A Technique for the Investigation of the Competitive Saprophytic Ability of Soil Fungi by the Use of Easily Decomposed Substrates

Abstract

THE method commonly used to determine the competitive saprophytic ability of soil fungi consists of exposing suitable substrates to colonization by the fungus in non-sterile soil. A serious disadvantage of this method is that only relatively resistant substrates, for example, wheat straw, can be used effectively because of the manipulation to which the substrate has to be subjected after incubation. Less-resistant substrates such as portions of non-woody roots, rhizomes and cylinders or disks of storage parenchyma of carrots or potatoes, etc., are so softened by the action of soft-rotting organisms in the soil that it becomes difficult to cleanse them adequately of adhering soil, and hence to state with confidence that fungi isolated from them have actually colonized the substrate in competition with the general soil microflora. It is very important to test the suitability of any potential substrate, for, as Garrett1 has pointed out in the case of root-infecting fungi, one is usually trying to demonstrate a negative character, that is, inability to compete as a soil saprophyte, and to obtain the strongest circumstantial evidence the killed host must be used as the substrate. A substrate such as wheat straw can probably only yield evidence relevant to fungi pathogenic to wheat or at most to cereals; hence the need to be able to employ any substrate that suggests itself. Since the chief obstacle to the use of soft tissues as substrates is the difficulty of removing the unwanted soil, the present method has been designed to prevent all unnecessary contact with the soil.