**2.1. Oomycetes**

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 with the propagule type [25, 26].

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.

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greenhouse-grown tomatoes [27], or even in tomato field crops, planted in the dry season in the Brazilian Midwest (unpublished).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) may favor disease development, stimulating spore germination [23, 28].
