**3.2. Fungi in soil**

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

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water deficit levels to the host during the vegetative stage and at 90% of soil capacity during the production phase (unpublished).

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 pathogens [66].

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 the fungus [68].

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