DISEASE FOCUS: Do you know what might be lurking in your irrigation water?

by Deborah Mathews

The Problem

Recycling irrigation water is becoming more common in nurseries and greenhouse operations due to a desire to be “green,” financial considerations, and legal requirements from local government agencies or water districts. While it looks good on paper to save water for all of these reasons, one aspect that is commonly overlooked is the risk of plant pathogen build up and transmission within your operation. Fungi, bacteria, viruses and nematodes are all readily transmitted in water. This article will make you aware of some of the problems and management strategies for dealing with this issue.

The Culprits

Fungi and oomycetes are the biggest threats to plants in irrigation water. Major fungal players here are Rhizoctonia, Botrytis, Fusarium oxysporum, Colletotrichum, Cylindrocladium and Sclerotinia, with the oomycetes Phytophthora and Pythium presenting a particular threat due to their production of swimming zoospores. Bacteria found in water include species of Erwinia, Pseudomonas and Xanthomonas.

You are probably aware that plant viruses are transmitted by multiple methods including mechanical means (touching by hands, tools, plant to plant contact, contaminated surfaces, etc), and insects, but you may not be aware that they can also be transmitted through water via root contact. At least 10 plant viruses have been reported to be transmissible through irrigation water including Tobacco mosaic virus (TMV), Cucumber mosaic virus (CMV) and Tomato spotted wilt virus (TSWV) (1, 2, 3).

Multiple plant pathogenic nematode species are found in recycled water as well as in water from natural ponds, lakes or canals. These include root-knot (Meloidogyne spp.), dagger, ring and lesion nematodes.

Management Options

Disinfection of contaminated water can be achieved by many methods. They include physical treatments — such as heat, UV light and filtration — and chemical treatments like chlorine and ozone. Each has its benefits and hazards.

Heat treatment for broad-spectrum pathogen control requires reaching a temperature of 140°F (60°C) for a minimum of 2 minutes, although many viruses require 185 to 194°F (85to 90°C) for 3 minutes. This strategy is simple to perform, but the energy use required to achieve these temperatures can be expensive, the water must be cooled prior to use and beneficial microbes can indiscriminately be eliminated as well.

UV Irradiation using short wavelengths between 200 to 280 nm, with an optimum of 254 nm, causes the genetic material (DNA and RNA) of the pathogen to degrade, rendering them harmless. Doses ranging from 100 to 250 mJ/cm2 are effective in removing virtually all pathogens. Water must be relatively clear for UV treatment to be effective so filtration prior to treatment is usually required.

Filtration to physically remove pathogens can be done with membranes with a defined pore size, with most fungi, bacteria and nematodes being removed with 0.1 ?m pores, but not viruses. This greatly reduces flow rates, however, and filters must be regularly replaced which can be expensive. Filtration using sand beds (slow-sand filtration, SSF) has been shown to be efficient at removing almost all pathogen types, not only through physical exclusion of larger propagules, but mainly through biological inactivation via a microbial layer of microorganisms that forms on the sand. My laboratory is currently working on exclusion of viruses using SSF in collaboration with Dr. Loren Oki of UC Davis who has shown effective elimination of Phytophthora using SSF.

Chlorine is the most common chemical control method, although there are numerous other methods for decontaminating water. Chlorine is usually applied using liquid bleach (sodium hypochlorite). It is very effective in killing all pathogens, but levels must be constantly monitored to maintain effectiveness; chlorine is dependent on pH, can negatively react with nitrogen compounds and can be phytotoxic.

Ozone is another common chemical control method that disrupts membranes and proteins of pathogens and is very effective against all pathogen types. However ozone is sensitive to pH and organic matter in the water. It must be generated onsite and surplus amounts must be inactivated with carbon filters, adding to the cost.

Integrated pathogen management. The most effective water management strategies for pathogen removal incorporate two or more of the above methods. Pre-filtration, followed by UV light treatment or a chemical method combination, is the most common.

Deborah Mathews is UC Cooperative Extension Specialist/Plant Pathologist for Ornamental Crops, Department of Plant Pathology and Microbiology, UC Riverside

References

1) Buttner C, Marquardt K, and Fuhrling M. 1995. Studies on transmission of plant viruses by recirculating nutrient solution such as ebb-flow. Acta Hort. 396:265-272.

2) Koenig, R. 1986. Plant viruses in rivers and lakes. Advances in Virus Research 31:321-333.

3) Stewart-Wade, SM. 2011. Plant pathogens in recycled irrigation water in commercial plant nurseries and greenhouses: their detection and management. Irrig Sci. 29:267–297.