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Infection Unit Method of Vesicular-Arbuscular Mycorrhizal Propagule Determination

USDA-ARS, Plant-Soil Symbiosis Group, Plant Development-Productivity Res. Unit, Western Regional Res. Ctr. (WRRC), Albany, CA 94710

* Corresponding author.

ABSTRACT

A method is presented for quantifying infectivity of vesicular-arbuscular mycorrhizal (VAM) fungal inocula by directly counting infection units in roots. Inocula of five isolates of VAM fungi [Glomus etunicatum Becker and Gerd., Glomus pallidum Hall, and Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe from California, Florida and Nevada] were assessed. Sorghum bicolor L. was grown for 14 d under controlled conditions in 100 mL growth tubes containing 48, 12, 3, or 0.75 g of each inoculum (96, 24, 6, or 1.5 for G. etuicatum) mixed with sterile soil to provide a total growth medium of 96 g. Discrete infection units were found for all isolates at all inoculum density levels, except for G. etunicatum (no infection at lowest density) and G. mosseae-California (coalescing infection units at highest density). The relationship between inoculum density and discrete infection units per gram root fresh weight was linear (P < 0.01). This relationship is particularly suitable for determining amounts of inocula of equivalent potential for use in short-term experiments.

Received for publication May 2, 1988.