Grower Reference
Fungicides & Mycorrhizae
Not all fungicides are compatible with mycorrhizal inoculants. Use this chart to identify which products are safe to use, which to avoid, and which need more data.
Applying an incompatible fungicide at or near the time of mycorrhizal inoculation can reduce or eliminate colonization. The chart below - sourced from Mycorrhizal Applications, Inc. - covers common horticulture and turf fungicides and their effects on both endo and ecto mycorrhizal fungi.
Why Some Fungicides Harm Mycorrhizal Fungi
Mycorrhizal fungi are fungi — so fungicides designed to kill pathogens can also kill beneficial spores if applied at the wrong time or concentration. The primary mechanism of harm depends on the fungicide class:
- Triazoles and sterol-inhibitors (propiconazole, triadimefon, fenarimol): These block ergosterol synthesis, a compound essential to all fungal cell membranes. Because mycorrhizal fungi rely on the same pathway, triazole-class fungicides are among the most harmful to endo colonization.
- Broad-spectrum contact fungicides (captan, folpet, PCNB): These disrupt cell membranes non-selectively. Most harm mycorrhizae at high rates but have limited effect at low rates, since spores in the root zone receive less exposure than surface-applied pathogens.
- Systemic oomycete fungicides (fosetyl-AL, mefenoxam): These target oomycete-specific pathways that mycorrhizal fungi do not share, making them generally safe for inoculant use.
Fungicide Compatibility Chart for Mycorrhizal Inoculants
OK to use
Avoid use
Insufficient data
| Trade Name | Active Ingredient | Endomycorrhizae | Ectomycorrhizae | ||||
|---|---|---|---|---|---|---|---|
| OK | Avoid | Insuf. Data | OK | Avoid | Insuf. Data | ||
| 3336 | Thiophanate-methyl | OK | ? | ||||
| Aliette | Fosetyl-AL | OK | OK | ||||
| Banner MAXX II | Propiconazole | Avoid | ? | ||||
| Banol | Propamocarb hydrochloride | OK | ? | ||||
| Banrot | Etridiazole + Thiophanate-methyl | ? | Avoid | ||||
| Bayleton | Triadimefon | Avoid | ? | ||||
| Captan | Captan | Avoid | No effect at low rate, tends to suppress at high rate. | ||||
| Chipco 26019 | Iprodione | OK | ? | ||||
| Daconil | Chlorothalonil | OK | No effect at low rate, tends to suppress at high rate. | ||||
| Dithane | Mancozeb | OK | ? | ||||
| Dorado | Propiconazole | Avoid | ? | ||||
| Eagle 20EW | Myclobutanil | OK | ? | ||||
| Fore | Mancozeb | OK | Avoid | ||||
| Fungo | Thiophanate-methyl | OK | ? | ||||
| Heritage | Azoxystrobin | OK | OK | ||||
| Lesan | Fenaminosulf | ? | Avoid | ||||
| Phaltan | Folpet | Avoid | Avoid | ||||
| Prodigy Signature | Fosetyl-AL | OK | OK | ||||
| Proplant | Propamocarb | OK | ? | ||||
| Rally | Myclobutanil | OK | ? | ||||
| Rubigan | Fenarimol | Avoid | ? | ||||
| Senator WSB | Thiophanate-methyl | OK | ? | ||||
| Sentinel | Cyproconazole | OK | ? | ||||
| Signature | Fosetyl-AL | OK | OK | ||||
| Strike | Triadimefon | Avoid | ? | ||||
| Subdue Maxx | Mefenoxam | OK | OK | ||||
| Systec 1998 | Thiophanate-methyl | OK | ? | ||||
| Systhane | Myclobutanil | OK | ? | ||||
| Terraclor | Pentachloronitrobenzene (PCNB) | No effect at low rate, tends to suppress at high rate. | Avoid | ||||
| Terraneb SP | Chloroneb | No effect at low rate, tends to suppress at high rate. | Avoid | ||||
| Terrazole | Etridiazole | ? | Avoid | ||||
| Teremec SP | Chloroneb | No effect at low rate, tends to suppress at high rate. | Avoid | ||||
| Tersan 1991 | Benomyl | Avoid | No effect at low rate, tends to suppress at high rate. | ||||
| Tersan 75 | Thiram | OK | Avoid | ||||
| Turfcide | Pentachloronitrobenzene (PCNB) | No effect at low rate, tends to suppress at high rate. | Avoid | ||||
| Ziram | Zinc dimethyldithiocarbamate | Avoid | Avoid | ||||
General Observations
- The longer you wait to apply any fungicide after mycorrhizal inoculation, the better for mycorrhizal fungi development.
- Most foliar sprays of any fungicide (except systemic products such as Bayleton) have little effect on mycorrhizal fungi.
- This chart covers horticulture and turf applications. Source: Mycorrhizal Applications, Inc. (updated May 2017).
Application Timing Recommendations
- Best: Apply mycorrhizal inoculant first, directly at the root zone. Allow 2–4 weeks before applying any soil-applied fungicide to give the fungal network time to establish.
- If concurrent treatment is unavoidable: Choose a compatible systemic fungicide (fosetyl-AL, mefenoxam, azoxystrobin) and apply as a foliar spray rather than a soil drench to minimize root-zone contact.
- Avoid: Soil-drenching with triazole-class fungicides (Banner MAXX, Bayleton, Dorado, Rubigan, Strike) within 4 weeks of inoculation — these are the highest-risk class for mycorrhizal suppression.
- Pre-treating the growing medium with a broad-spectrum fumigant before inoculation eliminates the mycorrhizal benefit entirely. Inoculate into untreated media or wait until fumigant residues dissipate.