Objective: All potting media must be heat treated unless components are certifiably pathogen free and have been handled and stored in a manner that precludes contamination.
What you need to know: Because nurseries are so conducive to development of Phytophthora infections, low amounts of Phytophthora spores in potting media can lead to a high rate of contamination in the nursery. However, unless potting media is very highly contaminated, it is difficult to effectively test large volumes of media to reliably determine whether it is free of Phytophthora. Contamination can be introduced to potting mix through contaminated components (sand, peat, compost, forest products), contaminated water, use of contaminated containers, or contact with contaminated tools, equipment, and surfaces. Commercial potting mixes that are not heat-treated can be sources of pathogens and weeds. Proper heat treatment eliminates Phytophthora, ensuring that you start with clean potting media.
Best practices:
4.1. All germination and potting media must be pathogen-free. Moist heat is the most effective means for killing Phytophthora. and many other soilborne pathogens. Heating specifications are discussed here.
4.2. Commercial vermiculite and perlite perlite are produced by high temperature processes, so sealed bags from the manufacturer can be assumed to be pathogen free. Other packaged potting media components should not be assumed to be heat treated unless specifically labeled as such.
4.3. Handle heat-treated potting media in a manner to prevent contamination. Store planting media in clean, covered bins. Stored heat-treated potting media should not come in contact with the ground or be exposed to water splash or runoff. Do not contaminate heat-treated potting soil by using nonsanitized tools, hands, gloves, or by walking on it.
4.4. Using the smallest size containers possible will reduce the amount of potting media that needs to be treated.
4.5. Use one-way flow control to prevent contamination of heat-treated potting media. Do not return excess clean media removed for filling containers, etc., to heat-treated media storage bins. Remove only as much media as will be used at a time from storage bins to minimize the need to discard excess media. Excess media may be recycled to stockpiles awaiting heat-treatment.
3/1/2021 - minor rewording for clarity. Added BMP 4.5.