Curative vs Protectant & Kick-Back Timing
Times the fungicide for apple scab
Has infection already started? Enter the disease, the hours since the infection event and the leaf-wetness conditions, and find out whether you need a protectant or a curative, whether the kick-back window is still open, the FRAC class to choose, and the resistance-rotation caution.
Timing vs the infection
An infection event = enough continuous leaf wetness at the right temperature for spores to germinate and infect.
12 h continuous wetness at 16°C vs Apple scab's ~9 h need (optimum 16°C). Treat as a real infection period.
Next: reach for a curative — Tebuconazole / Difenoconazole (DMI) (FRAC 3, DMI triazole), while you still have ~48 h of kick-back. Then rotate to a different FRAC group and tank-mix a multi-site protectant.
Mode-of-action timing follows the FRAC Code List 2024 + land-grant (Cornell/PSU/MSU/OSU) protectant-vs-curative guides; infection triggers use Mills/Smith-style leaf-wetness × temperature rules. Always confirm activity and timing on your own product label.
Fungicide timing — key facts
- Protectant
- must precede infection
- Curative kick-back
- hours of post-infection reach-back
- DMI (FRAC 3) kick-back
- ~72 h (longest here)
- SDHI (FRAC 7) kick-back
- ~48 h
- QoI (FRAC 11) kick-back
- ~24 h (shortest)
- Eradicant
- early lesions only (e.g. dodine)
- Multi-site anchors
- chlorothalonil (M05), mancozeb (M03)
- Privacy
- Runs in your browser; nothing uploaded
Before infection, or chasing it?
A fungicide can only help if it is applied inside its mode-of-action window relative to the infection event. Protectants form a barrier and must be on the leaf before spores germinate; once the fungus is inside, they do nothing. Curatives can reach back a limited number of hours after infection while the fungus is still in early latent growth — but that kick-back window is short and chemistry-specific, and it closes for good.
This tool puts a clock on that decision. It first confirms whether a wet period actually caused an infection event, then compares the hours since against each curative's kick-back hours to tell you whether you should still be on protectants, whether the curative window is open or closing, and which FRAC class fits — with the resistance-rotation caution every single-site product demands.
Fungicide class timing reference
11 FRAC classes with the forward protectant hours and the post-infection kick-back hours each provides — from the FRAC Code List 2024 and extension fungicide-timing guides.
| Chemistry (example) | FRAC | Mode of action | Activity | Protect. | Kick-back | Risk |
|---|---|---|---|---|---|---|
| Chlorothalonil (Bravo) | M05 | Multi-site contact | protectant | 168 h | 0 h | low |
| Mancozeb (Dithane) | M03 | Multi-site dithiocarbamate | protectant | 168 h | 0 h | low |
| Copper (fixed Cu) | M01 | Multi-site inorganic | protectant | 120 h | 0 h | low |
| Sulfur | M02 | Multi-site inorganic | protectant | 120 h | 0 h | low |
| Azoxystrobin (Quadris) | 11 | QoI strobilurin | curative | 240 h | 24 h | high |
| Tebuconazole / Difenoconazole (DMI) | 3 | DMI triazole | curative | 96 h | 72 h | medium |
| Boscalid / Fluxapyroxad (SDHI) | 7 | SDHI | curative | 168 h | 48 h | high |
| Cyprodinil (Vangard, AP) | 9 | Anilinopyrimidine | curative | 96 h | 48 h | medium |
| Mefenoxam (Ridomil) | 4 | Phenylamide (oomycete) | curative | 168 h | 48 h | high |
| Cymoxanil (Curzate) | 27 | Cyanoacetamide-oxime | curative | 24 h | 36 h | low |
| Dodine (Syllit) | U12 | Guanidine | eradicant | 96 h | 96 h | medium |
Disease infection-period models
| Disease | Pathogen | Wet hrs to infect | Optimum °C | Incubation |
|---|---|---|---|---|
| Apple scab | Venturia inaequalis | 9 h | 16 | 216 h |
| Late blight | Phytophthora infestans | 10 h | 18 | 96 h |
| Early blight | Alternaria solani | 12 h | 24 | 120 h |
| Grape downy mildew | Plasmopara viticola | 8 h | 22 | 144 h |
| Septoria leaf blotch | Zymoseptoria tritici | 24 h | 18 | 360 h |
| Wheat stripe rust | Puccinia striiformis | 6 h | 12 | 240 h |
How to use it — five steps
- 1Select the disease
Its infection threshold and incubation period load.
