Department of Plant Pathology, OARDC, Ohio State University, Wooster, OH 44691, USA

Several types of composts are used effectively for consistent control of diseases caused by soilborne plant pathogens in the field as well as in container agriculture. However, many factors must be controlled for composts to provide consistent effects. The raw materials from which composts are produced, the composting process itself, the degree to which the organic matter in composts has been stabilized, the types of microorganisms colonizing composts after peak heating, and chemical as well as physical properties of composts, all contribute to efficacy [1].

Control of soilborne plant pathogens such as Pythium and Phytophthora spp. is due to the activity of a broad spectrum of biocontrol agents present in composts. Composts which are high in salinity do not induce suppression unless they are applied several months ahead of planting to allow for leaching of salts. Most composts and compost-amended substrates effectively suppress these diseases. In contrast, a pathogen such as Rhizoctonia solani, which is controlled effectively by a narrow range of biocontrol agents in composts, is not controlled readily in compost-amended field soils or container media. Inoculation of composts, after peak heating but before substantial recolonization with mesophiles has occurred, with inoculants of specific biocontrol agents of R. solani overcomes this deficiency in natural composts.

The length of time that suppression induced by composts is sustained depends on the microbial carrying capacity of the substrate. This can be characterized by the rate of hydrolysis of fluorescein diacetate (FDA), by NMR or DR-FTIR spectroscopy. Practical guidlines for FDA activity are available for container and field agriculture [2].

Diseases of plant foliage may also be affected because some composts induce systemic resistance in plants. This property of composts is highly variable in nature, however. The talk will focus on successes and failures in disease control obtained with composts.

1. Hoitink HAJ, Stone AG, Han DY, 1997. Horticultural Science 32, 184-187.
2. Boehm MJ, Wu T, Stone AG et al., 1997. Applied Environmental Microbiology 63,162-168.