The physical properties of container media influence Phytophthora disease development. The most important property in this regard is the air-water relationship in the media’s pore spaces.

Some container media do not drain very well and thus do not have enough air space to discourage Phytophthora. Research has shown that an air-filled porosity (AFP) greater than 30% reduces Phytophthora root rot. Yet, when AFP gets too high, irrigation must be more frequent so plants will not dry out—and frequent overhead irrigations can encourage foliar Phytophthora disease.

Finding the right balance of water-holding capacity and air-filled porosity takes time and experience. The physical properties of container media can be estimated at the nursery using a simple procedure. For details, see Altand, Physical properties of container media.

In addition to managing water, cultural practices that help control Phytophthora disease include nursery sanitation and selective use of fungicides. These are discussed in the following sections.