Agrobacterium, Rhizobium spp.

Proliferation (Root mat) and root tumors (Crown gall)

General  

• Bacteria widely distributed in soils, associated with symptoms of root proliferations and tumors in tomato and eggplant rootstocks.
• Root proliferations described by the Anglo-Saxons as "Root mat" and associated with different names in the literature: Agrobacterium biovar 1, strains of Agrobacterium radiobacter, Rhizobium radiobacter (Ri).
  • Induced by the transfer and expression of the "Ri" ( plasmid root-inducing ) in the genome of tomato rootstock cells
  • Other bacterial species associated with Root mat and belonging to other bacterial genera such as Ochrobactum , Rhizobium , Sinorhizobium which would harbor this plasmid
  • Reported in several European countries and other continents.
  • Seems to affect only tomato rootstocks of grafted eggplants.
• Root tumors attributed to Rhizobium radiobacter * (Agrobacterium tumefasciens ) present in the rhizosphere of many plants.
  • Very polyphagous on dicotyledons, it attacks plants belonging to more than 60 botanical families, whether they are woody or herbaceous.
  • Rather little reported on tomato, even less on eggplant, tumors are especially observed above all on rootstocks, especially in soilless crops.
  • Occasionally associated with other soil-borne pests and in particular Meloidogyne spp. in crops in soil under shelter.

* The biology of this bacterium and its infectious process are quite original. The mechanism used to parasitize a plant involves the integration of part of its genome, a fragment plasmid Ti of the tumor-inducing (T-DNA), into that of the plant. This is why it is used in transgenosis as a vector making it possible to integrate desired genes into the genome of plants.
Several biovars of Rhizobium radiobacter were described. The biovars 1 of A. tumefaciens , of ' A. rhizogenes and ' A. radiobacter were grouped into a single taxon: A. tumefaciens . The crown gall of grape is caused by him biovar 3 d ' A. vitis ; it was spread all over the world by cuttings.

• Organs attacked  : mainly roots
•  Symptoms : 
  • Rather linear root proliferation in and on the surface of rock wool cubes and cakes (photo), formation of root mats ("root mat")
    • Increased vegetative growth of eggplants at the expense of fruiting.
    • Fruit size can be reduced.
    • The high density of the roots makes them much more susceptible to attack by fungi present in soilless crops, in particular chromists ( Pythium spp., Phytophthora spp.). In fact, drainage in cubes and breads can be greatly reduced and promote root asphyxia.

  • Small sizes on some roots, especially close to the surface of the soil or the substrate (figure).

    • Young tumors are smooth, whitish and rather spherical.
    • As they age, they suberise on the surface and gradually take on a more or less marked brown tint.
    • They have an irregular shape and a variable size, some reaching several centimeters in diameter (figure 3). They can become spongy and break down later.
    • The presence of these tumors on the roots does not appear to affect plant growth and fruit production.
    • Combined attacks of this bacteria and root-knot nematodes sometimes occur on the same root systems
• >>> More pictures
• Signs  :
• Possible confusion :

Biology

• Storage : These bacteria are able to live in the saprophytic state in the soil for at least several years, as well as on and in plant debris. They must be able to maintain themselves in the substrates if they are not replaced, and on various cultivated plants, especially vegetables, in addition to tomatoes and rootstocks: beets, turnips, etc. These plants, when they are cultivated near plots of grafted eggplant or produced in the same greenhouse, can constitute sources of inoculum, water and contaminated soils as well. Nurseries can be contaminated by the root proliferation agent, and therefore plants.
• Infection : Penetration into the plant by wounds forming on the roots or the stem near the surface of the soil or the substrate, as a result of cultivation operations, damage linked to pests, etc. Mobile bacterial cells are attracted to root chemotaxis as a result of the release of sugars and other compounds into the rhizosphere.
• Development - Dissemination : Once in place, they multiply abundantly and actively disrupt the physiology of the plant: rapid and anarchic division of the surrounding plant cells, overproduction of growth regulators which cause cell proliferation, hormonal imbalances, reduction in blood pressure. root absorption of water and nutrients, etc. Transmission by water and contaminated soil particles to other plants or crops. Workers in the course of their activities in the fields, with tillage machinery on wheels loaded with clods of earth, can ensure the dispersion of these bacteria. They are found in the substrates and nutrient solution of soilless crops. It is not impossible that it could be disseminated by the latter.
• Favorable conditions :
  • Favored by the conditions encountered in soilless crops and under shelters. High temperatures seem to promote tumor expression and possibly root-knot nematode attacks?

Protection  

• Very difficult to control root proliferation in soilless crops. Hygiene measures should be put in place to limit the conservation and proliferation of bacteria:
  • Thoroughly clean and disinfect nurseries and shelters with 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 conserved there and recolonizes it quickly;
  • Do not put the loaves in contact with the ground and suspend the drippers;
  • When the plastic covering the floor is replaced, be very careful that the new one is not soiled by soil dust on its upper face.
• When tumors are found on plants, it is unfortunately too late to intervene. No product, no measure, can effectively control this bacteriosis. Implement some hygiene measures to try to eliminate it from the farm:
  • In soilless culture, if a few plants are affected in a limited number of bags, it is desirable to remove them from the culture and destroy them.
  • Do not leave plant debris and particles of substrate in place.
  • Take out and destroy all the plants, and in particular their root systems, at the end of the crop.
  • Eliminate the substrates, carry out a complete cleaning of the greenhouse and the equipment used in it.
  • In soil, rotate for several years with monocots, cereals such as wheat or corn. It will be necessary to check that other root pests, and in particular root-knot nematodes, are not also associated with the root deformations observed in the crop. If this is the case, consideration should also be given to implementing specific protection methods for Meloidogyne spp.