Symbiotic Fungi: Principles and Practice (Soil Biology)

342 M. Vestberg and A.C. Cassells
a new quarantine zone; this however has not prevented the commercialisation of
European PGPR inoculants in North America (Fravel 2005) and vice versa.
21.3 Selection, Production and Formulation of Inoculants
Some, on ecological grounds, have suggested that microorganisms should be sought
in pathogen-suppressive soils, or at least from soils and environments similar to
those in which the inoculant will be used (Cook and Baker 1983). However,
economic constraints may influence the methods used to select potential inoculants
as inoculum producers seek to reduce costs by seeking ‘universal’ strains which
have wide soil and environmental tolerance (Schisler and Slininger 1997; Fravel
2005). In general, AMF inoculants show high host genotype-environment sensitivity,
whereas PGPR inoculants have a broad host range and wide environmental tolerance.
21.3.1 Arbuscular Mycorrhizal Fungi
Many species of AMF can infect a wide range of plant hosts. In early studies AMF
were therefore regarded non-specific also in terms of symbiotic functionality. In the
1970s and 1980s research with AMF was much focussed on finding some super
strains capable of increasing plant biomass under any environmental and soil
conditions (Vosatka and Dodd 2002). However, studies done during the last decade
have shown a high degree of AMF/host genotype and environmental specificity. It
is now recommended that AMF strains intended for commercial use should be
selected from the environments similar to those of their intended use. In an experiment
at the English Channel tunnel, this principle was demonstrated by Dodd et al.
(2002) who showed the superior effects of inoculation with native AMF on the
growth of Elymus pycnantus in comparison with commercially produced nonmycorrhizal
plants.
For screening and producing AMF inoculum, the first basic requirement is to
have a large collection of characterized AMF. This is very laborious since special
care has to be taken to keep the strains uncontaminated with environmental microorganisms.
Inoculum quality control is still one of the biggest bottlenecks in AMF
utilisation. Prediction of symbiotic effectiveness in terms of increased growth or
stress tolerance of the host is another obstacle to the successful commercial use of
AMF. Feldmann and Grotkass (2002) discussed this problem in connexion with a
directed inoculum production process (DIPP). They claim that in practice the
Mycorrhizal Effectivity Index (MEI, Bagyaraj 1994) should exceed 30% for creating
an interest in potential customers. The challenges of mycorrhiza technology are
treated in more detail by Vosátka et al. (2008).