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Fungicide Re-Spray Interval Calculator
Estimates how much of a fungicide coat rain removed and when to re-spray, from rainfast hours, rainfall and product residual life.
Fungicide Re-Spray Interval Calculator
Chlorova · updated for 2026
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Click “Calculate” to see your results.
Rainfast hours and residual days here are typical label and extension ranges, not your product's stated interval, and the label is the legal answer whenever it disagrees. Real loss depends on droplet drying speed, humidity, adjuvants and canopy interception more than on the rainfall gauge alone. Alternating active-ingredient groups matters more than any single interval, so keep a resistance-management plan rather than repeating one chemotype, and rotate protectants with systemics across the program.
A fungicide application is a coat of chemistry with a clock on it, and rain either shortens that clock or erases it depending on how long the spray had to dry and whether the product was sitting on the leaf or moving into it. This calculator takes the product group, the hours between spraying and the rain, the inches that fell, and how many days are already gone, then returns a re-spray date with the residual arithmetic shown. It is the follow-up question most growers ask after a storm and no chart answers it.
Working Out the Re-Coat
- Choose the fungicide group you actually applied last time.
- Enter the spray date so the calendar lines land correctly.
- Set how many days have passed since that application.
- Enter the inches of rain that followed the spraying.
- Add the hours between spraying and the start of rain.
- Read the rainfast verdict before the re-spray date itself.
- Note the protection left, which is the days worth of coat.
- Plan the next spray on the printed re-spray date.
- Switch the active-ingredient group when you re-spray again.
- Re-run after each storm instead of trusting the first answer.
Residual Life Minus Rain Damage
Every product carries a residual window in days, roughly fourteen for protectants and biology and three weeks for the systemics listed. From it the calculator subtracts the age of the coat, a rainfall penalty of eight days for every inch past a half-inch shower, and an earliness penalty of a quarter day for every hour the rain beat the rainfast time. Rain that arrives an hour after a protectant with a 24-hour rainfast costs six days of life before any inches fall.
The result is floored at zero, because a coat cannot have negative protection: once the arithmetic is exhausted, the answer is simply re-spray now. Dates travel as day counts internally, so the spray date plus the entered age reconstructs today, and adding the remaining window to today prints the re-spray day without any calendar library involved.
Worked Example: Half an Inch After Two Hours
- You sprayed azoxystrobin on June 12 over a brown-patch lawn.
- It is now June 20, so the coat is eight days old.
- Rainfall measured 0.6 in and arrived six hours after spraying.
- That product's rainfast is roughly two hours, so it had set.
- The margin reads plus four hours, which prints short grace.
- Rain past a half inch costs 0.1 times eight, about a day.
- No earliness penalty applies, because rain arrived after rainfast.
- Fourteen residual days minus eight minus 0.8 leaves 5.2 days.
- June 20 plus five days prints a re-spray target of June 25.
- A protectant instead would have lost far more at six hours.
- That is why group choice drives the whole interval answer.
Rainfast and Residual by Group
The figures driving the formulas, given as typical label and extension ranges for home-lawn use. Read them as planning values: your product's own rainfast statement overrides anything on this table, and so does a local diagnostic clinic's advice.
| Group | Type | Rainfast (h) | Residual (d) | Rain tolerance |
|---|---|---|---|---|
| Azoxystrobin | Systemic | 2 | 14-21 | High once dry |
| Propiconazole | Systemic | 2 | 21-28 | High |
| Chlorothalonil | Protectant | 8 | 14-21 | Moderate |
| Mancozeb | Protectant | 24 | 7-14 | Low |
| Copper | Protectant | 24 | 7-14 | Low |
| Bacillus | Biological | 24 | 5-7 | Very low |
Key Terms
- Rainfast period
- The drying time a spray needs before rain can no longer wash it off.
- Protectant
- A surface barrier fungicide with no curative action once fungus is in the tissue.
- Systemic
- A fungicide absorbed and translocated by the plant, so rain matters far less after uptake.
- Residual interval
- How long a coat keeps useful protective activity on the leaf.
- FRAC group
- The mode-of-action family used to rotate chemistries against resistance.
Program Thinking Over Spray Thinking
- Protectants fail in storms and systemics survive them, which is why a program that leans on one type collapses the first week it rains on schedule.
- The decision that matters most is not the interval but the rotation: two sprays of the same FRAC group in a humid week selects for exactly the strain you are spraying.
- Irrigation after a spray behaves like rain for a protectant, so a 0.2 in overhead soak counts against the coat even when the gauge stayed dry.
- Biology-based controls are curative only as prevention, and their five-to-seven-day clock in humid weather is a feature of living spores rather than weak product.
- Most home brown-patch calls are watering mistakes: evening irrigation keeps leaves wet for twelve hours, and no re-spray interval fixes that schedule.
Common Mistakes & Fixes
- Re-spraying the same chemotype because it is the bottle in the shed.
- Assuming that any rain at all means the whole application was wasted.
- Watering the lawn overhead the night after a protectant spray.
- Skipping the rainfast line and acting on the date alone.
- Spraying on a fixed calendar all season long with no disease actually present.
- Mixing to a weaker rate to stretch the tank and shortening residual.
- Treating a biological at weekly intervals only after damage shows.
When to Use This (and When Not To)
Reach for it the morning after a storm breaks a spray program, and before booking the next application in a humid disease season. It handles timing only: use the spray-tank mixing calculator for the dilution and rate, the sprinkler run-time calculator to move irrigation off the leaf, and send a sample in before assuming a fungicide is what the lawn needs.
Fungicide Re-Spray Interval Calculator: Frequently Asked Questions
How soon can rain wash off a fungicide?
Within the rainfast period, which runs from about two hours for systemics to a full day for protectants and copper products. Rain that arrives before that window has largely passed removes most of the coat, so the safe answer is to count the application as lost.
Do I have to re-spray after every rain?
No, only when rain beat the rainfast time or when the rainfall was heavy enough to cost more residual days than the coat has left. A quarter inch that falls two days after a systemic spray is a non-event, and re-spraying then buys resistance rather than protection.
What is the difference between a protectant and a systemic?
A protectant sits on the leaf surface as a chemical barrier and never enters the tissue, so losing the coat means losing the program. A systemic is absorbed and moves within the plant, which makes it far more tolerant of rain once it has dried in.
How long does a lawn fungicide last?
Roughly two to three weeks for most synthetics in a growing season, and under a week for biological products in humid weather. Heat, UV, leaf growth and irrigation all shorten the window, which is why labels quote intervals instead of a single expiry.
Should I add a spreader sticker to resist rain?
A sticker or oil adjuvant genuinely improves deposition and drying on waxy leaf blades, but it cannot rescue a protectant that was rained off at one hour. Read the label for the adjuvant it allows, since some products forbid them and some require one.
Is it worth spraying again if the disease is already visible?
Usually yes with a systemic, which still has curative action, and largely no with a protectant alone once lesions are established. The better move is one systemic now and a protectant program scheduled behind it.
Sources & Further Reading
- Integrated Disease Management for Turfgrass — Penn State Extension. Basis for protectant versus systemic rainfast behavior.
- Fungicide Resistance and FRAC Rotation — Rutgers Turf Program. Source of the rotate-the-group guidance.
- Home Lawn Disease Identification and Control — University of Maryland Extension. Supports residual intervals and irrigation cautions.
Chlorova Gardening Team
Builds, checks and documents every tool we publish
Chlorova editorial review
Checked against extension-service publications
Formula verified against published references · Updated