**2.5. Viruses**

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

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 sprin-

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

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

Bacteria, single-celled prokaryotes (1–2 μm in size) which reproduce by binary fission, are natural inhabitants on the rhizosphere or plant surfaces where they are mostly harmless as residents or epiphytes. The plant pathogenic ones will cause problems to a susceptible host only when conditions are favorable for their establishment, infection and multiplication. These conditions include high humidity and poor air circulation around plants. A film of free water on the leaf surface is the right condition for bacterial multiplication. Since they are microscopic, their presence is noticed only in large quantities, such as colonies in laboratory culture media or as viscous substances oozing from plant vessels and biofilms, or upon

moisture deriving from nighttime dew, without need for irrigations [6].

manifestation of symptoms of the diseases they induce.

any further on the host tissue colonization.

kler irrigation system [10].

128 Irrigation in Agroecosystems

cycle phases [36, 37].

**2.4. Bacteria**

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 action.

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 the virus, compounding losses.

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 similar example is during the infection of wheat by the *Barley yellow dwarf virus*; when the host is stressed from severe drought, the survival of the infected plant is increased, and it offers a more favorable growth for the aphid vector, *Rhopalosiphum padi* [45]. Turnip plants suffering from water deficiency stress can increase the transmission of *Cauliflower mosaic virus* (CaMV) by 34%, while the transmission of the *Turnip mosaic virus* may be increased by 100%. The increase in transmission was not related to higher virus tittering but for a rapid response by CaMV in producing transmissible morphs [43].

**3.1. Oomycetes in soil**

dispersed by soil water.

tion [51].

Many of the previously addressed factors in topic 2.1 can be applied for oomycetes causing disease in lower plant parts. The dependence on water still exists, although, different from the aerial organs, soil tends to be more stable for physical factors in general, and for temperature

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As for various pathogens, the epidemiology of a given oomycete is bound to irrigation or rainfall intensity and frequency. *Phytophthora capsici,* for example, during seasons of intense rainfall, causes much faster epidemics than when in conditions of moderate rainfall or irriga-

Soil oomycetes are in general highly adapted to survive in soil, with varying times of survival accordingly to temperature and a few other abiotic factors. Irrigation water plays an especially important role on the dispersal of oomycetes, due to their flagellate zoospores. "True fungi" (those in the Kingdom Fungi) do not have flagellate spores, and so are less efficiently

As discussed earlier, irrigation water and free soil water aid pathogens that are immovable, as non-flagellate bacteria which go with the water flux, but also for zoospores of oomycetes, flagellate spores that may dislocate in water [50]. Zoospores are also capable of host plant detection, allowing chemotaxis to the host and a quick attachment to the host tissue and the initiation of the infection process. *Phytophthora parasitica,* a pathogen of citrus, is one of those organisms that uses water for dispersal: irrigation spreads this pathogen not only within one field, but to an entire region, affecting growers that use the same water source [52]. The same pattern is found for *Phytophthora capsici,* in bell pepper, tomato and squash fields: for this pathogen, furrow irrigation has been shown to carry sporangia and zoospores to long distances. The number of infected plants along an irrigation line is attributed to the collection of secondary inoculum produced by the first infected plants [53]. *Phytophthora capsici* and *P. parasitica* were readily dispersed in furrow irrigation water up to 70 m from the point sources of inoculum in Solanaceae and Cucurbitaceae [54], and the mere reduction of furrow irrigation frequencies drastically reduced Phytophthora wilt on squash [53] and sweet pepper [55].

Frequent irrigations saturate soils and keep humidity for long periods of time, favoring propagule dispersal. Bowers et al. [56] and Ansani and Matsuoka [57] showed that in warm conditions (15–25°C), *P. capsici* resists for several days, even buried at several depths in the soil. In addition, soil moisture may render some hosts more predisposed to oomycete infection [58]. However, this has not been confirmed for all oomycete pathosystems, as for *P. capsici* in bell pepper [59]. Constant soil moisture at saturation or low saturation levels is not as positive for disease development as fluctuations of soil moisture [60]. Therefore, a lesser number of irrigation events are usually a form of disease control. For *Pythium aphanidermatum* in petunia, low and constant irrigation reduced plant infection, in contrast with constantly saturated soils or

Different irrigation methods may increase or reduce diseases caused by oomycetes in soil. Gencoglan et al. [62] showed that drip irrigation was the most efficient system to avoid *P. capsici*, with only 1.7% of incidence, versus 3.1% and 3.2% for furrow and sprinkler

soils submitted to a cycling of wetting and drying [61].

and humidity in particular, while it is a generally more competitive environment.

The main effect of irrigation on plant virus diseases concerns its effects on the vectors. Irrigation may affect the vectors, by altering its feeding habits, the efficiency of virus acquisition from an infected host, and, especially, by physically removing or disturbing the feeding of the insect. This latter effect is most noticeable by the application of water by sprinkler irrigation, which can reduce the population when compared to other irrigation methods in experimental plots [46]. These findings were confirmed not only for whiteflies (*Bemisia tabaci*) [27, 47] but also for *Myzus persicae* [48], each of which are important vectors of numerous plant viruses worldwide.
