**3.1. Oomycetes in soil**

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

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

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

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

In the following sections, the same group of pathogens addressed previously is discussed for

movement following the water flow on soil, as when furrow irrigation is used.

CaMV in producing transmissible morphs [43].

viruses worldwide.

130 Irrigation in Agroecosystems

**3. Crown and root diseases**

caused by pectolytic bacteria.

the development root and crown diseases.

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

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 irrigation [51].

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 dispersed by soil water.

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

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 irrigations, respectively, and lastly and most prejudicial, basin irrigation, which caused 93.9% dead plants. Several authors have confirmed that drip irrigation is the most efficient irrigation method for oomycete control [63, 64].

(*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

Management of Plant Disease Epidemics with Irrigation Practices

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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

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

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

Nematodes infect root systems of a great number of plants species and are one of the most difficult plant pathogens to control. Some parasitize upper plant organs, causing galls or lesions on leaves and seeds. However, most nematodes are root pathogens that not only act as plant parasites, but also facilitate infections by other soil pathogens, that penetrate through lesions

Nematode populations usually keep a steady growth if a susceptible host is available, soil texture is ideal and irrigation is not excessive (reducing oxygen availability), or restricted

The influence of water in this group of plant pathogens is mostly related to dissemination and movement in soil. Soil moisture, depending on the nematode species is essential to allow movement of juveniles and adults from colloid to colloid on water films around soil

In addition to active movement, eggs, juveniles and adult nematodes can be carried passively by irrigation water to short or long distances. Nematode spreads through large field areas, if

(preventing movement), as reported for *Meloidogyne enterolobii* in guava [74].

multiplication.

and dispersal [73].

**3.4. Nematodes**

particles.

of the pathogen in the furrow is taken into account.

host rhizosphere, to the detriment of *Streptomyces* species.

caused by the nematodes on the root systems.
