it was first described in the 1970s in the UK, and associated with a strain of Agrobacterium biovar 1 cucumber crops in soil under shelter. It was only observed in this country on tomatoes from 1997, mainly in soilless cultivation where the number of affected plants varied between 1 and 50%. Strains of A. radiobacter carrying a " plasmid Ri " ( root-inducing ) were responsible. In fact, the root symptoms are induced by the transfer and expression of this plasmid in the genome of tomato cells. More recent work has made it possible to demonstrate that the “Ri” plasmid is also present in other bacterial species associated with the root mat and belonging to other genera: Ochrobactum, Rhizobium, Sinorhizobium . These species could play a significant epidemiological role in the disease. Note that root mat is now reported in several other European countries such as the Netherlands, France ...

Affected plants show rather linear root proliferation in and on the surface of rock wool cubes and cakes (Figures 1 and 2), which leads to increased vegetative growth of tomatoes at the expense of fruiting. Note that the size of the fruits can be reduced. In addition, the density of the roots makes them much more susceptible to attack by fungi present in soilless crops, in particular chromists ( Pythium spp., Phytophthora spp.).

These root proliferations are due to the effects of strains of A. radiobacter biovar 1 harboring a fragment of circular DNA, a cucumopine plasmid “Ri”, a root inducer. In fact, this bacterium, as in the case of A. tumefaciens , acts as a vector and transmits this plasmid which will be inserted into the genome of plant cells. Subsequently, the production of roots is totally disrupted, the plants producing in an anarchic manner. The incubation period of this disease seems to be quite long, approximately 4 to 8 weeks. The isolation and identification of this bacterium is quite difficult and requires the intervention of a specialized laboratory (the plasmid can be detected by PCR).

It is quite difficult to control this disease in soilless tomato crops. In fact, there is no product allowing effective and approved treatments to be carried out during cultivation. Only several hygiene measures can be implemented in the nursery and during cultivation to limit the conservation and proliferation of this bacterium:
- carefully clean and disinfect nurseries and shelters with a disinfectant. Between two crops, the material used in greenhouses should not be stored on the ground or in a dirty environment because it could become contaminated on this occasion;
- disinfection of the soil will often be illusory because this bacterium is easily preserved there and recolonizes it quickly;
- the breads must not be in contact with the ground and the drippers must be suspended;
- when the plastic covering the floor is replaced, special care should be taken so that the new one is not soiled by soil dust on its upper face.

Attempts to steam disinfect breads have been found to be effective in killing the bacteria, but not the infectious plasmid. Note that this solution is not recommended in countries affected by root mat .

Note that a somewhat similar condition, called " syndrome thick roots " ( thick root syndrom ) and whose origin is unknown, is reported in particular in the Netherlands, mainly cucumber nurseries, pepper, tomato. It results in excessive growth of the roots which also tend to curl irregularly and take on a glassy appearance. Their diameter is sometimes considerable (at least ten times larger than that of a normal root), and they eventually decompose. Note that they are much more sensitive to attacks by Pythium spp.