Infectivity and effectiveness of vesicular arbuscular mycorrhizal fungi: Effect of inoculum type

Plants, inoculated with four vesicular arbuscular mycorrhizal (VAM) fungi (Glomus fasciculatus, G. monospovus and two isolates of Acaulospova laevis), were grown from 4 to 16 weeks, and the development of infection and spores was followed. Infected roots from pot cultures of different ages were used to examine the effect of mycorrhiza development on the infectivity of each fungus. The effectiveness of each fungus was assessed by measuring its ability to increase the growth of subterranean clover on a phosphate-deficient soil. For all fungi, the percentage of root length infected increased rapidly up to 10 weeks after sowing, and thereafter it either increased only slightly or decreased. Infectivity of root inocula increased with increasing percentage of root length infected in the inoculum for all fungi, except where large numbers of mature spores (24/g infected root) had been produced by one isolate of A. laevis. The infectivity of inoculum roots from pots containing mature spores of this isolate declined rapidly, although it was not decreased by the onset of sporulation by A. laevis. For all fungi, irrespective of the inoculum used, the fresh weight of tops of subterranean clover grown on a phosphate-deficient soil was very closely correlated with the percentage of its root length infected at an early stage of plant growth. That is, the effectiveness of the species of fungi examined at increasing phosphorus uptake into plants was related to the infectivity of the inoculum used.