Main symptoms

The many symptoms caused by Clavibacter michiganensis subsp. michiganensis are expressed on all the aerial organs of the tomato. They are influenced by many factors: the plant (the variety, its stage of development ), the bacteria (the aggressiveness of the strain present), the type of crop (under shelter, open field), climatic conditions surrounding areas and cultural practices. This bacteriosis can go unnoticed at certain times of the year and be explosive at others, which makes its identification particularly difficult. Despite this, the early detection of its symptoms could make it possible to limit its area of ​​extension and sometimes to succeed in its eradication in a crop.

Transmitted by seeds , the bacteria can already appear in the nursery. Seedlings may wither quickly, remain dwarf, or appear healthy. In the latter case, the most formidable, latent contaminations may be expressed later, after planting and during cultivation.

The very first symptoms are sometimes difficult to notice when they reflect a vascular infection. By observing the plants with a lot of attention, one notes, at the edge of the blade or between the veins of certain leaflets , the presence of patches of pale green tissue , even livid, and sometimes humid . Subsequently, these moribund tissues quickly necrotize and dry out (Figures 1 to 3). These areas of necrotic tissue expand and become coalescent, destroying large portions of the leaflets and even drying out the latter and entire leaves. The curling of low leaves and their reversible wilting during the night is also indicative of the presence of this vascular bacteria. Pulling up a plant shows that the root system is healthy.

Longitudinal (Figures 4 to 6) or cross sections (7 and 8), made at several levels of the stem , reveal more or less marked symptoms depending on the stage of the disease. Thus, we can see:
- discreet yellowing and browning of the vessels , but especially of adjacent tissues. The latter sometimes only present a diffuse yellowish tint and a few tiny cavities that are rather difficult to locate;
- marked browning of the xylem and adjacent tissues, the phloem and the pith. The latter may in some cases turn brown or be dry and mealy.

plants Heavily affected produce smaller fruits , poorly colored or prematurely dropping . A cross section shows often discolored vascular vessels. Although the disease can occur at any point in the tomato's development cycle, it is especially when the plant begins to bear fruit that it is particularly expressed. The time between contaminations and the appearance of the first symptoms varies mainly according to climatic conditions. It can be quite short on the order of 2 to 3 weeks and sometimes reach at least 3 months. In many cases, several contiguous plants in the same row are affected (Figure 9). This line distribution results from the transmission of the bacteria during cultivation operations (pruning, leaf stripping, harvesting, etc.).

Rare and tiny canker spots , initially white then taking on a beige tint as they become suberized, may appear on the leaflets , their petioles , stems (Figures 10 and 11) and fruits momentarily wet (Figures 12 and 13). They materialize local aerial contaminations of C. michiganensis subsp. michiganensis . On the fruits, the spots have a very characteristic shape reminiscent of a bird's eye. They are white, raised, a few millimeters in diameter (3 to 4 mm), and have a dark brown center. When they are close to each other, they can coalesce.