Mefenoxam Sensitivity in Phytophthora cinnamomi Isolates

Area of Science:

• Plant Pathology
• Mycology
• Agricultural Science

Background:

• Phytophthora cinnamomi is a significant pathogen affecting ornamental plants.
• Mefenoxam is a key fungicide used to manage this disease.

Purpose of the Study:

• To evaluate the sensitivity of Phytophthora cinnamomi isolates from ornamental nurseries to mefenoxam.
• To assess the efficacy of mefenoxam in protecting Lupinus angustifolius from P. cinnamomi infection.
• To investigate the potential for mefenoxam resistance development in P. cinnamomi.

Main Methods:

• In vitro evaluation of 65 P. cinnamomi isolates for mefenoxam sensitivity using agar-based assays.
• Determination of EC50 values for sensitive and intermediately sensitive isolates.
• In vivo assessment of mefenoxam efficacy on lupine seedlings challenged with P. cinnamomi.
• Induction of mefenoxam resistance through UV mutagenesis and mycelial adaptation.

Main Results:

• Most P. cinnamomi isolates (57/65) were highly sensitive to mefenoxam; 8 were intermediately sensitive.
• Mefenoxam provided over 96% disease reduction in lupine seedlings against all tested isolates.
• UV mutagenesis and adaptation generated less sensitive isolates, but with reduced growth rates.

Conclusions:

• Phytophthora cinnamomi populations in Virginia ornamental nurseries exhibit high sensitivity to mefenoxam.
• Mefenoxam is effective in controlling P. cinnamomi in ornamental plant production.
• While resistance can be induced, it may be accompanied by fitness costs, suggesting continued utility of mefenoxam.