COUNTING RHIZOBIUM SP (PHASEOLUS) BY A PLANT-INFECTION METHOD USING ASMALL-SEEDED CULTIVAR OF PHASEOLUS-SILVESTRE L GROWN ENCLOSED IN TESTTUBES

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 p ure cultures using Morelos 628 grown in test tubes and a larger commer cial variety of P. vulgaris L. grown in Leonard jars, and with colony counts on yeast-mannitol agar (YMA) plates. Morelos 628 did not nodula te in bacteria-free conditions when inoculated with unrelated strains of root nodule bacteria. After 28 days, MPN estimates of R. sp. (Phase olus) were not significantly different from counts made after 3 days o n 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 d ays.