Symbiotic Fungi: Principles and Practice (Soil Biology)

350 M. Vestberg and A.C. Cassells
21.5 Holistic Strategies for the Use of Inoculants
Often, inoculants fail to reproduce in commercial production the beneficial results
obtained in greenhouse microcosm experiments (Gurr and Wratten 2000). This
has led to the evaluation of a range of strategies for the deployment of inoculants.
These vary from the use of mixed formulations of inoculants, e.g. AMF or PGPR
formulations containing two or more AMF species or isolates or PGPR species
(Yan et al. 2002; Mia et al. 2005; Domenech et al. 2006) or isolates (Bora et al.
2004); combinations of AMF and PGPR (Rodriquez-Romero et al. 2005); multiple
combinations of AMF and PGPR (Vestberg et al. 2004); combinations of
AMF, PGPR and Trichoderma harzianum (Srinath et al. 2003); AMF, PGPR and
other bacteria (Bashan 1998); AMF, PGPR and nitrogen-fixing rhizobacteria, with
stimulants of AMF spore germination and plant colonization, plant stimulants and
fertilizers (Handelsman and Stabb 1996). The results of the use of multiple/combined
inoculants have been variable. Either the inoculants, or some combinations
thereof, may interact positively (Vestberg et al. 2004; Domenech et al. 2006) or
show no additive effects (Tahmatsidou et al. 2006), or sometimes the plants may
suffer from biological overload and show reduced yield (Cordier et al. 2000).
Another alternative strategy to the use of single inoculants, which approaches
the use of complex mixtures of inoculants, has been the attempt to establish a
‘beneficial microflora’. This may be through the use of relatively specific soil
adjuvants, e.g. chitin, which is a substrate for lytic enzyme-producing microorganisms
(including the widely used Trichoderma inoculants) (Rafferty et al. 2003) or
the use of compost, green manure and other organic amendments (Hoitink and
Boehm 1999; Rafferty and Cassells 2003). The concept of the beneficial microflora
is the essence of the buffered microflora characteristic of traditional farming
systems (Altieri 1995) and aspired to by farmers converting to organic farming,
which results in pathogen-suppressive soils (Cook and Baker 1983).
The main emphasis in the use of biocontrol inoculants has been for the control of
soil-borne pathogens, which are relatively unaffected by foliar fungicides (Handelsman
and Stabb 1996). Indeed, the market niche for inoculants has expanded with
the withdrawal of the soil sterilant methyl bromide (Ristaino and Thomas 1997;
USDA 2000; Tahmatsidou et al. 2006). The withdrawal of methyl bromide is but
one aspect of the general debate on the sustainability of intensive agriculture/
horticulture. On energy, food safety and environmental pollution grounds there is
increasing pressure to adopt more sustainable, environmentally friendly production
methods (Chrispeels and Sadava 2003). The fungicide industry has responded by
developing synthetic ‘plant activators’ (Strange 2003; Agrios 2004). Commercial
plant activators include reactive oxygen species, analogues of the SAR (systemic
acquired resistance) signalling-compounds salicylic acid and of ISR-signalling
compounds jasmonic acid and ethylene (for reviews see Oostendorp et al. 2001;
Gozzo 2003). For reviews on fungal and bacterial elicitors of plant defence
responses, and for the signalling pathways involved see: Gatz (1997); Chang and