Fungistasis as a Factor in the Rhizosphere Phenomenon

Abstract

IN the course of work in progress on soil inhibitory factors, it has been observed that the spores of certain fungi which are unable to germinate when introduced into fresh fertile soil on glass slides coated with 0.5 per cent peptone in 2.0 per cent agar, owing to the presence in the soil of a fungistatic factor, may germinate readily when seedling roots grow in their immediate vicinity, that is, within a distance of some 2 mm. This observation has been made with conidia of Gliocladium roseum and Paecilomyces marquandii on glass slides buried beneath germinating peas so that the seedling roots grew into contact with the slides; and also with Fusarium solani and two unidentified isolates of Fusarium in similar experiments using seedlings of pea, wheat and lettuce. Fusarium solani and one of the unidentified Fusaria show peculiar behaviour; in the presence of soil their conidia germinate normally, but usually produce terminal chlamydospores on short germ tubes and cease further development. Park¹ has reported similar formation of chlamydospores by Fusarium roseum in response to an inhibitory factor produced by Bacillus macerans. In the presence of seedling roots the chlamydospores germinate and growth is resumed (Fig. 1). According to the abstract of a thesis by Burke² on root rot of Phaseolus vulgaris, Fusarium solani f. phaseoli may exhibit the same type of behaviour in naturally infested root-rot soils. It has further been observed with Gliocladium roseum and with the Fusaria studied that the germ tubes show a strong tropic growth towards the roots, although those of Paecilomyces marquandii do not appear to show such directed growth.