Activation of Systemic Resistance in Plants as a
Disease Control Practice

Caroline M. Press

Activation of systemic resistance (SAR) is a mechanism whereby plant cells can be stimulated to turn on their defenses to fight invasion by plant pathogens. This activation is essentially a form of ‘plant immunization’ and can be compared to immunization against disease in mammals. Plants are treated with activating agents to stimulate plant defense responses that form chemical or physical barriers that are used by the plant to ward off diseases. Inducing agents include pathogens, biocontrol agents, certain types of composts, and plant activating compounds. SAR was originally described using weak pathogens as activators of plant defense responses. However, the utility of this form of plant inoculation is limited in the field because of the likelihood of pathogen spread following application. The discovery that certain biocontrol agents and specific plant activators could induce resistance has made the use of SAR as a disease control strategy an achievable goal.

One of the advantages of using biocontrol agents, composts, or plant activating compounds to stimulate SAR is that they will not cause disease. A key advantage to SAR is that the plant, once immunized, has its defense mechanisms turned on for an extended period of time, usually until flowering, thereby limiting the number of applications necessary for effective disease control. Immunization is generally broad spectrum, effective against an array of different pathogens and pathogen types (i.e. fungi, nematodes, viruses and bacteria).

SAR has been demonstrated to occur in most crops including many row crops, vegetable crops, and nursery and tree crops and can provide protection against pathogens for which there is no effective disease control measure currently available. There is a low risk of pathogen populations developing resistance to SAR since that would entail overcoming an array of plant defense mechanisms. Finally, the use of biocontrol agents as inducing agents can provide other advantages to the plant such as plant growth promotion which is often associated with certain types of biocontrol agents. Growth enhancement has also been reported for a plant activator.

SAR is initiated when an inducing agent signals the plant to ‘activate’ the production of plant defense compounds and additional signal compounds. The signal compounds travel systemically within the plant and activates defense responses throughout the plant. The defense compounds remain localized and form structures that are resistant to pathogen invasion, actively degrade fungal and bacterial cell walls or are toxic to the pathogen.

The mechanisms by which biocontrol agents cause SAR are still relatively unknown although there are some similarities with inducing compounds. Certain Pseudomonads produce salicylic acid that is an important determinant involved in SAR. Salicylic acid is known to activate plant defense responses and is thought to function as a signal molecule within the plant after the onset of resistance. However, bacterial salicylic acid production is the not the only determinant of SAR since there are examples of biocontrol agents that can activate resistance but do not produce salicylic acid. It is thought that some biocontrol agents activate resistance at low iron concentrations through the production of iron-scavenging compounds called siderophores. Additionally, certain plant growth-promoting biocontrol agents have demonstrated altered insect feeding patterns in cucumber, making treated plants less susceptible to predation than non-treated plants and limiting the spread of pathogens that are vectored by the insects. Field experiments with several of these biocontrol agents have demonstrated that SAR is a viable disease control strategy.

Currently, both plant activating compounds and specific biocontrol agents are being pursued as commercial products for activation of SAR. ActigardTM (Novartis) and MessengerTM (Eden Bioscience) are currently in EPA registration as pesticides. In addition, several biocontrol agents are being commercially developed as SAR activators for application as seed treatments or root dips. The introduction of registered plant activators in the near future will provide producers with an entirely new disease control strategy that is environmentally friendly, safe and ideally suited to sustainable agricultural systems.

Caroline M. Press
Research Associate
Dept. Botany and Plant Pathology
Oregon State University
2082 Cordley Hall
Corvallis, Oregon 97331
Tel: (541) 752-9457

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