SECTION III Benefits of Compost Utilization in Horticultural Cropping Systems © 2001 by CRC Press LLC CHAPTER 12 Spectrum and Mechanisms of Plant Disease Control with Composts Harry A J Hoitink, Matthew S Krause, and David Y Han CONTENTS I Introduction II Fate of Biocontrol Agents During Composting III Mechanisms of Suppression in Composts IV Biological Energy Availability vs Suppressiveness V Compost for Control of Foliar Diseases VI Disease Suppression — Future Outlook References I INTRODUCTION During the 1960s, nurserymen across the U.S explored the possibility of using composted tree bark as a peat substitute to reduce potting mix costs Improved plant growth and decreased losses caused by Phytophthora root rots were observed as secondary benefits in the nursery industry Today composts are recognized to be as effective as fungicides for the control of such root rots (Hardy and Sivasithamparam, 1991; Hoitink et al., 1991; Ownley and Benson, 1991) Therefore, the ornamental plant industry relies heavily on compost products for control of diseases caused by these soil-borne plant pathogens Composts have replaced methyl bromide in this industry (Quarles and Grossman, 1995) In field applications of composts similar results have been obtained (Hoitink and Fahy, 1986; Lumsden et al., 1983; Schüler et al., 1993) Examples of diseases controlled by composts were reviewed by Hoitink © 2001 by CRC Press LLC and Fahy (1986) A summary of the types of diseases suppressed by various types of composts is presented in Table 12.1 Composts must be of consistent quality to be used successfully in biological control of diseases of horticultural crops, particularly if used in container media (Inbar et al., 1993) The rate of respiration is one of several procedures that can be used to monitor stability of composts (Iannotti et al., 1994) Variability in compost stability is one of the principal factors limiting its widespread utilization Maturity is less important in ground bed or field agriculture as long as the compost is applied sufficiently ahead of planting to allow for additional stabilization; however, lack of maturity frequently causes problems here as well Table 12.1 Summary of Literature on Suppression of Plant Diseases by Various Types of Peats and Composts Peat or Compost Typez Disease Suppressed Pythium + Phytophthora Rhizoctoniay Fusariumy Root Rots Diseases Wilts Spagnum peat H4 – – – Sphagnum peat H 2, H + – – Pine bark + + + Hardwood bark + + + Yard/green wastes + + Grape pomace + + Cow manure + + Biosolids + + z y References Boehm and Hoitink, 1992; Chen et al., 1988b; Mandelbaum and Hadar, 1990 Boehm and Hoitink, 1992; Tahvonen, 1982; Wolffhechel, 1988 Boehm and Hoitink, 1992; Chen et al., 1988b; Ownley and Benson, 1991; TrillasGay et al., 1986 Chen et al., 1988a, 1988b; Kuter et al., 1983; Nelson et al., 1983; Trillas-Gay et al., 1986 Grebus et al., 1993; Rÿckeboer et al., 1998; Schüler et al., 1993; Tuitert et al., 1998 Gorodecki and Hadar, 1990; Mandelbaum and Hadar, 1990 Gorodecki and Hadar, 1990; Hoitink and Fahy, 1986 Chen et al., 1988a, 1988b; Kuter et al., 1988; Lumsden et al., 1983 Indicates peat decomposition level on the von Post scale (Puustjärvi and Robertson, 1975) or raw materials from which compost was prepared Requires inoculation with biocontrol agents or long-term curing of composts for consistent induction of suppression Effects of chemical properties of composts on soil-borne disease severity often are overlooked (reviewed by Hoitink et al., 1991) Highly saline composts enhance Pythium and Phytophthora diseases unless they are applied months ahead of planting © 2001 by CRC Press LLC to allow for leaching Composts prepared from municipal biosolids have a low carbon to nitrogen (C/N) ratio They release considerable amounts of nitrogen (N) and enhance Fusarium wilt (Hoitink et al., 1987) On the other hand, composts from high C/N materials such as tree barks immobilize N and suppress Fusarium diseases if colonized by an appropriate microflora (Trillas-Gay et al., 1986) High ammonium and low nitrate nutrition increases Fusarium wilts (Schneider, 1985) Perhaps biosolids composts enhance Fusarium diseases because they predominantly release ammonium (NH4) II FATE OF BIOCONTROL AGENTS DURING COMPOSTING The composting process is often divided into three phases The initial phase occurs during the first 24 to 48 hr as temperatures gradually rise to 40 to 50°C, and sugars and other easily biodegradable substances are destroyed During the second phase, when high temperatures of 55 to 70°C prevail, less biodegradable cellulosic substances are destroyed Thermophilic microorganisms predominate during this part of the process Plant pathogens and seeds are killed by the heat generated during this phase (Bollen, 1993; Farrell, 1993) Compost piles must be turned frequently to expose all parts to high temperature to produce a homogeneous product free of pathogens and weed seeds Unfortunately, most beneficial microorganisms also are killed during the high temperature phase of composting Curing begins as the concentration of readily biodegradable components in wastes declines As a result, rates of decomposition, heat output and temperatures decrease At this time, mesophilic microorganisms that grow at temperatures