Induced Disease Resistance in Crop Health Management
With the introduction of the first reliable chemical activator of broad disease resistance for several crops, an additional tool is available for the design of more sustainable disease control strategies (6). The goal remains to integrate all available genetic, cultural, biological, and chemical methods for disease control in a way to optimize their benefits and minimize their risks for producers, consumers, and the environment. To achieve this, it is necessary to avoid ideologically motivated either-or discussions that pit genetic, biological, or chemical solutions in crop health management against each other. The choices have to be based on the reliability of the disease control and the demonstrated safety for the environment and the consumers. It is reasonable to assume that best results from a benefit/risk point of view, as well as for the sustainability of crop health management systems, can only be achieved if all technologies for the maintenance of plant health are combined in an optimal way. What are the main features of chemical activators and how can they be integrated?
Chemically Induced Disease Resistance in Plants
Of the many natural defense mechanisms plants have evolved to survive in nature, only a few can be triggered by biological or chemical agents without deleterious side effects and in a sufficiently controlled way for practical use. The best-known example is systemic acquired resistance (SAR), which in nature is activated by localized infections with necrogenic pathogens or non-pathogens including viruses, bacteria, or fungi (2). The activated state of SAR is characterized by broad-spectrum resistance against viruses, bacteria, and fungi, and by a set of biochemical responses; both the spectrum of protection and biochemical responses vary according to plant species. Acibenzolar-S-methyl (ASM), a derivative of benzothiadiazole (BTH), is the first commercially available product that activates the same defense responses as the biological inducers of SAR, with the same spectrum of protection and the same changes on the biochemical level (3,5,6). ASM is itself systemic and substitutes for the natural SAR signal molecule salicylic acid (SA) that is essential for biological activation of SAR (4). The resistance reactions of activated plants to infections often resemble those of genetically resistant cultivars. ASM activates disease resistance at very low rates on many dicot and monocot crops including wheat, rice, bananas, tobacco, and vegetables (6). Most of these crops had not been shown previously to contain a SAR response system and ASM proved to be a convenient tool to demonstrate the wide distribution of related inducible defense systems in the plant kingdom. However, the spectrum and duration of resistance activation by ASM are very crop specific, with the duration being generally longer in monocots than in dicots. Optimal integration of ASM with other disease control measures, therefore, needs to be carefully evaluated for each crop (6).
Many experimental chemicals have been shown to activate SAR earlier than ASM or SA in the signal transduction pathway by mimicking biological inducers through the formation of necrotic lesions (5). By such a mechanism these other activators always cause some tissue damage and may therefore not have been useful for practical application. An altogether different defense signal pathway is induced by rhizosphere bacteria in several dicots (7). This pathway is not dependent on SA accumulation but requires functioning jasmonic acid and ethylene responses for activation. No synthetic chemicals have yet been identified for this type of induced resistance and no clear biochemical markers have been established for the induced state. However, there is increasing evidence for interactions between the different systemic defense signaling pathways, including those against insects (1). These interactions can be positive or negative and depend on the growth conditions of the plants. To define optimal use in practice for chemicals that activate any part of the plant resistance network it is important, therefore, to evaluate carefully all of their effects on field-grown plants, much as a breeder would do for potentially new cultivars.
Integration with Other Methods to Maintain Plant Health
Chemical activation of SAR is not the new silver bullet against plant diseases, any more so than was the case for genetic resistance or other biological or chemical products for disease control. The experience with ASM has shown that SAR activation on most crops is best used in combination with other methods of disease control, including genetic disease resistance of all types and sound crop management that can provide additional reduction of the disease pressure. Where less resistant cultivars are preferred for yield, quality, or agronomic reasons, resistance activators can stimulate the plants to better protect themselves against some pathogens. Against other pathogens and where the level of genetic and activated resistance is not sufficient, fungicides or, where available, biological products can help assure healthy crops and high produce and food quality. In some cases, mixtures of activators with reduced rates of appropriate fungicides have given excellent disease control, with the fungicide providing curative and short term protection and activated resistance providing long-term protection (3,5). In other cases, activators can lower the fungicide load per season, thereby reducing the selection pressure for resistance against modern selective fungicides. Similarly, it could be shown that the use of ASM slows the build-up of new races of bacterial pathogens that overcome the genetic resistance of cultivars (5). Little information is available for the integration of biological disease control methods with induced resistance and with fungicides, except for copper and sulfur.
Outlook for Chemical Activators of Disease Resistance in Crop Protection
Research on biological and chemical activation of disease and insect resistance has taught us that plants possess complex networks of inducible defense pathways that can interact with each other. Of these pathways, the SA dependent SAR pathway seems to be the most robust to be exploited for practical crop protection, as evidenced by the development of ASM. With the increasing set of Arabidopsis defense pathway mutants it will be much easier in the future to determine if and how novel chemical disease control agents interact with the plant's own defense network. This information will help the optimal utilization of the various signaling pathways for practical crop protection through novel genetic, biological, or chemical solutions. Unfortunately, no such model system is available yet in monocots, where much less is known about the signaling pathways involved in biological or chemical stimulation of disease resistance. However, the experience with the chemical plant activators available so far suggests that some basic inducible broad-spectrum defense responses are conserved across the plant kingdom.
Chemical activation of disease resistance in plants represents an additional option to maintain healthy crops and to prevent losses due to plant diseases. Against some pathogens, like bacteria and viruses, it may be the only chemical control option where genetic resistance is not sufficient. Against dynamic fungal pathogens with a history of resistance to fungicides or of adaptation to resistant cultivars, resistance activators can help prevent the emergence of adapted pathogen populations. Sustainable crop production systems need all methods available to manage plant health, so that in each case the growers, together with their customers, can make the right choice by weighing costs, benefits and risks for their specific cropping situation. Growers then will have one more tool and sustainable disease control has a greater chance, where resistance activators become available.
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