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FutureStarrMycorrhizal Fungal Inoculum
Arbuscular mycorrhizal (AM) fungi are the most important mycorrhizae in agricultural ecosystems due to the fact that they colonize the majority of crop plants. Known as “obligate symbionts,” AM fungi must associate with plant roots to survive; it is this association that begins a mutually beneficial between the fungi and the plant. In return for sugars from a plant, the long, thread-like structures of fungi, the hyphae, act as an extension of a plant’s root system and increase a plant’s access to immobile nutrients including phosphorus (P), zinc and copper. While plant root hairs extend 1-2 mm into the soil, the mycorrhiza’s hyphae explore a greater volume of soil and can extend up to 15 cm from the plant’s roots. The relationship between mycorrhizae and crop plants often enhances plant growth and yield, but even when no growth enhancement occurs, the majority of P uptake can be attributed to mycorrhizae. Mycorrhizae have also been credited with increasing a plant’s disease resistance, improving a plant’s ability to grow under drought conditions, and improving soil structure.
We have also experimented with the type of diluent and quantified the effect of perlite, vermiculite and peat based horticultural potting media on inoculum production (Douds et al., 2010). Inoculum can be successfully produced with all of the media amendments; spore production by all AM fungi studied was not significantly different among mixes using different diluents. However, a most probable number bioassay, a technique used to determine the density of infectious propagules, showed that vermiculite did tend to produce more overall propagules than peat based potting media. We hypothesize that the laminar sheets of the vermiculite may be an ideal environment for the growth and persistence of mycorrhizal hyphae. The similar spore populations and root colonization between the three diluents support this theory.
While we most often propagate isolates of a specific AM fungus species for research, one of our first trials highlighted the possibility of using the on-farm method to propagate indigenous mycorrhizal fungi. In this early trial, contaminant fungi were found in what should have been a single species inoculum. These contaminant fungi most likely came from soil that was mixed into the compost during turning the previous year. This inadvertent propagation of native AM fungi illustrated that the on-farm method could be used to create an inoculum that was not only cheaper than single-species commercial inoculum, but also had the added benefit of containing a diverse group of locally-adapted mycorrhizal fungi that could be used to boost a farm’s native populations (Douds et al. 2005).To obtain a locally adapted and taxonomically diverse inoculum, field soil can be mixed into the dilute compost mix as a source of native AM fungi (Douds et al. 2010). To ensure a diverse sampling of mycorrhizae, soil should be collected from a natural area of the farm such as a wood lot or fence row. Collecting soil from these areas is preferable since it should contain a diverse and healthy mycorrhizal fungus population that has not been impacted by agricultural practices. Collecting soil from a production field is also an option, but the field should not have been used in the past two years to grow the crop that will be inoculated. This precaution is to avoid introducing pathogens to the inoculum. Additionally, due to the fact that mycorrhizal fungi can be distributed irregularly, pooling four to five samples is suggested. Most mycorrhizae are found in the top 10 cm of soil, so samples do not need to include soil deeper than this layer. (Source: rodaleinstitute.org)