REVIEW Practical application of induced resistance to plant diseases : an appraisal of effectiveness under field conditions D.R. WALTERS* AND J.M. FOUNTAINE Crop and Soil Systems Research Group, Scottish Agricultural College, King’s Buildings, West Mains Road, Edinburgh EH9 3JG, UK (Revised MS received 19 February 2009; First published online 23 June 2009) SUMMARY Plants resist pathogen attack through a combination of constitutive and inducible defences. Different types of induced resistance have been defined based on differences in signalling pathways and spectra of effectiveness. Systemic acquired resistance (SAR) occurs in distal plant parts following localized infection by a necrotizing pathogen. It is controlled by a signalling pathway that depends upon the accumulation of salicylic acid (SA) and the regulatory protein NPR1. In contrast, induced systemic resistance (ISR) is promoted by selected strains of non-pathogenic plant growth-promoting rhizobacteria (PGPR). ISR functions independently of SA, but requires NPR1 and is regulated by jasmonic acid (JA) and ethylene (ET). Resistance can be induced by treatment with a variety of biotic and abiotic inducers. The resistance induced is broad spectrum and can be long-lasting, but is rarely complete, with most inducing agents providing between 0 . 20 and 0 . 85 disease control. In the field, expression of induced resistance is likely to be influenced by the environment, genotype, crop nutrition and the extent to which plants are already induced. Unfortunately, understanding of the impact of these influences on the expression of induced resistance is rudimentary. So too is understanding of how best to use induced resistance in practical crop protection. This situation will need to change if induced resistance is to fulfil its potential in crop protection. INTRODUCTION Induced resistance Plants protect themselves from attack by pathogens using a complex array of mechanisms involving rec- ognition, attack and defence. In the early stages of the interaction between the plant and the pathogen, elicitor molecules are released. These elicitors can be of plant or pathogen origin and include carbohydrate polymers, peptides, lipids and glycopeptides (Walters et al. 2005). Plant perception of these elicitor mol- ecules leads to activation of a signalling pathway and ultimately to the production of plant defences. These defences include production of reactive oxygen species (ROS), phytoalexin biosynthesis, accumulation of pathogenesis-related (PR) proteins and cell wall re- inforcement (Hammerschmidt 1999). In race-specific resistance to a pathogen, the major gene controlling this resistance (R gene) codes for a product that recognizes the product of a matching avirulence (Avr) gene in the pathogen. In this situation, the plant quickly recognizes the pathogen and there is rapid activation of defences, e.g. a hypersensitive response (HR). In contrast, if the pathogen does not possess an Avr gene that is recognized by the host plant, HR is not activated and the pathogen is kept in check by a range of non-specific defences. This is known as polygenic or basal resistance. It is well established that, following infection by a microbial pathogen, susceptible plants can develop an enhanced resistance to further infection (Kuc´ 1982; Hammerschmidt 2007). This is known as in- duced resistance and can be split broadly into two * To whom all correspondence should be addressed. Email : dale.walters@sac.ac.uk Journal of Agricultural Science (2009), 147, 523–535. f Cambridge University Press 2009 523 doi:10.1017/S0021859609008806 Printed in the United Kingdom