Is Azoxystrobin Systemic or Contact?
Understanding the Terminology: “Systemic” vs “Contact”
Before explaining how azoxystrobin behaves inside the plant, it’s essential to clarify the key differences between systemic and contact fungicides:
- Contact (Protectant) fungicides remain on the surface of leaves and stems. They prevent fungal spores from germinating on treated tissue but do not move inside the plant. If new leaves grow after spraying, those new tissues are unprotected unless another application is made.
- Systemic fungicides are absorbed into plant tissues and move internally, providing protection beyond the point of contact. Depending on their transport mechanism, systemic fungicides can move upward through the xylem (acropetal movement), downward through the phloem (basipetal), or across the leaf surface from one side to another (translaminar).
In this continuum, azoxystrobin does not behave as a fully systemic product like true phloem-mobile compounds. Instead, it is a partially systemic, xylem-mobile fungicide—absorbed and transported within the plant, yet not freely mobile throughout all tissues.
How Azoxystrobin Moves Inside the Plant
Azoxystrobin belongs to the QoI (quinone outside inhibitor) or strobilurin class of fungicides. These molecules are designed to balance absorption into plant tissues with persistence on the surface. Their mobility profile can be broken down into three main behaviors:
1. Translaminar Movement
After application, azoxystrobin penetrates the leaf cuticle and moves through the leaf blade from one side to the other.
This means both the upper (adaxial) and lower (abaxial) leaf surfaces gain protection—even if only one side was sprayed. This property makes it ideal for crops with dense canopies or overlapping foliage.
2. Xylem Systemic Movement (Upward Only)
Once absorbed, azoxystrobin travels upward (acropetally) through the xylem—the plant’s water transport system.
It moves with the transpiration stream, protecting newer leaves and growing tips to some extent.
However, it does not move downward (basipetally) through the phloem. Therefore, roots and older leaves below the application zone are not significantly protected.
3. Mesostemic / Surface Redistribution
Azoxystrobin also binds lightly to the leaf’s waxy surface, creating a thin, protective film that resists washing off by light rainfall.
Over time, small amounts of the active ingredient can re-distribute along the surface or re-enter tissues, enhancing the rainfastness and residual protection typical of strobilurin chemistry.
Why It’s Considered “Systemic with Contact Activity”
The combination of these behaviors makes azoxystrobin a hybrid between systemic and contact modes:
| Property | Systemic / Contact Character | Explanation |
|---|---|---|
| Absorption | ✅ Systemic | Penetrates leaf tissue quickly after spraying |
| Translaminar | ✅ Systemic | Moves through the leaf to the opposite surface |
| Xylem movement | ✅ Systemic (upward only) | Moves in the transpiration stream to new growth |
| Phloem movement | ❌ Not systemic | Does not move downward or toward roots |
| Surface film | ✅ Contact-like | Forms a residual layer that resists wash-off |
| Rainfastness | ✅ Partial contact behavior | Maintains efficacy after light rainfall |
Thus, azoxystrobin functions internally to protect existing and some new growth, while also leaving external residues that prevent reinfection—two layers of protection that explain its broad, durable performance in the field.
Practical Implications of Its Systemic Nature
1. Coverage Still Matters
Even though azoxystrobin moves within tissues, thorough spray coverage is vital.
The internal redistribution is limited and localized, so untreated leaf sections or new growth far from the treated area will not be fully protected.
2. New Growth Protection
Because azoxystrobin is xylem-mobile, it provides partial protection to developing leaves above the treated zone. However, this effect is moderate, and continued coverage through repeat applications ensures full canopy protection.
3. Preventive Action Dominates
Its primary value lies in preventing fungal infection before it occurs.
Azoxystrobin interferes with fungal respiration in mitochondria, stopping spore germination and mycelial growth early.
It is less effective as a curative product once the disease has fully established.
4. Rainfastness and Residual Activity
The dual systemic/contact nature gives azoxystrobin good rainfastness—better than purely contact fungicides, though not fully immune to wash-off under heavy rain.
Its mesostemic film slowly releases active ingredient, extending protection for multiple days or even weeks depending on environmental conditions.
Frequently Asked Questions
Q1. Is azoxystrobin systemic or contact?
Azoxystrobin is mainly systemic—absorbed and translocated within the plant—but also provides contact-like surface protection. It’s best described as locally systemic with residual protectant behavior.
Q2. Does azoxystrobin move throughout the whole plant?
No. It moves upward through the xylem and across leaf tissue (translaminar), but not downward through the phloem. Therefore, it is not fully systemic.
Q3. Is azoxystrobin rainfast?
Yes, relatively. Because it penetrates leaf tissue and binds partially to the waxy surface, it remains active after light to moderate rain. However, very heavy rainfall can still reduce its performance.
Q4. Why does it have both systemic and contact properties?
This balance improves field reliability—systemic movement protects internal tissues, while surface residues defend against new spore landings. Together, they extend duration and broaden coverage.
Conclusion
Azoxystrobin represents the modern systemic-protectant hybrid model of fungicide design. It’s not purely contact and not fully systemic—but a locally systemic compound that moves through the leaf and upward through the xylem while maintaining an external protective layer. This dual behavior allows it to act both inside and on the surface of the plant, ensuring robust, preventive control against a wide range of fungal pathogens under varied field conditions.
