Counting Rhizobium sp. (Phaseolus) by a plant-infection method using a small-seeded cultivar of Phaseolus silvestre L. grown enclosed in test tubes

We report laboratory experiments that tested the suitability of a wild-type small-seeded cultivar of Phaseolus silvestre L., Morelos 628, for counting Rhizobium sp. (Phaseolus) by the most probable number method (MPN). There was good agreement between MPN estimates of numbers in pure cultures using Morelos 628 grown in test tubes and a larger commercial variety of P. vulgaris L. grown in Leonard jars, and with colony counts on yeast-mannitol agar (YMA) plates. Morelos 628 did not nodulate in bacteria-free conditions when inoculated with unrelated strains of root nodule bacteria. After 28 days, MPN estimates of R. sp. (Phaseolus) were not significantly different from counts made after 3 days on YMA, at either low or high cell concentrations in pure culture or in unsterile soil. Nodulation of Morelos 628 was at a maximum after 21 days.