Quick Answer: What is the difference between SAR and ISR?

Systemic acquired resistance (SAR) and induced systemic resistance (ISR) are both whole-plant immune responses that increase crop resistance to pests and diseases—but they are triggered differently and rely on different signaling pathways.

• SAR is typically triggered by pathogen infection or salicylic-acid-type compounds and is strongly associated with salicylic acid (SA), and pathogenesis-related (PR) proteins.

• ISR is typically triggered by beneficial microbes and is associated with jasmonic acid (JA) and ethylene (ET) signaling and works primarily through defense priming.

SAR vs. ISR at a glance

Simple takeaway: SAR is the plant’s “systemic warning response,” while ISR is its “systemic readiness state.”

Why growers should care

Modern crop production is no longer about solving one problem at a time. It is about managing overlapping pressures—disease, insects, heat, water stress, regulatory constraints, and resistance management—all at once.

In this context, SAR and ISR are not abstract biology. They are practical tools that allow growers to shift from purely reactive management to proactive plant conditioning.

Instead of only asking: "What kills the pathogen?"

Growers can also ask: "What helps the plant respond faster, stronger, and more consistently?"

That shift is where induced resistance becomes valuable in IPM programs. Research consistently shows that SAR and ISR can:

• broaden protection across multiple stressors

• reduce reliance on single modes of action

• improve timing flexibility

• complement conventional crop protection tools

What is SAR?

Quick Answer: What is systemic acquired resistance?

SAR is a salicylic-acid-driven systemic immune response triggered by infection or chemical elicitors, leading to whole-plant resistance through activation of defense genes and PR proteins.

The plant’s systemic warning system

SAR begins with recognition of a threat.

A pathogen infects a leaf, or an elicitor mimics that signal. The plant responds locally—but more importantly, it sends a systemic signal throughout its tissues. This signal activates defense pathways in parts of the plant that have not yet been attacked.

The result is not immunity, but heightened resistance across the entire plant.

This process is strongly associated with:

• accumulation of salicylic acid

• activation of the NPR1 gene

• production of pathogenesis-related proteins

These proteins help reinforce the plant’s defensive capacity, particularly against biotrophic pathogens.

Practical SAR triggers

1. Acibenzolar-S-methyl (ASM / BTH)

A well-established SAR activator used in crops like tomato and pepper for bacterial disease management.

Key idea: ASM mimics the SAR response without requiring actual infection.

2. Harpin proteins

Protein-based elicitors that stimulate plant defense signaling.

• Used in crops like strawberry

• Linked to increased defense enzyme activity

• Associated with reduced disease and mite pressure in some studies

3. Chitosan (context-dependent)

Chitosan is best described as a broad-spectrum elicitor that can activate plant defense pathways.

• In grapevine: activates salicylic-acid-linked defenses

• In strawberry: reduces Phytophthora crown rot

• In tomato: primes defense against Botrytis

Important nuance: Chitosan does not belong strictly to SAR or ISR—it can activate overlapping pathways depending on crop and context.

What is ISR?

Quick Answer: What is induced systemic resistance?

ISR is a microbially triggered, jasmonic-acid- and ethylene-associated defense state that primes plants to respond faster and more effectively to future stress.

The plant’s readiness state

ISR is not typically triggered by damage. It is triggered by beneficial relationships.

Certain microbes colonize plant roots and interact with plant signaling systems. Instead of activating full defense immediately, they “prime” the plant.

This means:

• defenses are not constantly turned on

• energy is conserved

• response is faster and stronger when needed

Practical ISR triggers

1. Pseudomonas spp.

Classic ISR-inducing rhizobacteria.

• Improve root colonization

• Influence nutrient availability

• Enhance defense signaling

2. Bacillus spp.

Widely used in agriculture.

• Promote growth and root development

• Induce systemic resistance

• Fit easily into existing crop programs

3. Trichoderma spp.

Beneficial fungi with ISR effects.

• Support root health

• Enhance stress tolerance

• Influence plant defense signaling

4. Crop examples

ISR-related benefits have been observed in:

• tomato, pepper, cucumber → reduced disease incidence

• berries and specialty crops → improved resilience and growth

• ornamentals and turf → enhanced stress tolerance

Key takeaway: ISR is especially valuable in systems where root health and repeated stress events drive outcomes.

Where jasmonates fit

Quick Answer: Is methyl jasmonate SAR or ISR?

Methyl jasmonate is part of the jasmonate signaling pathway and aligns more closely with ISR-type induced resistance, though real plant responses involve significant hormone crosstalk.

Jasmonates as defense signals

Jasmonates are central to plant responses to:

• insect feeding

• necrotrophic pathogens

• physical stress

Applications of methyl jasmonate (MeJA) have been shown to:

• activate defense genes

• increase antioxidant activity

• enhance resistance to pathogens like Botrytis

• improve postharvest quality in crops like grapes

Important nuance

Plant immunity is not binary.

While SAR and ISR are useful frameworks:

• SA, JA, and ET pathways interact

• responses depend on crop, timing, and stress type

Practical takeaway: Think in terms of dominant pathways, not rigid categories.

Why SAR and ISR matter for IPM

Quick Answer: Do SAR and ISR replace pesticides?

No. SAR and ISR do not replace pesticides—they complement them by improving plant readiness and resilience.

The real value in the field

SAR and ISR provide:

• preventative protection rather than reactive control

• diversified modes of action for resistance management

• improved plant consistency under pressure

They are most effective when:

• used before peak pressure

• integrated with scouting and chemistry

• applied consistently, not reactively

Where they matter most

These pathways are especially valuable in:

• berries and grapes → repeated infection cycles, quality sensitivity

• vegetables (tomato, cucumber, pepper, greens) → high disease pressure

• tree crops and nuts → long-term stress accumulation

• turf and ornamentals → continuous stress environments

The real lesson

SAR and ISR are not competing ideas. They are complementary strategies within the plant’s immune system.

SAR represents systemic activation after a perceived threat.
ISR represents systemic readiness built through beneficial interaction.

Together, they expand crop protection from:

“What kills the problem?”

to:

“What keeps the crop functioning under pressure?”

Key Takeaways

• SAR and ISR are systemic plant immune responses with different triggers and signaling pathways.

• SAR is salicylic-acid-driven and associated with PR protein activation.

• ISR is microbially driven and associated with jasmonic acid and ethylene signaling.

• Both pathways improve crop resilience and fit into IPM strategies.

• Elicitors like ASM, chitosan, and jasmonates—and microbes like Bacillus and Pseudomonas—can activate these systems.

FAQ

What is the simplest difference between SAR and ISR?

SAR is triggered by pathogens or SA-type signals and activates systemic defense directly. ISR is triggered by beneficial microbes and primes the plant for faster response.

 

Is chitosan SAR or ISR?

Chitosan is a defense elicitor that can activate both SAR-like and ISR-like responses depending on crop and conditions.

 

Is methyl jasmonate a plant defense activator?

Yes. Methyl jasmonate activates jasmonate signaling pathways involved in defense against insects and certain pathogens.

 

Do induced resistance pathways work in specialty crops?

Yes. Evidence exists in strawberries, grapes, tomatoes, peppers, cucumbers, and other specialty crops, though responses vary by system.

 

When should SAR/ISR tools be applied?

They are most effective when applied before or early in stress or disease pressure, allowing the plant to prepare rather than react.