Isolates of Glomus macroncarpum Tul. & Tul. grown in soil cultures from single spores were tested at known numbers of propagules ml−1 soil for their efficacy in promoting growth of bell pepper (Capsicum annuum L.) seedlings. The soil cultures, propagated under similar growth conditions, had from 0.02 to 25 propagules ml−1 as measured by the most probable number technique. Seven cultures with similar levels of inoculum density (a median of 7 propagules ml−1) were used undiluted, and at 10−1 and at 10−2 dilutions to test efficacy of the isolates. Also, cultures with originally dissimilar propagule levels (0.02 to 15 ml−1) were diluted to uniform inoculum density levels and tested for efficacy.
Large intra-species variation in efficacy occurred in isolates collected from different soils and in those isolated from the same soil. Differences in the growth enhancement ability of isolates were evident from four weeks after seeding until harvest. The best isolates were superior at all inoculum levels tested but the ranking of isolates differed with the inoculum density.
A significant positive correlation was found between initial G. macrocarpum propagules ml−1 soil and all plant height and weight parameters measured. Growth of mycorrhizal plants was significantly greater than that of non-mycorrhizal plants when more than 0.5 propagule ml−1 was present. With more than 5 ml−1, 70% of the isolates were effective. A minimum level of 1 propagule ml−1 is suggested for infestation of pepper seed beds; greater benefits can be expected from higher levels up to at least 10 to 15 propagules ml−1.