Clavibacter michiganensis subsp. michiganensis , a bacterium transmissible by seeds and during cultivation operations, is particularly devastating. It is reported in many production areas around the world, both in field tomato crops and in greenhouses, in soil as well as in soil. In France, it is particularly feared and its damage is often very damaging.
 

In glasshouses, bacterial canker often manifests itself, at least at the start of its development, by the appearance, on the leaflets, of livid interveinal spots which quickly become necrotic (figure 1). Often, they are followed by leaf wilting , the leaflets tending in this case to curl downwards. Eventually, seedlings, like adult plants, may wilt and dry out entirely. In some situations, leaf wilting is sectoral and accompanied by local yellowing of the blade (photo 2). Regarding the leaves, only certain leaflets located on the same side can be affected. These symptoms are, in this case, more typical of vascular disease. Climatic conditions greatly influence the expression of the symptoms of this bacteriosis. Also, it can go unnoticed for a long period of time, express itself suddenly and evolve rapidly, or vegetate and allow plants to produce. These symptoms are expressed on leaflets located indifferently at the bottom, top or halfway up the vegetation of the plants.
 

In the open field, the leaf symptoms are a little different: wilting and drying out develop at the edge of the blade. Brown lesions, in the shape of an inverted "V" and often haloed with a more or less wide yellow halo, can be observed on certain leaflets . They evolve and reach the entire limbus, which ends up drying out entirely. Many leaves are affected and the plants may eventually die.
 

Whether the tomato crop is grown in a greenhouse or in the field, the distribution of diseased plants is often the same: in more or less long lines (Figure 3). This very characteristic distribution is linked to the mode of transmission of C. michiganensis subsp. michiganensis which occurs from plant to plant during cultivation operations, and in particular during the pruning of axillary buds.
  

The foliar wilting described above is not always sufficient to diagnose this bacteriosis, it is also necessary that other more specific symptoms be present on diseased plants:

- portions of altered vessels, observable after having made a longitudinal or transverse section of the stem. Thus, creamy, whitish to yellow lines, gradually turning brown, are visible along the vessels and on nearby cortical tissues and marrow (Figures 4 and 5) . Depending on the situation, internal stem symptoms can be very difficult to detect and require cuts along the entire length of the stem or, on the contrary, be particularly marked with conspicuous brown vascular streaks. An observation under a photonic microscope of histological sections made in the vascular zone of the stem makes it possible to observe the presence of a bacterial mucus (FIG. 6);

- small lesions a few millimeters in diameter, initially whitish and quickly becoming canker, appear on the aerial organs of the tomato. These small superficial cankers, which slowly necrotize, are rather rare on leaflets , more frequent on the stem (figure 7), petioles and fruits (figure 8). On the latter, they often have a particular appearance: small circular white spots and flat on the periphery and whose center is more in relief, sometimes burst, beige to ocher in color. These characteristics give them the appearance of a bird's eye spot (figure 8). In addition, the emission of adventitious roots can take place on the stem.
 

Only the combined observations of leaf symptoms and more specific lesions will allow you to identify this bacteriosis with certainty.

For additional information on this bacterium, you can consult the Clavibacter michiganensis subsp. michiganensis .