Predation of Plant Pathogens by Mycophagous Soil Amoebae

Abstract

Since soil amoebae were found to feed on and destroy soilborne plant pathogens, the amoebae have been attracting a considerable interest as antagonists of the plant pathogens and as potential agents for the biological control of root diseases. This paper deals with the morphology, taxonomy, feeding behaviour, ecology of the amoebae, and the possibility of biological control of root diseases by the amoebae. More than ten species of mycophagous amoebae have been identified including a wide range of taxonomical classes, e. g. Filosea, Lobosea, Acarpomyxea and Granuloreticulosea in the subphylum Sarcodina. The feeding process of propagules of the pathogens by mycophagous amoebae is initiated by attachment of trophozoites to the propagules following engulfment and digestion of the propagules. Emptied walls of the propagules are left after feeding and perforations varying in size are observed in the walls of the emptied propagules. Size of perforations, time required for feeding and feeding behaviour depend on the species of amoebae. Mycophagous amoebae are ubiquitous and show a worldwide distribution. In Japan, the amoebae are widespread in agricultural soils irrespective of the crop species, cultivation method, and soil texture from Hokkaido to Okinawa. The number of amoebae in soil amounts to 3 to 70 individuals/g of soil and the amoebae are more numerous in the upper layers of soil. Soil environmental factors strongly affect the activity of the amoebae. The optimal matric water potential for amoebal activity in soil varies with the soil specimens and the activity can be detected in a matric water potential ranging between -10 and -250 mb. Osmotic water potential is also an important factor for the amoebal activity. The activity is not detected at an EC above 1500μmho/cm in soil and osmotic water potential below -800 mb in liquid medium. The activity is maximal at soil pH6.5 to 7. High amoebal activity is detected in the suppressive soils of wheat take-all and Phytophthora root rot of avocado. A larger number of amoebae is detected in the wheat rhizosphere of the take-all suppressive soils and mycophagous amoebae are considered to play an important role in inhibiting the infection of the pathogen in the wheat rhizosphere of the suppressive soils. The amoebae inhibit the pathogen by colonizing the plant root and growing ectotrophically on the root surface. As a result, the severity of the root diseases is reduced and the height and top dry weight of the plants increase.