Reports of Non-Viable Mycorrhizal Products

There are several reports circulating the industry regarding mycorrhizal products that are not viable. This is due, in part, to the use of short-lived root fragment inoculum for commercial purposes, and to the unfamiliarity with the extended time requirement for assaying spore-based inoculum.

Different Propagule Types Have Different Shelf Lives

There are several fungal components that are commonly used commercially as VA mycorrhizal (VAM) fungal propagules. These include spores, colonized root fragments, and dry fungal mycelium from the root zone. There is considerable difference in the effectiveness over time of each of these types of propagules.

Root Fragments and Mycelium Fragments

Colonized root fragment propagules are pieces of plant roots containing VAM fungal mycelia. In the production of most brands of commercial VAM inoculum, colonized roots are removed from the plant and soil, dried, diced up into small pieces, and mixed into a dry carrier. Mycorrhizal root fragments and dried mycelium fragments are very rapid and effective propagules as long as they are reasonably fresh. As they age, their effectiveness is rapidly lost. There are published reports of root fragments and dried mycelia losing their viability after only a few weeks of routine storage. Most research involving mycorrhizal fungi inoculum has been done using these types of propagules, and many techniques for evaluating inoculum viability have been based on the rapid infectivity of these propagules.

Spores

Spores are actually reproductive and survival structures produced by mycorrhizal fungi. VAM fungi produce spores underground on roots in the soil. Usually, VAM fungal spores grow singly at the ends of fungal strands, and are typically produced outside of the root (but there are exceptions, with some spores being produced inside root tissue). The spores near roots germinate and colonize new roots, thereby propagating the fungal species.

Duration of Viability Assays

Spores are much more long-lived than root fragments and other propagules, but they require a much longer time period to colonize roots (four to six weeks). This is a critical point. Due to the fact that most university research on VAM inoculum is done using the rapidly infective root fragment inoculum, the more common techniques for testing inoculum viability assume similar rapid infectivity, even when assaying spore inoculum. As a result, some tests on commercial inoculum viability are completed before spores have had adequate time to germinate and infect roots. So the results are poor or negative, with the hasty conclusion that the inoculum is not viable.

If the inoculum is entirely comprised of dried hyphae and colonized root fragments, then the conclusion is correct. However, if the inoculum includes spores, then the conclusion is in error, since the testing method does not provide enough time for spore germination and root colonization. Still, the mere presence of some spores is not sufficient to characterize root fragment inoculum as having extended viability. The fact is that spores are the minority propagule in this type of inoculum, occurring only by chance. After extended storage, only the spores will remain viable, as most of the root fragments die off rather quickly. It is disingenuous (and perhaps even dishonest), for a company who sells root fragment inoculum to characterize it as being viable for 2 to 3 years simply because it happens to contain a few spores. That claim ignores the fact that the bulk of the inoculum loses viability rapidly, while only a few incidental spores remain alive.

New root growth is also needed for mycorrhizal colonization to occur. Mycorrhizal fungi prefer to colonize young, fine feeder roots, rather than older roots. When fast germinating seeds are used to assay inoculum, this is not a significant factor, assuming that significant time is allowed for significant root development to occur. Recently planted plants typically need several weeks time to recover from transplant shock before new roots are produced. This too affects the time required for colonization to occur, and should be factored into the experimental design where transplants are used.

Spore-Based Inoculum

The production of spore-based inoculum is designed to produce and harvest only spores. As a result, spores are the dominant propagule, while root fragments and dried hyphae may occur as incidentals. Such inoculum is properly characterized only by its spore count, with incidental non-spore propagules being excluded from the count. Spores do have an extended shelf life of 3 years or more in dry, room temperature conditions, long after any non-spore propagules have expired.

Spores Live Longer, But Also Take Longer to Colonize

As usual, there is a “but.” While spores are much longer lived, they do require considerably more time to colonize a plant compared to living root fragment inoculum. Root fragment inoculum can “infect” a root within 1 to 2 weeks. Spores take longer than that to germinate, respond to roots, find them and colonize. This is not a big problem in the field. But it has created a major problem in the lab. Since most research involving mycorrhizal fungi inoculum has been done using root fragment propagules, the techniques for evaluating inoculum viability are based on the rapid infectivity of those propagules. So most viability tests are completed in 2 to 3 weeks, long before spores have had enough time to weigh in. As a result, spore-based inoculum typically does poorly in these rapid infectivity trials, and this poor result is too often due to the rapid methodology, and not to the actual condition of the inoculum.

Recommendations

1. In order to properly test the viability of inoculum, particularly spore-based inoculum, the infectivity tests should be assessed at least 6 weeks after inoculation. Results obtained sooner will not assess the viability of most VAM spores.
2. The shelf life of any inoculum should be based on the longevity of the dominant propagule appearing in the counts shown on the labels.
3. Commercial spore-based inoculum should be characterized by spore count, with non-spore propagules being excluded from enumeration.

References

Bellgard, S.E. 1992. The propagules of vesicular-arbuscular mycorrhizal (VAM) fungi capable of initiating VAM infection after topsoil disturbance. Mycorrhiza 1(4): 147-152.

Cortid, L. et al. 2004. Assessing the infectivity of commercial mycorrhizal inoculants in plant nursery conditions. J. Environ. Hort. 22(3): 149-154.

Smith, S.E. and D. J. Read. 2008. Mycorrhizal symbiosis. (3rd Ed.) Academic Press, NY. 787 p.