Induced resistance for plant disease control - Disease Control in Crops - page 64

acid (JA) and ethylene (ET) sensitive pathway (Pieterse et al., 1998; Pieterse & Van Loon,
2007). ISR is phenotypically similar to SAR in that it acts unspecifi cally against taxo-
nomically different pathogens (Zehnder et al., 2001; Pieterse & Van Loon, 2007). Further
support for a general similarity of ISR and SAR came from the demonstration that a func-
tioning NPR1 is required for the successful establishment of ISR (Figure 4.1) (Pieterse
et al., 1998) and from the fi nding that ISR can be associated with the accumulation of
PR-proteins and/or phytoalexins and with changes in cell wall composition, all traits that
also characterise SAR (Ramamoorthy et al., 2001). Another feature of induced resistance
that is common to both SAR and ISR is a phenomenon called priming, whereby plant
defences are not directly activated by the inducing agent but instead are potentiated for
enhanced expression upon subsequent pathogen attack (Benhamou et al., 2000; Conrath
et al ., 2006; Beckers & Conrath, 2007).

4.2 Induced resistance in practice

Numerous biotic and abiotic agents have been reported to activate plant defence responses
thereby rendering treated plants more resistant to pathogenic infection (da Rocha &
Hammerschmidt, 2005). The commercialisation of resistance 'activators' has created a
new generation of crop protectants including, Bion ® /Actigard ® (acibenzolar- S -methyl
(ASM), Syngenta), Milsana ® ( Reynoutria sachalinensis extract , KHH BioScience Inc.
USA), Elexa ® (chitosan, SafeScience, USA), and Messenger ® (harpin protein, Eden
Bioscience, USA). In this section, we describe the implementation of activators in dif-
ferent cropping systems and examine the contribution of induced resistance to disease
management. Our discussion will focus primarily on the performance of products that
were specifi cally developed as activators. Therefore, fungicides such as probenazole
(Oryzemate ® , Meiji Seika) and fosetyl-al (Aliette ® , Bayer Crop Science), and 'benefi cial
microbes', including certain Bacillus spp., Pseudomonas spp., and Trichoderma spp., that
have been shown to activate systemic resistance in plants will not be discussed in detail.

4.2.1 Fruit and vegetable production

4.2.1.1 Tomato

Tomato is a major global crop and production is estimated to have a farm gate value worth
more than US$1 billion in the US alone (Florida Tomato Committee, 2005). Several
pathogens can attack tomato leaves and fruit during production and disease management
involves the integrated use of different bactericides and fungicides. A considerable num-
ber of studies have evaluated the use of inducing agents for disease control in tomato.
Actigard ® (ASM), in particular, has been the focus of intensive research as a management
option to control bacterial spot ( Xanthomonas axonopodis pv. vesicatoria ) and bacterial
speck ( Pseudomonas syringae pv . tomato ) (Louws et al., 2001). In a series of glasshouse
and fi eld studies carried out over a four-year period in eastern North America, ASM was
as effective or superior to copper-based bactericides against bacterial spot and bacterial
speck (Louws et al., 2001). The authors proposed that ASM should prove to be par-
ticularly useful in areas with copper-resistant pathogenic strains. Actigard is now estab-
lished as a recommended component of spray schedules for management of bacterial spot