Re: Soil- and waterborne Phytophthora species linked to recent outbreaks in Northern California restoration sites by Matteo M. Garbelotto, et al. (vol. 72, no. 4, October–December 2018)

I am curious if any researcher has linked the inadvertent introduction of Phytophthora to restoration areas to the current practice of using dead plant tissue (compost) as part of the growing media.

If the plant material is grown in a sterile highly permeable mineral media, such as mined pumice, Phytophthora organisms will not be promoted.

Organic media eventually, if not immediately, promote Phytophthora when utilized as a growing medium. As organic substrate particles continue to decompose, the permeability of the medium decreases. The decomposition also consumes oxygen creating conditions perfect for Phytophthora.

In agriculture the hydroponic researchers realize the importance of promoting adequate oxygen levels in the rootzone. In floriculture the same concerns have been addressed. They are aware that dead (or alive) organic matter anywhere in the rootzone or irrigation system can result in oxygen levels that are too low for ideal root health.

Horticulture has to follow suit.

Gary Matsuoka
Laguna Hills Nursery

Matteo Garbelotto, UC Cooperative Extension specialist and adjunct professor at UC Berkeley, responds:

We published a paper in California Agriculture in 2015 on the risks of using products that are in between true compost (which is normally truly Phytophthora-free) and mulch (see volume 69, issue 4; http://calag.ucanr.edu/archive/?article=ca.v069n04p237 ). Also, we have found that soil and mulch used for trail-making can be chock full of Phytophthoras, and we are alerting stakeholders about the risks of using these two media. Your comments were right on.