Nurseries that use surface water for irrigation or recycle excess water can assume that Phytophthora is in the water. If you grow Phytophthora-susceptible plants, your water management plan should include water testing and treatment.

At the nursery, a practical method to test water for Phytophthora is to use rhododendron leaf baits. Place leaves from a Rhododendron (such as ‘Cunningham’s White’) in nylon mesh bags and suspend them in drainage ditches or recycling ponds. After several days, the leaves can be inspected for Phytophthora leaf spots (see Module 1). An ELISA test on these leaves will confirm that Phytophthora is present. Be aware that some species of Pythium can also cause a positive result. If no leaf spots are found, leaves should be wrapped in a moist paper towel and placed in a plastic bag. Keep these leaves at room temperature for a few more days to see if symptoms develop.

Treating irrigation water to remove or kill Phytophthora is a complex task requiring careful planning, a sound design, and proper management of the system.

Chlorine (as a gas or as sodium hypochlorite or calcium hypochlorite), ozone, ultraviolet light, and filters are used to treat irrigation water at nurseries for Phytophthora and other disease-causing organisms (see Table 1). How effective a particular method is depends on several factors, such as exposure time and initial water quality.

Water that contains suspended solids and/or dissolved organic matter is more difficult to treat than clear water. Sand filters are good at cleaning water, but they can’t handle the volume of water most nurseries use.

No one treatment method is best in all cases; you must decide which one best matches your nursery's needs.

A detailed table describing options for disinfesting water is available here. For additional information visit the Water Education Alliance for Horticulture.

Table 1.  Summary of nursery water treatment methods to kill or remove Phytophthora.*

Water Treatment Method Costs Water Volume** System Considerations^ Worker Safety^ Environment^
Filter - Sand Medium Low Clogging of filter Few concerns Disposal of backwash
Filter - Micro High Low Excessive clogging Few concerns Disposal of backwash
Chlorine - gas Low High Two stage injection system Highly toxic gas requires monitoring Residual chlorine in treated water
Chloring - sodium hypochlorite Medium High Simple injection system Hazardous material Residual chlorine and sodium in treated water
Chlorine - calcium hypochlorite High High Simple passive flow or injection system Fewest concerns of chlorine materials Residual chlorine in treated water
Ozone High Reduced Complex system Harmful if leaked from system Atmospheric risk if leaked from system
Ultraviolet High Reduced Complex system Few concerns None known

* In practice, poor implementation or system design and inadequate monitoring will lower the effectiveness of any water treatment method.

** High water volume (250-300 GPM per acre) necessary to operate typical overhead sprinkler irrigation systems used by nurseries.  Reduced and low flow water treatment methods may not be adequate to provide coverage or operate specific irrigation systems.

^ Nursery management concerns