In general, mains and borehole-derived water are safe to use so long as they are stored properly and the irrigation system is kept clean, whilst surface-derived waters (ponds, ditches, reservoirs, rivers and runoff) carry moderate to very high disease risks.
Treatment of surface-derived water to control plant (and potentially human) pathogens before use for irrigation is therefore highly desirable/recommended. Pasteurisation, ozonation, hydrogen peroxide, UV, slow sand filtration, chlorination and the use of chlorine dioxide (distinct from the use of chlorine as it does not form hypochlorous acid in water) are all established methods of water treatment (use of any chlorine product must consider the risk of chlorate and perchlorate residues in plant tissue, details can be found in AHDB Horticulture project CP 154a ‘Chlorine and its oxides - chlorate and perchlorate review’).
Related research projects
AHDB Horticulture project CP 126 ‘A desk-study to review global knowledge on best practice for Oomycete root-rot detection and control’ gives a summary of best practice treatment for different water sources in terms of oomycete pathogens with advantages and disadvantages of the treatments. Also useful are AHDB Horticulture Factsheets 21/15 ‘Testing water for plant pathogens’ 22/15, ‘Methods of water treatment for the elimination of plant pathogens’ and 23/15 ‘Hygiene and disease avoidance underpin the management of Oomycete stem and root rots’. An earlier 2004 project FV 248 ‘Assuring the microbiological quality of water used to irrigate salad crops: an assessment of the options available’ has further information on the different system types and there is an electronic grower guide to the use of slow sand filtration Download .
An AHDB Horticulture Factsheet 15/06 was produced specifically for ornamental crops ‘Water quality for irrigation of container ornamentals’. There is also a specific AHDB grower guide ‘Water harvesting and recycling in soft fruit’ published in 2013. AHDB ran a series of workshops called ‘Keep it Clean’ in 2016 and further supporting information is available on that web page. A ‘Keep it Clean - assessing the risk of microbial contamination’ DVD is available on request. There are also three web-based risk assessment decision matrices (on water quality, manure inputs and soils and worker hygiene) which are hosted by Food Standards Scotland.