Metalaxyl-M: A Systemic Fungicide for Oomycete Control

Definition:
Metalaxyl-M, also known as Mefenoxam, is a systemic fungicide belonging to the phenylamide group. It is the enantiomerically pure (R-isomer) form of Metalaxyl, offering higher biological activity and improved disease control at lower doses. It is primarily used for the prevention and control of diseases caused by Oomycete fungi such as Phytophthora, Pythium, and Peronospora.

Chemical Characteristics of Metalaxyl-M:

• Chemical Name: (R)-methyl N-(2-methoxyacetyl)-N-(2,6-xylyl)-DL-alaninate

• Molecular Formula: C15H21NO4

• Molecular Weight: 279.34 g/mol

• Appearance: White crystalline solid (technical grade).

• Solubility: Moderately soluble in water; soluble in organic solvents.

• Stability: Stable under normal storage conditions; degrades under strong alkaline or acidic conditions.

Mode of Action:

Metalaxyl-M works by inhibiting RNA polymerase I activity in Oomycete fungi, which disrupts RNA synthesis and protein formation, leading to the death of the pathogen. It is absorbed by plant roots or leaves and translocated systemically throughout the plant, providing both protective and curative activity.

Uses and Applications:

1. Target Pathogens:

   • Effective against Oomycete fungi, including:

     • Phytophthora spp. (blight, root rot)

     • Pythium spp. (damping-off)

     • Peronospora spp. (downy mildew)

2. Crops:

   • Widely used in crops such as potatoes, tomatoes, grapes, cucurbits, tobacco, and ornamental plants.

   • Also applied for seed treatment to protect seedlings from soilborne diseases.

3. Application Methods:

   • Foliar spray for leaf infections.

   • Soil drench or seed treatment for root and seedling diseases.

Benefits of Metalaxyl-M:

• High Efficacy: More active than racemic Metalaxyl, allowing reduced application rates.

• Systemic Action: Provides internal plant protection and prevents new infections.

• Versatile Application: Suitable for foliar, soil, and seed treatment use.

• Curative and Preventive: Effective against early-stage infections and for disease prevention.

Limitations:

• Resistance Risk: Overuse may lead to resistant strains of Oomycete fungi.

• Narrow Spectrum: Active only against Oomycetes; ineffective on true fungi.

• Combination Needed: Often tank-mixed with contact fungicides (e.g., Mancozeb) to delay resistance development.

Recommended Application:

• Foliar Spray: 50–150 g active ingredient per hectare, depending on crop and disease pressure.

• Seed Treatment: Typically 0.1–0.3 g active ingredient per kg of seed.

• Precautions:

  • Rotate with fungicides of different modes of action.

  • Avoid excessive use to minimize resistance risk.

  • Ensure good coverage and proper application timing.

Storage and Handling:

• Storage: Store in original containers in a cool, dry place away from sunlight.

• Shelf Life: Stable for 2–3 years under recommended storage conditions.

• Safety: Low acute toxicity; use personal protective equipment (PPE) during handling and application.

FAQ on Metalaxyl-M:

Q1: What is the difference between Metalaxyl and Metalaxyl-M?
A1: Metalaxyl-M is the R-enantiomer of Metalaxyl, offering higher activity at lower doses, improved disease control, and reduced environmental load.

Q2: Can Metalaxyl-M control powdery mildew or rust?
A2: No, it is specific to Oomycete fungi and does not control diseases caused by true fungi.

Q3: Is Metalaxyl-M systemic?
A3: Yes, it is absorbed and translocated within the plant, providing internal protection.

Q4: How can resistance be managed?
A4: Rotate with fungicides from different chemical classes and use in mixture with protectant fungicides like Mancozeb or Chlorothalonil.