Biology, epidemiology

• Conservation, sources d'inoculum
  

Pseudomonas syringae pv. tomato (Figure 1) is able to maintain the n s or on plant debris for at least seven months . In production areas where possible, it survives on plants grown from volunteer seedlings or regrowth. This bacterium seems to be able to persist on the roots and foliage of some weeds ( Amaranthus retroflexus, Chenopodium album, Galinsoga parviflora, Hibiscus trionum, Lamium amplexicaule, Portulaca oleracea, Sinapis arvensis, Stellaria media, Polygonum lapathifolium ). Its range of cultivated hosts is said to be very narrow, limited to tomato among horticultural crops. This bacterium can be present on seeds and persist there.

• Penetration and invasion

The bacterial cells present on the surface of the tomato (epiphytic development) penetrate in the leaflets by way of natural openings such as stomata and hydathodes or via various injuries (broken trichomes, ingrowth slots, injuries following the effects of sand or wind ). Subsequently, the bacteria invade the tissues (especially the intercellular spaces) and multiply there in large quantities. If conditions are very favorable, the first symptoms appear in approximately less than a week . In 24 hours, several million bacterial cells are produced, which is a definite asset for their dissemination. Note that this bacterium would have been associated with the hairs (glandular or not) of the ovaries during the period of anthesis. These hairs, gradually disappearing after this step, give way to openings on the epidermis of the young fruits serving as the site of infection with P. syringae pv. tomato .

• Multiplication and dissemination

It occurs mainly through water projections, or even aerosols, occurring during rains and sprinkling irrigations . These splashes, these microdroplets, can be carried over long distances by the wind. Thus, these bacterial cells are projected to other surrounding organs and plants. Workers working and circulating in crops with wet foliage contribute to its dispersal. The seeds and seedlings polluted or contaminated also ensure its spread to other farms or to other areas or countries of production.

• Conditions favorable to its development

The relatively low temperatures and the presence of moisture are conditions very conducive to outbreaks of P. syringae pv. tomato . This bacterium thrives at temperatures between 13 and 28 ° C, the range 18-24 ° C is particularly favorable for it, and its activity is greatly reduced above 30 ° C. Periods of dew or fog, rains or sprinkler irrigation leave a film of water on the plants which is very favorable to contamination. Six hours are enough for an epidemic to begin. It should be noted that the conditions surrounding the crop would play a more important role in the development of the disease than contamination in the nursery.