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 it is very highly contaminated, it is difficult to effectively test large volumes of soil to determine whether they are 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. Proper heat treatment eliminates Phytophthora, ensuring that you start with clean potting media.
4.1. Always use use heat-treated germination media and potting media. Moist heat is the most effective for killing Phytophthora. Heating specifications are discussed in Phytosanitary Procedures for BMPs for Producing Clean Nursery Stock under “2.4. Heat treatment of potting media”.
4.2. Commercial vermiculite and perlite in sealed bags from the primary manufacturer or bagged potting media that has been heat-treated should be pathogen-free if it has been handled in a manner to prevent contamination.
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.