Understanding the Terminology: “Systemic” vs “Contact”
Before explaining how azoxystrobin behaves inside the plant, it’s essential to clarify the key differences between systemic and contact fungicides:
- Contact (Protectant) fungicides remain on the surface of leaves and stems. They prevent fungal spores from germinating on treated tissue but do not move inside the plant. If new leaves grow after spraying, those new tissues are unprotected unless another application is made.
- Systemic fungicides are absorbed into plant tissues and move internally, providing protection beyond the point of contact. Depending on their transport mechanism, systemic fungicides can move upward through the xylem (acropetal movement), downward through the phloem (basipetal), or across the leaf surface from one side to another (translaminar).
In this continuum, azoxystrobin does not behave as a fully systemic product like true phloem-mobile compounds. Instead, it is a partially systemic, xylem-mobile fungicide—absorbed and transported within the plant, yet not freely mobile throughout all tissues.
How Azoxystrobin Moves Inside the Plant
Azoxystrobin belongs to the QoI (quinone outside inhibitor) or strobilurin class of fungicides. These molecules are designed to balance absorption into plant tissues with persistence on the surface. Their mobility profile can be broken down into three main behaviors:
1. Translaminar Movement
After application, azoxystrobin penetrates the leaf cuticle and moves through the leaf blade from one side to the other.
This means both the upper (adaxial) and lower (abaxial) leaf surfaces gain protection—even if only one side was sprayed. This property makes it ideal for crops with dense canopies or overlapping foliage.
2. Xylem Systemic Movement (Upward Only)
Once absorbed, azoxystrobin travels upward (acropetally) through the xylem—the plant’s water transport system.
It moves with the transpiration stream, protecting newer leaves and growing tips to some extent.
However, it does not move downward (basipetally) through the phloem. Therefore, roots and older leaves below the application zone are not significantly protected.
3. Mesostemic / Surface Redistribution
Azoxystrobin also binds lightly to the leaf’s waxy surface, creating a thin, protective film that resists washing off by light rainfall.
Over time, small amounts of the active ingredient can re-distribute along the surface or re-enter tissues, enhancing the rainfastness and residual protection typical of strobilurin chemistry.
Why It’s Considered “Systemic with Contact Activity”
The combination of these behaviors makes azoxystrobin a hybrid between systemic and contact modes:
| Property | Systemic / Contact Character | Explanation |
|---|---|---|
| Absorption | ✅ Systemic | Penetrates leaf tissue quickly after spraying |
| Translaminar | ✅ Systemic | Moves through the leaf to the opposite surface |
| Xylem movement | ✅ Systemic (upward only) | Moves in the transpiration stream to new growth |
| Phloem movement | ❌ Not systemic | Does not move downward or toward roots |
| Surface film | ✅ Contact-like | Forms a residual layer that resists wash-off |
| Rainfastness | ✅ Partial contact behavior | Maintains efficacy after light rainfall |
Thus, azoxystrobin functions internally to protect existing and some new growth, while also leaving external residues that prevent reinfection—two layers of protection that explain its broad, durable performance in the field.
Practical Implications of Its Systemic Nature
1. Coverage Still Matters
Even though azoxystrobin moves within tissues, thorough spray coverage is vital.
The internal redistribution is limited and localized, so untreated leaf sections or new growth far from the treated area will not be fully protected.
2. New Growth Protection
Because azoxystrobin is xylem-mobile, it provides partial protection to developing leaves above the treated zone. However, this effect is moderate, and continued coverage through repeat applications ensures full canopy protection.
3. Preventive Action Dominates
Its primary value lies in preventing fungal infection before it occurs.
Azoxystrobin interferes with fungal respiration in mitochondria, stopping spore germination and mycelial growth early.
It is less effective as a curative product once the disease has fully established.
4. Rainfastness and Residual Activity
The dual systemic/contact nature gives azoxystrobin good rainfastness—better than purely contact fungicides, though not fully immune to wash-off under heavy rain.
Its mesostemic film slowly releases active ingredient, extending protection for multiple days or even weeks depending on environmental conditions.
Frequently Asked Questions
Q1. Is azoxystrobin systemic or contact?
Azoxystrobin is mainly systemic—absorbed and translocated within the plant—but also provides contact-like surface protection. It’s best described as locally systemic with residual protectant behavior.
Q2. Does azoxystrobin move throughout the whole plant?
No. It moves upward through the xylem and across leaf tissue (translaminar), but not downward through the phloem. Therefore, it is not fully systemic.
Q3. Is azoxystrobin rainfast?
Yes, relatively. Because it penetrates leaf tissue and binds partially to the waxy surface, it remains active after light to moderate rain. However, very heavy rainfall can still reduce its performance.
Q4. Why does it have both systemic and contact properties?
This balance improves field reliability—systemic movement protects internal tissues, while surface residues defend against new spore landings. Together, they extend duration and broaden coverage.
Conclusion
Azoxystrobin represents the modern systemic-protectant hybrid model of fungicide design. It’s not purely contact and not fully systemic—but a locally systemic compound that moves through the leaf and upward through the xylem while maintaining an external protective layer. This dual behavior allows it to act both inside and on the surface of the plant, ensuring robust, preventive control against a wide range of fungal pathogens under varied field conditions.