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

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 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

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

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

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,

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

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].

and reduced tuber size, or excess, which increases the intensity of many diseases [5].

irrigation had much lower disease levels and higher yields [7, 8].

area must be done before choosing the most adequate irrigation method.

especially of bacterial and most fungal spores.

*Phytophthora infestans* [14].

124 Irrigation in Agroecosystems

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 loss of photoassimilates.

These pathogens, different from the soil-habitant ones, must be resilient to adverse environmental conditions such as dehydration, large temperature fluctuations, nutrient scarcity in an epiphytic phase, incidence of UV light, among other physical, chemical or biological harmful factors [18].

Other oomycetes can be controlled by drip irrigation, as for *Phytophthora infestans* infecting greenhouse-grown tomatoes [27], or even in tomato field crops, planted in the dry season in the Brazilian Midwest (unpublished). *P. infestans* requires 2 to 6 h of leaf wetness (depending on temperature); nevertheless, high humidity levels inside the greenhouse (due evaporation)

Management of Plant Disease Epidemics with Irrigation Practices

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Fungus is one of the most diverse Kingdoms, with many species pathogenic to plants. Most fungi do not require water for spore dispersion, being easily dispersed in the dry air. However, numerous fungi, including important plant pathogens, are dependent on water splash for the dissemination. Commonly, this kind of fungi produces conidia associated to a gelatinous matrix in asexual sporulation structures such as picnidium (*Ascochyta*, *Phoma, Septoria*) or

If one fungus species requires water splash for dispersion, again the type of irrigation has a strong effect on such group of pathogens. The size and amount of the water drops may alter its capacity of spore dispersion, since smaller drops are unlikely to dislocate and disseminate

An example of the effect of irrigation method on fungi dissemination are the high severities of gummy stem blight (*Didymella bryoniae*) and anthracnose (*Colletotrichum gloeosporioides* f. sp. *cucurbitae*) of watermelon irrigated by overhead sprinkler, which presented reduced productivity and fruit quality. When shifting overhead to furrow irrigation, both diseases were drastically reduced [6]. These changes were associated with strong reductions of the foliar and fruit wetness periods, resulting in less dispersion and germination of spores. The same pattern was seen for anthracnose (*Colletotrichum acutatum*) in strawberry, when drip irrigation leads to very low disease incidence, postponing disease onset, and, therefore, reducing loses [30]. The same pattern has been observed for sweet pepper anthracnose, caused by *Colletotrichum* spp. (unpublished) and *Septoria lycopersici* on tomato [31]. For the septoria leaf spot, disease progress rates varied widely in the sprinkler, microsprinkler, drip and furrow irrigated plots, and severity increased most in treatments that kept leaves wet the longest.

The concept of leaf wetness is also an issue for *Glomerella cingulata* in apple. This pathogen requires high RH (>99%) and foliar wetness duration of 2.76 h, for significant germination of conidia. Additionally, the spore release from the acervuli and subsequent dispersal need rain or irrigation water for the splash-dispersal effect. Therefore, in the absence of these condi-

Several species in the Fungi Kingdom reproduce asexually by producing dry conidia, with no

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

tions, lesions are sparse and do not spread, even within a single host plant [15].

gelatinous matrix, and may or may not be affected by irrigation management.

may favor disease development, stimulating spore germination [23, 28].

**2.2. Gelatinous matrix fungi**

acervulus (*Colletotrichum*).

**2.3. Dry propagule fungi**

spore from one spot to another [29].

While wind plays a critical role on the dispersion of plant pathogens, irrigation water and rain, provide conditions for spore germination, avoiding desiccation of fungal and bacterial cells or, in some instances, damaging propagules sensible to water.

Many reports have indicated that more frequent sprinkle irrigations increase disease incidence of several foliar diseases [6, 14]. The understanding of the dynamics of each pathosystem is therefore mandatory for choosing the method of irrigation to be implemented in a given situation.

While oomycetes, fungi, bacteria and viruses all infect aerial parts of plants and are affected by irrigation, the latter is indirectly influenced because water affects insects and other vectors which transmit them.
