THE SPREAD OF MYCORRHIZAL INFECTION BY GIGASPORA CALOSPORA FROM A LOCALIZED INOCULUM

The lateral spread of Gigaspora calospora (Nicol. and Gerd.) Gerdemann & Trappe was examined in Trifolium subterraneum L. (subterranean clover) grown on a steamed lateritic podsol. Plants were sown in a sward in pots 29 cm long × 12 cm wide and inoculum of G. calospora was placed at one end. Mycorrhizal infection was estimated 21, 35, 63, 91 and 119 d after sowing at points 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24 and 26 cm from the inoculum. The lateral spread of Glotnusfasciculatum (Thaxter sensu Gerd.) through roots of T. subterraneum was measured 35 d after sowing. The percentage root length infected by G. calospora declined exponentially with distance from the inoculum. In contrast, infection by G. fasciculatum spread with a distinct front and hence was best described by a Gompertz curve. The number of spores of G. calospora recovered at increasing distances from the point of inoculation at the final harvest was correlated with the proportion of roots infected at an earlier harvest, and therefore provided a useful measure of the pattern of spread of G. calospora within roots.