The label is the law. Rates and restrictions below are taken from the product labels listed under Sources. Your product's own label governs; read it before you mix or apply. Mixing calculator · Disclaimer
- Active ingredient
- Mono- and di-potassium salts of phosphorous acid (potassium phosphite)
- Chemical class
- Phosphonate; FRAC Group P07, the host plant defense inducers, a group FRAC moved to P07 from its former code 33 in 2018.[2]
- Mode of action
- FRAC Group P07: Host plant defence induction: phosphonates (phosphorous acid and salts)[2]
- Signal word
- Agri-Fos Systemic Fungicide (residential sub-label; also registered as Agri-Fos 400): Caution[1]
Measuring small amounts
The label's conversion table helps with small batches. One-third fluid ounce equals 2 teaspoons, half an ounce is 3 teaspoons, and 1 fluid ounce is 2 tablespoons or 6 teaspoons.[1]
Indoor plants
When treating houseplants, don't overspray and keep it on the target plants. If you can't, move the plants outdoors to spray and dry before bringing them back in.[1]
Where it is used
Before treating a plant species for the first time, spray a few plants, wait 3 days, and check for leaf burn.[1]
Fill the sprayer a quarter to half full of water, add the product slowly while mixing, then top it off. Keep agitating while you spray.[1]
Apply before disease develops. Don't treat plants that are heat or moisture stressed or dormant, and never spray at intervals shorter than the label allows.[1]
Besides foliar sprays, the label allows soil drenches, mixing into potting media, basal bark sprays and bare-root dips, each at its own rate.[1]
Products and registrations
| Product | Registrant | EPA Reg. No. | Active % | Formulation | Signal word |
|---|---|---|---|---|---|
| Agri-Fos Systemic Fungicide (residential sub-label; also registered as Agri-Fos 400)[1] | Liquid Fertiliser Pty. Ltd. (trading as Agrichem) | 71962-1 | 45.8% | liquid concentrate | Caution |
Label rates
| Product | Use | Pests | Rate |
|---|---|---|---|
| Agri-Fos Systemic Fungicide (residential sub-label; also registered as Agri-Fos 400) | Home ornamentals such as azaleas, rhododendrons, roses, boxwood, hibiscus and indoor foliage plants | Downy mildew, powdery mildew, bacterial blight, Phytophthora root rot, black spot | 2–4 tsp product per gallon of water[1] Wet all foliage. Bacterial blight sprays repeat every 7 to 14 days; downy and powdery mildew sprays repeat every 14 days. Powdery mildew and black spot uses are not for California. |
| Agri-Fos Systemic Fungicide (residential sub-label; also registered as Agri-Fos 400) | Home lawns | Pythium; Rhizoctonia; suppression of anthracnose | 5–10 fl oz product per 1,000 sq ft[1] For Pythium, mix in 1 to 5 gallons of water and repeat every 14 to 21 days. For Rhizoctonia and anthracnose, use 2 gallons of water every 7 to 14 days. Those two uses are not for California. |
Safety and environmental hazards
People
Signal word CAUTION: harmful if swallowed, absorbed through skin or inhaled, and it causes moderate eye irritation. Avoid breathing spray mist and wash with soap and water after handling.[1]
Pets
The hazard statement covers people and domestic animals alike. On lawns, the label says not to irrigate or mow until the spray has completely dried.[1]
Bees and pollinators
The residential label has no bee or pollinator statement.[1]
Fish and water
Keep it out of storm drains, ditches, gutters and surface water. Spray in calm weather with no rain forecast for 24 hours so it isn't blown or washed off the treated area.[1]
Common questions
- Is potassium phosphite the same as phosphorous acid?
- The active ingredient is the mono- and di-potassium salts of phosphorous acid. Each gallon of Agri-Fos holds 5.17 lb of the salts, equal to 3.35 lb of phosphorous acid.[1]
- How much Agri-Fos per gallon for roses and shrubs?
- 2 to 4 teaspoons per gallon of water as a foliar spray for most listed ornamental diseases.[1]
- Will diseases become resistant to phosphite?
- FRAC lists the phosphonates, including phosphorous acid and its salts, as low resistance risk.[2]
Sources
- U.S. EPA Pesticide Product Label System, “Agri-Fos Systemic Fungicide label, EPA Reg. No. 71962-1 (EPA-stamped, Dec. 18, 2020).” www3.epa.gov/pesticides/chem_search/ppls/071962-00001-20201218.pdf (accessed 2026-10-06)
- Fungicide Resistance Action Committee, “FRAC Code List 2025.” www.frac.info/media/ljsi3qrv/frac-code-list-2025.pdf (accessed 2026-10-06)