**2.3. Dry propagule fungi**

Several species in the Fungi Kingdom reproduce asexually by producing dry conidia, with no gelatinous matrix, and may or may not be affected by irrigation management.

Powdery mildew, for example, caused by a number of species on the *Erysiphaceae* (Ascomycota), can infect several hosts, and is characterized by the presence of a whitish growth (mycelium, conidiophores and conidia), mainly in the adaxial leaf surface. Still fairly dependent on humidity as several other pathogens, its development may increase until a maximum of 80% RH as reported for *Uncinula necator* in grapevine [32]. Nonetheless, different from other fungal diseases, sprinkler irrigation is harmful for powdery mildews disease progress. The mechanical impact of water droplets harms the fungal structures, hindering disease progress. This phenomenon was previously found by Ruppel et al. [33] who observed lower disease incidence on sprinkler-irrigated sugar beet fields when compared to furrow irrigated ones. The effect of free water in powdery mildew conidia was analyzed by Shomari and Kennedy [34] in conidia of *Oidium anacardii*, a pathogen of cashew, by immersion of infected leaves in water, exhibiting a significant reduction on spore germination after an immersion period of 4 h. This interaction with conidia is only seen before germination: after that phase, leaf wetness does not influence any further on the host tissue colonization.

As for the diseases caused by oomycetes and true fungi, bacterial diseases in plants may occur in the aerial plant parts, including leaves and fruits, causing several symptoms such as cankers, pustules, blights, spots and specks. The symptomatology may vary with plant variety,

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Bacterial diseases are strongly affected by irrigation. Water, because it is necessary for the epidemiological processes of dispersal, infection and colonization, is considered one of the most, if not the most, important inputs that move bacterial disease expression on most crops. Leaf wetness is essential for bacterial infection and colonization of aerial parts of the plants. Bacteria penetrate through wounds or natural openings such as stomata and hydathodes. From diseased plants, bacterial cells are dispersed within and among fields through aerosols, insects, windblown soil and sand particles, movement of plant propagules and water flow.

For instance, bacterial spot (*Xanthomonas euvesicatoria*) is a recurrent disease that can devastate pepper fields whenever warm, wet weather is present. The pathogen is seed borne and is responsible for the formation of leaf spots that harbors large number of bacterial cells. Upon impacting on lesions, droplets from rain or overhead irrigation disperse bacterial cells through many micro-droplets from infected plants to neighboring healthy plants, especially under windy conditions. In addition, when foliage is wet, farm operations allow bacterial cells from infected plants to be carried to healthy plants within or between field areas [39].

In this example, which applies to many other bacterial spot diseases, switching from overhead to drip irrigation will warrant necessary moisture accessible to the roots while keeping the foliage dry. It is necessary to keep in mind that, as discussed elsewhere in this chapter, other diseases and pests might be favored by one particular kind of irrigation. An overall analysis of the crop management is necessary for the decision-making process, in a way to

Viruses are intracellular pathogens not capable of reproducing outside a living cell but possessing the genetic means for the manipulation of the host replication machinery for such

Vectors of plant viruses have a major role on the epidemics of plant virus because they are needed for the transportation and introduction of the virus particles into the host plant cell [40]. Most plant viruses can be transmitted by one of several groups of insects. A minority may also be vectored by other organisms such as mites, nematodes and pseudofungi (as those from kingdom Protozoa) [41, 42]. Nematodes that disseminate plant viruses will be addressed below. In some cases, diseases of complex etiology combine damages from the nematode with

Irrigation water does not affect the several viral pre-infection stages that are found within the fungi and bacteria life cycles. When lacking or in excess, water and irrigation may cause physiological host changes, which may accentuate or attenuate symptoms or alter the relationship of the vector with the virus and the host plant [43]. In some cases, the virus may protect its host from severe drought by avoiding irreversible wilt, as reported by Xu et al. [44]. Another

cope with different diseases and obtain desirable yields.

**2.5. Viruses**

the virus, compounding losses.

action.

host age and climatic conditions [38].

Other examples of the irrigation effects over powdery mildew may be seen with *Leveillula taurica* in tomato, which displays a critical increase of incidence when the crop is drip-irrigated, due to the absence of free water on leaves [27]. On pumpkin, powdery mildew is progressively reduced with increasing water volumes applied by the conventional overhead sprinkler irrigation system [10].

Conversely, *Alternaria solani,* the causal agent of tomato and potato early blight, does not suffer any negative effects of sprinkler irrigation. In fact, *A. solani*, as the great majority of plant pathogens that form dry propagules, benefits from the increased leaf wetness duration delivered by irrigation systems that wet aerial plant parts. Processes such as spore production and germination rates are favored. Reduced amounts of water may not markedly affect the development of *Alternaria* diseases, since its dark, thick-walled, multicellular spores are resilient to desiccation. In addition, germination of *A. solani* can take place with the only source of moisture deriving from nighttime dew, without need for irrigations [6].

Fusarium head blights (*Fusarium graminearum, F. culmorum, F. avenaceum*) of maize, wheat and other Poaceae, are economically devastating diseases not only for the direct losses of reduced grain yield but also for the accumulation of mycotoxins in the produce. Timing of irrigation is determinant for avoiding the occurrence of these diseases, and water should be avoided before anthesis and early grain fill periods [35]. Irrigation or rain water stimulates spore production, dispersion and germination of the *Fusarium* and of its sexual form (*Gibberella zeae*). High humidity levels (>94%) are also a requirement for most of the disease cycle phases [36, 37].
