Infection Unit Method
This is a direct assay because it measures primary mycorrhizal colonization (infection units formed from single entry points) prior to initiation of secondary spread (and coalescing infection units). The advantage of this assay is that the results reflect a 1:1 correspondence between number of infective propagules and the number of infection units.
Various inocula are diluted with the goal of obtaining a linear relation between the dilution (or mass of inoculum per total mass of growth medium) and number of infection units. Franson and Bethlenfalvay (1989), who developed this procedure, used irregular dilutions rather than a consistent dilution series for reasons that were not stated. Roots are stained and the number of infection units along the total length of root are counted.
This approach is laborious, but worth doing if the goal is obtain a direct measure of infective propagules. These assays are necessarily of short duration (14 days) to insure that only primary infections are measured (and secondary colonization does not occur). This limitation may introduce some variability, the amount of which depends on types of propagules present in different inocula, mixing procedures, host species etc. All of these factors influence probability of propagule-root contact. The only practical solution in overcoming this problem is to set up extra replications and monitor for the onset of secondary colonization.
Comparisons can be made from the slopes of the regression equations, a mid-point value in the range of dilution or some other reference point. Standard errors in the data tend to be relatively low, at least based on results of Franson and Bethlenfalvay (1989).
Few researchers have used this method, so its utility remains an open question. We don’t use the method because the fungi in the collection vary so much in aggressiveness that a lot of work would be required to adjust harvest times to avoid the complications of secondary colonization.
Franson, R. L. and G. J. Bethlenfalvay. 1989. Infection unit method of vesicular-arbuscular mycorrhizal propagule determination. Soil Sci. Soc. Am. J. 53: 754-756.