The infectivity of ten commercial mycorrhizal inoculants was examined in nursery conditions. Corn plants were grown in a soil-based medium and in two different soilless substrates, a potting mix prepared with redwood bark, pine sawdust, calcined clay and sand, and the commercial Sunshine #5 mix, mainly composed of Canadian sphagnum peat moss. The percentage of mycorrhizal colonization obtained with the different mycorrhizal inoculants ranged from 0 to 50%. This variation might reflect the presence or absence of viable propagules, the difference in the recommended rates applied as well as the difference in the content and type of infective propagules of each product. However, the infectivity of each mycorrhizal inoculant was also influenced by the growing media. Two products promoted higher values of mycorrhizal colonization in the Sunshine mix, three in the nursery mix and one in the soil-based medium. Mycorrhizal colonization did not enhance plant growth. Only the plants inoculated with the products that did not promote mycorrhizal colonization increased their growth relative to the non-inoculated controls, suggesting the presence of other growth promoters in the inoculum products. Based on these results, nurseries should conduct preliminary tests to determine which inoculants will perform in their potting mixes to assure the best fit of inoculum with their particular conditions.

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Author notes

We would like to thank the participation of Sheila Kee, Griselda Hernandez and Diane Greene; Ramiro Rodriguez, Salvador Zamarripa, Haydée Rodriguez, Sinfarosa Tampa, Abby Harned and Antonio Rodriguez. We are also grateful to Tom Zink for allowing us to use the facilities at San Diego State University and to the commercial mycorrhizal inoculum producers who kindly donated their products. This project was funded by The Horticultural Research Institute, 1000 Vermont St., NW, Suite 300, Washington, DC 20005.

2Plant Ecologist. Tree of Life Nursery. P.O. Box 635, San Juan Capistrano, CA 92693. Corresponding author: <[email protected]>.

3Natural Resources Cooperative Extension Specialist. Professor of Plant Ecology, University of California-Riverside.

4Assistant Environmental Horticulture Extension Specialist, University of California-Riverside.

5Professor of Plant Pathology, Director of the Center for Conservation Biology, University of California-Riverside.

6Environmental Horticulture Advisor. University of California Cooperative Extension, Ventura County, 669 County Square Dr. 100, Ventura, CA 93003.

7Tree of Life Nursery, San Juan Capistrano, CA 92693.