Metalaxyl-M (Mefenoxam): A Systemic Fungicide for Oomycete Disease Control

Technical Introduction

Metalaxyl-M (also marketed as Mefenoxam) is the (R)-enantiomer of metalaxyl, belonging to the phenylamide class of fungicides. It was developed to provide the same disease control as metalaxyl but at half the application rate, due to its higher biological activity.

• Mode of Action: Inhibits RNA polymerase I in oomycete fungi, preventing ribosomal RNA synthesis. This stops fungal growth and reproduction.

• Target Pathogens: Specially effective against Oomycetes (Phytophthora, Pythium, Peronospora, Plasmopara).

• Systemic Properties: Absorbed by roots, stems, and leaves, then translocated both upward and downward in plants (xylem and phloem systemic).

Applications

• Crops Protected:

  • Vegetables (tomato, potato, cucumber, onion, brassicas)

  • Grapes

  • Ornamentals

  • Tobacco

  • Oilseed crops

  • Turfgrass

• Diseases Controlled:

  • Phytophthora infestans (late blight in potato and tomato)

  • Plasmopara viticola (downy mildew in grapes)

  • Peronospora destructor (downy mildew in onion)

  • Pythium spp. (damping-off in seedlings and turf)

  • Peronosclerospora spp. (downy mildew in maize)

• Application Methods:

  • Foliar spray

  • Soil drench

  • Seed treatment

  • Granule formulations for turf and ornamentals

Advantages

• Enantiopure (R-isomer): Twice as active as racemic metalaxyl, so lower doses are needed.

• Systemic Action: Moves within the plant for preventive and curative protection.

• Versatile: Suitable for foliar, soil, and seed treatments.

• Low Toxicity: Considered safe for mammals, birds, and beneficial insects.

• Rainfastness: Remains effective after rainfall due to rapid absorption.

Specifications (Typical Parameters)

FAQ

Q1: What makes Metalaxyl-M different from Metalaxyl?
A: Metalaxyl-M is the R-isomer of metalaxyl and is more biologically active, so it achieves the same efficacy at about half the dosage.

Q2: Which pathogens does it control best?
A: Oomycetes such as Phytophthora, Pythium, and Plasmopara. It is not effective against true fungi like Fusarium or Alternaria.

Q3: Can Metalaxyl-M be used preventively?
A: Yes, it provides both preventive and curative activity, though preventive application is most effective.

Q4: Is resistance an issue?
A: Yes, resistance to phenylamides (like metalaxyl and mefenoxam) has been documented in downy mildews and late blight pathogens. It is recommended to use in rotation or mixtures with fungicides of different modes of action.

Q5: What are common mixtures?
A: Frequently combined with mancozeb, copper oxychloride, or chlorothalonil to delay resistance and broaden disease control.