FARM ADVISORS Control of Root Rot in the Landscape with Mulches Jim Downer, University of California, Ventura
he University of California has a long history with biological control of pests. In 1963 the first international meeting on soil-borne plant pathogens was held in Berkeley, California. This landmark effort presented papers dealing with the micro-ecological balance in relation to soilborne plant disease and presented a view of biological control that related “largely to natural phenomena of enormous potential waiting to be used if only the right approaches or organisms could be found.” In 2010 with fumigants that have made agriculture enormously profitable now on the slate for removal, the promise of biological control is again sought by agriculture, but we are by no means closer to finding the right organisms for all disease systems. There are two ways to rapidly stimulate biological control in cropping systems– these are: crop rotation and addition of organic amendments to soil to stimulate microbial antagonism. Indeed much of the need for soil fumigation is to replace crop rotations as a disease control method (Cook, 1990). With tree crops either as an agricultural commodity or as a landscape feature, neither crop rotation, except in a long view, nor fumigation is a viable method of disease control. This leaves only palliative methods and biological methods that increase the organic matter and its constitutive control. Dr. Cook’s thoughts from 1990 remain current, organic matter added to soil increases biological control of many plant pathogens. Questions still remain. Why are pathogens controlled and how is organic matter best added where trees are concerned? Understandably, the great allure of biological control is to find an organism that will attack a pathogen rendering it dead or at least reducing its ability to make its spores or interrupt its ability to attack the host. The most direct way to do this is for an organism to parasitize its host. There are many instances of fungi in soil that attack other fungi. These are called hyperparasites. They literally grow into and eat the contents of other fungi. Many hyperparasites lead a dual lifestyle acting as saprophytic fungi– living off soil organic matter when they are not killing pathogenic fungi. Thus increasing the organic content of soils favors their development. Fungi, bacteria and some yeasts also are capable of producing antibiotics that can inhibit the growth of other fungi. This classical antibiosis is the basis for many of the fungicides that we now use commercially. These are most often visually estimated or screened by looking at inhibition
32 December 2010
Figure 1. Phytophthora cinnamomi attacked by a huyperparasite (coiling hypha). Photo by Jim Downer.
Figure 2. Phytophthora cinnamomi is inhibited by a mulch fungus. Photo by Jim Downer.
zones in Petri dishes. The pathogen is often inhibited, or its growth stopped in culture by antibiotics that diffuse from the microbe through the agar affecting the pathogen. Many fungi that grow in mulch piles or in soils when organic matter is added will produce such substances. Other mechanisms of control include using (sequestering) iron or other nutrients in soils by the control organ-