**3.4. Nematodes**

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

True fungi in soil must not only survive humidity and temperature fluctuations but also the competitive environment that prevails in the rhizosphere. The effect of irrigation is different from what is commonly seen on above soil plant organs, and here, diseases may be favored by

Some plant pathogenic soil fungi have a complex relationship with the host, and infection may be hampered at low soil moisture, while high soil moisture may reduce symptom expression and improve yields. For example, the most effective management strategy to reduce Verticillium wilt, without decrease of dry matter production, is to irrigate at water deficit levels to the host during the vegetative stage and at 90% of soil capacity during the production

Accumulation of water in soil due to irrigation is increased when field soil is compacted (e.g., as a consequence of intensive agrotechnical operations) and/or native pedosphere properties (e.g., texture heavier soils). Several pathogenic soil fungi are favored by this condition of reduced aeration, such as *Fusarium oxysporum* pv. *solani, F. oxysporum* pv. *phaseoli, Rhizoctonia* spp. and *S. sclerotiorum* [65]. For *Rhizoctonia* infections causing root dieback in *Pinus* nurseries, excessive water interacts negatively with the host due to lack of root aeration, reducing growth and favoring the fungal infection. The ensuing root decay and water accumulation further stimulates the development of other secondary plant

Irrigation may also aid on the propagule dispersion and disease development. For example, Fusarium root rot (*Fusarium solani* f. sp. *phaseoli*) in beans is greatly reduced when sprinkler irrigation is used, contrarily to the negative effects of furrow or drip irrigations on the disease [67]. For *Sclerotinia minor,* the causal agent of lettuce drop, drip irrigation has a suppressive effect on the pathogen, while furrow increases substantially the sclerotial population. Irrigation not only provided humidity but also lowered the soil temperature, with furrow irrigation allowing the establishment of a more suitable temperature (18°C) for

As several other group of pathogens, fungi can also enter a new area by means of irrigation water. Previous studies on *V. dahliae* in irrigated olives showed a great dispersion of propagules [69] while its survival is also remarkable, with reports of up to 15,000 propagules of per

Soil-associated bacteria are highly influenced by soil moisture. For most plant pathogenic bacteria, high humidity favors disease onset and development. Incidentally, bacterial wilt

drip irrigation due to the large availability of water next to the host roots and crowns.

method for oomycete control [63, 64].

**3.2. Fungi in soil**

132 Irrigation in Agroecosystems

phase (unpublished).

pathogens [66].

the fungus [68].

**3.3. Bacteria in soil**

liter of water in ponds used for irrigation [70].

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 caused by the nematodes on the root systems.

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

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 particles.

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 water is collected from the same infested source [75]. Also, intensive irrigation is conducive to high nematode population levels, due to its effect on soil texture remodeling, altering abiotic conditions as aeration and particle arrangement creating new niches for protection [76]. Nematode locomotion depends on water, as studied for the J2 of *Meloidogyne incognita,* which could not travel against the water flow, limiting itself to resist the flow, trying to remain static. In sand substrate tests, when water percolated, the nematode moved with the water flow, resulting in the distribution of the nematode along irrigated areas [77].

Nematodes are already plant parasites *per se* but can also act as vectors for viruses as *Xiphinema index* (and other species) capable of transmitting *Grapevine fanleaf virus* into grapes [78]. Two nematode orders are known as vectors of plant viruses, Dorylaimida and Triplonchida [79]. For these nematode vectors, and several other species, soil is not required to be saturated, if humidity is kept at "normal levels" the parasite can survive and still act as a vector even 4 years in the absence of its hosts [80]. Also, *X. index* can be disseminated by contaminated irrigation water [81]. In some cases, these parasites are highly resistant to dehydration, in a survival strategy termed anhydrobiosis. Anhydrobiosis has been observed in many nematodes, such as among *Pratylenchus* (the lesion nematodes), one of the most important plant pathogenic nematode genera [82].

Differences among irrigation methods have not been very well explored for this group of plant pathogens. However, taking into consideration the effect of water flow and irrigation on the nematode's movement and displacement, drip irrigation could result in lesser dispersal and consequently, less infected plants in the fields.
