Method for the determination of competitive saprophytic colonization of soil fungi

Abstract

The competitive saprophytic colonization (CSC) of Macrophomina phaseolina (Tassi) Goid. on stem pieces (substrate units) of soybean ( Glycine max ) and maize ( Zea mays ), was estimated from the calculated inoculum density required to colonize 50% of the substrate units (EID 50). The EID 50 was found to be an inverse function of CSC. EID 50 values for M. phaseolina were low when inoculum, prepared by the Cambridge Method was used to determine CSC, suggesting that M. phaseolina had a high CSC. However, when sclerotia or infected plant material were added to soil as inoculum, EID 50 values were high, suggesting low CSC. The Cambridge Method is considered invalid for estimating the CSC of a plant-pathogenic, soil-borne fungus. The CSC of M. phaseolina was reduced by increasing the organic matter and hence the microbial population of the soil, and was also influenced by soil type, source of inoculum and substrate. Soybean and maize stem pieces colonized by Fusarium roseum were not colonized by M. phaseolina . A substance inhibiting M. phaseolina was produced by F. roseum in the substrate M. phaseolina grew from colonized stem pieces in soil-infected roots of soybean seedlings.