**2.2. Gelatinous matrix fungi**

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

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

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

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

The oomycetes, long treated as fungi and studied by mycologists due to their morphological, functional and ecological similarities with the Fungi Kingdom actually belong to the Chromista Kingdom and are more closely related to algae than to fungi [19]. They include Phytophthora wilts and blights, the downy mildews caused by the Peronosporales, the white rusts (genus *Albugo*) and root, crown and fruit rots by the genus *Pythium* and *Phytophthora*.

In general, oomycetes are greatly dependent on high humidity levels for all stages of the life cycle, including sporangia formation [20], and especially so for the indirect germination of sporangia in the form of zoospores, a process of great epidemiological consequence which requires not only high humidity levels, but actual free water [21]. High relative humidity (RH) can be achieved in several ways, including the method of application of irrigation water, high plant density and reduced plant spacing [22]. Shtienberg [23] also warned about the use of polyethylene mulch as a means to increase irrigation efficiency by reducing water evaporation. Irrigation may also be responsible for the short or long-distance introduction of oomycete inoculum into new growing areas, which was reported for the first time in 1921 [24]. Ranging from 6 to 45 days, the survival of plant pathogen propagules on irrigation water varies accordingly to the pathogen species, other abiotic conditions (temperature, pH, etc.) and especially

Free water on leaves, generally reported as leaf wetness duration, is a combined consequence of rains, irrigation events and microclimatic conditions prevailing in the plant canopy. Due to the strong dependence of oomycetes to leaf wetness, the ones infecting aerial plant parts can be controlled by the choice of irrigation method in favor of the systems that reduce leaf wetness. This has been shown for *Peronospora sparsa,* the causal agent of the blackberry downy mildew [22]. Mildew severities of 97% were recorded in the sprinkler overhead irrigation, compared to less than 10% in the drip system. Greater severity was associated with larger

periods of time of leaf wetness durations, in the sprinkler irrigated treatment.

cells or, in some instances, damaging propagules sensible to water.

factors [18].

126 Irrigation in Agroecosystems

given situation.

**2.1. Oomycetes**

which transmit them.

with the propagule type [25, 26].

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 acervulus (*Colletotrichum*).

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 spore from one spot to another [29].

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