**3. Crown and root diseases**

Crown and root diseases are caused by soilborne pathogens and usually result in great losses since control measures are more difficult because the "enemy" is protected by the soil layers. Frequently, soilborne pathogens lead to the abandonment of an infested field or make the whole farm improper for the cultivation of particular crops. These soil pathogens belong to different taxa in the fungi, oomycetes, bacteria and nematodes, and infect roots and crowns. They spend most of their life cycle in soil, with high resilience to changes in the physical environment and enhanced competitive skills. They are generally facultative pathogens, with good saprophytic activity. The dispersal of these pathogens is mostly associated to soil movement, adhered to implements and machines, even though spores of some may be dispersed by wind and water [49]. In tropical and subtropical conditions, these pathogens are favored, given the lesser oscillations in the soil physical parameters [50]. Soil is considered an environment that favors organisms which use water for movement, as the flagellate zoospores from oomycetes, flagellate bacterial cells, and nematodes that move in water films. Evidently, all these organisms may be passively transported even faster, and further, in flows of water.

The way irrigation methods affect crown and root diseases, and their causal agents vary accordingly to the group of microorganisms and other characteristics, such as the capacity for facultative anaerobiosis (in flooded or water-logged soil), which is conducive to soft rots caused by pectolytic bacteria.

Nematodes, mostly soilborne pathogens, are highly affected by water availability, typically by the aid of water for active movement in the root zone. Also, water allows for the passive movement following the water flow on soil, as when furrow irrigation is used.

In the following sections, the same group of pathogens addressed previously is discussed for the development root and crown diseases.
