Symbiotic Fungi: Principles and Practice (Soil Biology)

352 M. Vestberg and A.C. Cassells
21.6 Conclusions
In view of the high cost of micropropagation, there is a need to improve the quality
and disease resistance of microplants to ensure 100% establishment, rapid growing
on and improved resistance to pests and diseases, especially damping-off diseases.
There is evidence from the literature (see above) of the beneficial effects of
inoculants for both growth promotion and improved disease resistance, but not all
inoculants or more complex formulations of inoculants have been shown to be
positive (e.g. see Vestberg et al. 2004).
Given the high cost of micropropagules, they are usually grown on in intensive
propagation systems. There has been much concern about the use of pesticides in
intensive greenhouse production and there is the universal concern about the
sustainability of agriculture, problems of chemical residues in produce and the
environmental impact of pesticides. This poses the challenge as to whether
biological strategies can reduce pesticide usage or, indeed, substitute for pesticides.
Holistic biocontrol strategies, involving the use of inoculants to protect against soilborne
diseases and to promote plant growth and bioprime the plants defences,
combined with the use of plant activators, ideally natural elicitors, to activate the
primed defences for the control of haulm diseases, have been proposed (O’Herlihy
et al. 2003). These strategies require the development of methods for screening
potential inoculants for the traditional traits of pathogen suppression, for growth
promotion and for their ability to prime plant defences. Also, it will be necessary to
screen plant activators for their ability to augment primed defence response (Table
21.1). In the latter regard, it is important to recognise that salicylate and its
analogues, signal specific components of the biotic stress response, i.e. SAR
proteins, whereas optimisation of the defence response may require elicitation of
a broader response (McDowell and Dangl 2000; Conrath et al. 2002).
The combination of adding value to the micropropagules by improving establishment
and to the plants derived from them by influencing ontogenetic development
(Duffy and Cassells 1999) and by improving disease resistance has to be offset
against the cost of the inoculants and saving in the cost of pesticides and, possibly,
fertilizers. It remains to be determined which isolates or combination of isolates,
and which plant activators, will provide these benefits and whether pathogen- and
genotype-independent strategies can be developed to enable access to a large
market for the products.
References
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Agrios GN (2004) Plant pathology, 5th edn. Wiley, New York
Al-Karaki GN, Clark RB (1998) Growth, mineral nutrition, and water use by mycorrhizal wheat
under water stress. J Plant Nutr 21:263–276