**1. Introduction**

Plant diseases are one of the main constraints for agricultural production, leading to great loses annually all around the globe [1]. Plant pathology evolved along with agriculture, starting with the earliest farmers competing against plant pathogens with religious, supernatural or other

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

practices [2] to come to the modern era, where science is used to track the conditions which favors pathogens and consequently allows growers to how to avoid them on a rational basis.

The pathogen success in establishing itself in the aerial plant parts is highly dependent on the duration of foliar wetting, which is directly affected by irrigation timing and other factors [16]. If the moisture provided by irrigation is enough to retain free water in the plant surface for the minimum time required for infection, it will lead to more intense disease epidemics. For many years, we have observed that processing tomato in Central Brazil display significantly lower incidence of diseases caused by *Phytophthora infestans, Septoria solani, Xanthomonas* spp.

Management of Plant Disease Epidemics with Irrigation Practices

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In addition to water availability, the evaporation process must be considered. Evaporation is affected directly by relative humidity, air temperature, wind speed, air vapor pressure [4] and plant tissue position. For example, within Israeli climatic conditions, sprinklerirrigated tomato leaves take from 5 min (external leaves, strong wind, 36°C, 16% RH) to 4 h (internal leaves, no wind, no direct sun, 17°C, 16% RH) to dry. In the latter climatic conditions, the leaves may remain wet until dew formation at nighttime, completing a total 20 h of total humidity [17]. A similar phenomenon occurs in the dry season (April–September) in Central Brazil, when almost all processing tomato and potato crops are grown. Both crops are hosts of late blight (caused by the oomycete *Phytophthora infestans*) and early blight (caused by the true fungus, *Alternaria solani*). These pathogens have different resistance levels to dryness and widely different temperature requirements, serving as illustrative models for the discussion on infection and the influence of the leaf microenvironment

The way plant pathogens relate to irrigation and water availability depends on a diverse number of characteristics intrinsic of each group of microorganisms. In the present review, diseases and their respective causal agents were grouped according to their primary niche in the plant, either diseases of aerial plant parts or as crown and root diseases. Other divisions were made below for clarifying the effect of the water on each plant part or phase of the disease cycle. Oomycetes, for example, are very well adapted to the availability of free water, while other fungi, as the *Erysiphaceae,* (the powdery mildews) have a negative interaction resulting in damage of conidia when overhead irrigation is used. Bacteria are also highly dependent on water to prevent desiccation (which usually causes sharp decrease on their populations) and then to allow multiplication until they reach the threshold numbers necessary for invasion and infection. Fungi with a gelatinous matrix also respond differently when compared to other fungal groups: For instance, aerial transport by wind does not play an essential role for these organisms, whereas sprinkler irrigation typically provides the main

Fungi, oomycetes, virus and bacteria infect aerial parts of susceptible host plant (leaves, stem, flowers and fruits) resulting in diseases responsible for losses due to direct damage to the commercial produce or to yield reduction as a consequence of impaired photosynthesis and

and *Alternaria* spp. under drip when compared to sprinkle irrigation.

on disease severity.

dispersal method.

loss of photoassimilates.

**2. Diseases of the aerial plant parts**

The irrigation efficiency not only ensures the most efficient crop growth, but it is also essential for high-quality production of seeds, food, textiles and other produce with increasing perception of the economical and environmental impacts. It is estimated that 30–40% of the world food production is from irrigated agriculture [3, 4]. Its importance can be exemplified by reports on potato production which indicate that variations as low as 10% of the potato water need result in significant yield losses, either from water deficiency, leading to deformation and reduced tuber size, or excess, which increases the intensity of many diseases [5].

Choice of the irrigation system in itself, regardless of the volume of the water supply, affects plant development as well as disease onset, pathogen dispersal and rates of disease progress. For example, furrow irrigation which requires large amounts of water, usually demands higher rates of nitrogen fertilization which can predispose the plant to many diseases; in addition, soil borne pathogens easily spread in the irrigation furrows following water flow [6]. In areas infested with *Ralstonia solanacearum*, the furrow and some drip irrigation systems increased tomato wilt incidence and reduced yield, while conventional overhead sprinkler irrigation had much lower disease levels and higher yields [7, 8].

