• Conservation, sources d'inoculum
  

Once introduced into a plot of tomatoes located in the open field or in a greenhouse, Clavibacter michiganensis subsp. michiganensis is able to easily maintain itself there for several months , or even more than a year, depending on the temperature and humidity conditions of the soil. Plant debris, especially if they are dry, contribute to its conservation for up to 7 months when buried (some authors speak of 5 years), and at least two years when they are on the surface. This bacterium could be stored for 2 years in compost and about ten months in dead stems. In the presence of moisture, the decomposition of debris is much faster and its persistence reduced. A study on the durability of this bacterium in fragments of stems left on the ground or buried in contrasting climatic situations showed that it persisted there for several hundred days, always shorter when they were buried.

It is also found on the equipment used (pots, drippers, containers, gutters, strings, stakes, etc.), tools, shelter structures, etc. For example, in certain production areas, bamboo used to support plants. are also a source of primary inoculum; it stays there for at least a hundred days. It can also be present in the nutrient solution of soilless crops.

A number of alternative hosts have been reported, such as pepper, eggplant, potato, tobacco, and several weeds: Solanum nigrum, S. douglasii, S. triflorum, Chenopodium spp… C. michiganensis subsp. michiganensis also appears to be able to remain as an epiphyte on several other non-nightshade plants.

The seeds of tomato are likely to retain and disseminate this blight, its external or internal tissues can be polluted or contaminated.

• Penetration and invasion

The first contaminations occur in the nursery, from the seeds (via small wounds present on the cotyledons) or from the environment of the young plants. Secondary contaminations can occur during cultivation operations likely to injure plants in the nursery: repotting, grafting, handling of plants.

During cultivation, bacteria enter plants either through natural openings such as hydatodes or stomata, or through various wounds located on the roots or vegetation (broken trichomes, disbudding wounds, leaf stripping …). They gain and invade the vessels (xylem and phloem) - hence the name “vascular bacteriosis” - and the contiguous tissues (cortex and marrow), and multiply there. They can also descend into the plants and reach the roots, from where they can be released into the nutrient solution of soilless crops.

Many bacteria are also present on the surface of plants; they are responsible for the small canker spots that can be seen on leaflets, stems and fruits.

• Multiplication and dissemination

C. michiganensis subsp. michiganensis is widely disseminated following rains , sprinkler irrigation, phytosanitary spraying and during cropping operations. By disbudding and removing leaves from the plants, the workers ensure contamination, from plant to plant, responsible for the characteristic line mortalities in the plots.

In soilless culture, the nutrient solution also contributes to the spread of C. michiganensis subsp. michiganensis , especially in NFT. Many bacteria are released from the roots and carried by the nutrient solution. They would then enter other healthy plants through their root system. In contrast, C. michiganensis subsp. michiganensis does not appear to multiply in solution. Fine debris and dust from contaminated soil would be disseminated by the wind, and would therefore constitute sources of inoculum.

The bacteria can also be transported by workers who have just worked in an already contaminated greenhouse (their hands and clothes), by the use of soiled tools or equipment (trolleys, boxes, etc.) in other areas of the environment. the greenhouse, by selling plants that are contaminated but do not yet show symptoms ...

• Conditions favorable to its development

This bacteriosis is largely favored by humid climatic periods . If the temperature range in which this bacterium thrives is between 12.8 and 33.7 ° C, its optimum is between 24 and 27 ° C , with more than 80% humidity. The bacteria are particularly resistant to drought.

The reduced light intensities and unbalanced manuring, by weakening the plants, would contribute to its development. Plants having received an excessive supply of nitrogen in the form of nitrates, and therefore very vigorous, would also be more sensitive.

The incubation period can last between 12 and 34 days depending on climatic conditions. The intensity of symptoms is lower in cold climatic conditions and on older plants. Symptoms appear earlier on young plants than on older plants. They are also more severe on plants grown in sandy soils than in organic soils.