**3.3. Bacteria in soil**

Soil-associated bacteria are highly influenced by soil moisture. For most plant pathogenic bacteria, high humidity favors disease onset and development. Incidentally, bacterial wilt (*Ralstonia solanacearum*) was first known as the "moisture disease" of potatoes, before the causal agent was identified [71]. In fact, the disease is prevalent during the wet summers, when high temperatures and high humidity are combined in a perfect condition for bacterial multiplication.

When comparing irrigation methods on bacterial wilt, Marouelli et al. [7] found that disease was significantly higher when processing tomato in Central Brazil was drip-irrigated, with an average of 42.5% wilted plants, 65 days after seedling transplant, in comparison with 5.0% incidence with sprinkle irrigation. Frequency of drip irrigation did not affect bacterial wilt incidence. It is believed that drip irrigation maintains the plant rhizosphere close to field capacity, thus favoring the disease, contrasting with the sprinkle irrigation, which provides periods of dry and wet conditions. Furrow irrigation was not studied, but it would most probably have an effect similar to the drip irrigation, or even more pronounced, if dispersion of the pathogen in the furrow is taken into account.

Contrasting with bacterial wilt, potatoes are affected by common scab, induced by *Streptomyces* spp. In this case, however, low soil humidity during tuber growth phase favors scab formation, what makes irrigation management recognized as one of the most efficient scab control measures. According to Wharton et al. [72], keeping soil moisture near field capacity for a few weeks at the beginning of tuberization substantially inhibits pathogen infection and disease development. The most likely explanation for this phenomenon is that the maintenance of high soil moisture is a condition that favors a more varied and competitive microbiota in the host rhizosphere, to the detriment of *Streptomyces* species.

Overall, because plant pathogenic bacteria may be viable in water for long periods of time, irrigation deserves special attention for two important epidemiological processes: survival and dispersal [73].