Drip irrigation, in addition to a more efficient water use, is usually recommended to avoid wetting of aerial plant parts and generally results in less foliar diseases [9]. On the other hand, the direct (mechanical) and indirect (environmental) effects of delivering irrigation water droplets onto the leaf surfaces have been demonstrated to significantly reduce powdery mildews on Cucurbitaceae [10], Fabaceae [11] and Solanaceae [12] while also depressing virus vector movement [13]. These two situations indicate that diseases vary as to their response to irrigation. Therefore, a precise determination of the disease frequency and intensity in a given area must be done before choosing the most adequate irrigation method.

The sprinkle irrigation systems usually allow for better water distribution to the crop, at reasonable economic costs. It is generally more efficient than furrow irrigation, but it promotes foliar wetting, required for many pathosystems, and is favorable to propagule dispersion, especially of bacterial and most fungal spores.

In addition to the choice of the irrigation method, other factors must be taken into consideration, such as irrigation timing. Most fungal plant pathogens produce spores during nighttime, being dispersed after dawn. Consequently, morning irrigations are prone to dislodge and disperse spores, also offering humidity and free water for germination at the leaf surface. Some fungal pathogens may form spores or propagules later in the day and are thus favored by afternoon irrigations, while night irrigation will reduce spore dispersion, as reported for *Phytophthora infestans* [14].

With exception of the members of the Erysiphales (Ascomycota), fungi and bacteria need free water on the leaf surface to initiate infectious processes. In fact, the leaf wetness duration has been considered the most determinant microclimatic variable for disease establishment and progress, and it is one of the main variables monitored in disease prediction systems [15].

The pathogen success in establishing itself in the aerial plant parts is highly dependent on the duration of foliar wetting, which is directly affected by irrigation timing and other factors [16]. If the moisture provided by irrigation is enough to retain free water in the plant surface for the minimum time required for infection, it will lead to more intense disease epidemics. For many years, we have observed that processing tomato in Central Brazil display significantly lower incidence of diseases caused by *Phytophthora infestans, Septoria solani, Xanthomonas* spp. and *Alternaria* spp. under drip when compared to sprinkle irrigation.

In addition to water availability, the evaporation process must be considered. Evaporation is affected directly by relative humidity, air temperature, wind speed, air vapor pressure [4] and plant tissue position. For example, within Israeli climatic conditions, sprinklerirrigated tomato leaves take from 5 min (external leaves, strong wind, 36°C, 16% RH) to 4 h (internal leaves, no wind, no direct sun, 17°C, 16% RH) to dry. In the latter climatic conditions, the leaves may remain wet until dew formation at nighttime, completing a total 20 h of total humidity [17]. A similar phenomenon occurs in the dry season (April–September) in Central Brazil, when almost all processing tomato and potato crops are grown. Both crops are hosts of late blight (caused by the oomycete *Phytophthora infestans*) and early blight (caused by the true fungus, *Alternaria solani*). These pathogens have different resistance levels to dryness and widely different temperature requirements, serving as illustrative models for the discussion on infection and the influence of the leaf microenvironment on disease severity.

The way plant pathogens relate to irrigation and water availability depends on a diverse number of characteristics intrinsic of each group of microorganisms. In the present review, diseases and their respective causal agents were grouped according to their primary niche in the plant, either diseases of aerial plant parts or as crown and root diseases. Other divisions were made below for clarifying the effect of the water on each plant part or phase of the disease cycle. Oomycetes, for example, are very well adapted to the availability of free water, while other fungi, as the *Erysiphaceae,* (the powdery mildews) have a negative interaction resulting in damage of conidia when overhead irrigation is used. Bacteria are also highly dependent on water to prevent desiccation (which usually causes sharp decrease on their populations) and then to allow multiplication until they reach the threshold numbers necessary for invasion and infection. Fungi with a gelatinous matrix also respond differently when compared to other fungal groups: For instance, aerial transport by wind does not play an essential role for these organisms, whereas sprinkler irrigation typically provides the main dispersal method.
