The most effective way to produce Phytophthora-free nursery stock is following an integrated systems approach to excluding these pathogens through the entire plant production cycle. A systems approach to clean plant production takes constraints and properties of the nursery into account. Various approaches may be used to meet clean production standards, but in the end, the complete system needs to address all of the risk factors (aka critical control points) that may allow Phytophthora or other serious pathogens or plant pests to infest nursery plants.
Many features of plant nurseries put them at high risk for pest and disease issues, especially Phytophthora root diseases and foliar blights (see Plant nursery conditions favor Phytophthora root rots). The risk of these diseases is not related to a single factor; it arises from a number of interrelated factors that confer risk. For this reason, it is not possible to identify a few simple practices that will mitigate these risks. It is necessary to consider the entire plant production cycle in the nursery from beginning to end and examine all of the elements that pose a risk of introducing pathogens into the system and allowing them to propagate.
This concept is not new. Baker (1957) clearly stated that production of healthy nursery stock requires an integrated approach. He also noted that it was necessary to adopt the entire clean production system to achieve the goal of producing healthy plants. As an analogy, if you have a bicycle tire with many holes, it will not hold air if you decide to only patch some of them. In the same way, if you allow Phytophthora to contaminate your plant material at any point in the production cycle, you cannot end up with clean plants even if you follow some good production practices.
A widely-used systems approach for preventing contamination of products is the Hazard Analysis of Critical Control Points (HACCP) system. Parke and Grunwald (2012) present a discussion of the development of HACCP systems and discuss the application of this approach to clean nursery production. In short, a HACCP system involves conducting a hazard analysis to identify Critical Control Points, i.e., points in the production process where contamination can occur. Measures and limits are developed to address these points, and ongoing monitoring and record keeping are used to verify that the controls are adequate and working. To return to the bicycle tire analogy, the HAACP system determines where all the holes are, specifies a patch for each one, and checks to see that all of the holes are properly patched and that the tire holds air.
A structured system such as this becomes more important as the size and complexity of a production system increases and if many different people are involved in parts of the system. Smaller, simpler nursery operations, can also benefit by using a HACCP system, but may be able to take a somewhat less formal approach. To successfully implement a clean nursery production system, it is most important that all nursery personnel understand how contamination can be introduced into the nursery, and how to prevent that from occurring. Much of this can be accomplished by consistently applying several basic rules of thumb, as described below.