- 2Enter hours since the infection event
Negative values mean before infection — still fully preventable.
- 3Enter leaf-wetness hours and temperature
The tool confirms whether an infection event actually occurred.
- 4Read the verdict
Protectant, curative open, curative closing, or too late.
- 5Pick the FRAC class and rotate
Take the recommended class and follow the resistance-rotation caution.
Frequently Asked Questions
What is the difference between a protectant and a curative fungicide?+
A protectant must be on the leaf BEFORE spores germinate and infect; it forms a barrier and has no effect once the fungus is inside. A curative (or kick-back) fungicide can reach back a limited number of hours AFTER infection while the fungus is still in early latent growth and arrest it. Once that kick-back window closes, only an eradicant — or nothing — is left, so the time since the infection event decides which class you need.
What is the fungicide kick-back window?+
The kick-back window is the number of hours after an infection event during which a curative fungicide can still stop the developing infection. It varies by chemistry — a DMI triazole offers roughly 72 hours of kick-back, an SDHI about 48, and a QoI strobilurin around 24. This calculator compares the hours since infection against each curative's kick-back hours to tell you whether the window is still open, closing, or shut.
Has infection already started — do I need a curative?+
If the leaf-wetness duration and temperature met the disease's infection threshold and time has since passed, infection has begun and a protectant alone is too late. In that case you need a curative with enough kick-back hours remaining for the time elapsed. Enter the disease, the hours since the infection event and the wet-period conditions, and the tool confirms whether infection occurred and which curative still fits.
How does the tool know an infection event happened?+
It uses a Mills/Smith-style infection model: each disease needs a minimum number of continuous leaf-wetness hours at its optimum temperature to complete infection, and a cooler or warmer period needs proportionally more wet hours. If your observed leaf-wetness hours meet that temperature-adjusted requirement, the tool marks the infection as confirmed; if not, a protectant ahead of the next wet period is still the right call.
Which fungicide has the longest kick-back?+
Among the curatives in this dataset, the DMI triazoles (FRAC 3) offer the most kick-back at about 72 hours, followed by SDHIs (FRAC 7), anilinopyrimidines (FRAC 9) and phenylamides (FRAC 4) at roughly 48 hours, with QoI strobilurins (FRAC 11) shortest at about 24 hours. Dodine (FRAC U12) acts as an eradicant with extended reach-back. The tool picks the curative with the most kick-back hours remaining for your elapsed time.
What does the resistance-rotation caution mean?+
Most curatives are single-site fungicides (FRAC 3, 7, 9, 11) at medium-to-high resistance risk, so using one repeatedly selects for resistant strains and the product fails. The caution reminds you to use the curative once, then rotate to a different FRAC group and tank-mix or alternate with a multi-site protectant such as chlorothalonil or mancozeb, which carry low resistance risk and anchor the program.
Can I just use a protectant for everything?+
Protectants are excellent and low-resistance-risk, but only if they are applied before infection. If a wet period has already triggered infection, a protectant cannot reach the fungus inside the leaf and the disease will develop. The right strategy is to lead with protectants ahead of forecast infection periods and keep a curative in reserve for when an infection event slips past your protective cover.
What happens after the kick-back window closes?+
Once no curative has enough kick-back left for the time since infection, the tool reports it is too late to arrest that infection — an eradicant may still touch very early lesions, but you should focus on protecting the new, uninfected growth and rotating FRAC groups going forward. Lesions already established will run their course; the goal becomes preventing the next cycle of disease.
Does temperature change the infection requirement?+
Yes. Each disease has an optimum temperature at which the listed leaf-wetness hours complete infection; outside a tolerance band around that optimum the pathogen works more slowly, so it needs more wet hours to infect. The tool inflates the wet-hours requirement when the temperature is well off the optimum, which can mean a marginal wet period did not actually cause an infection event.
Is this timing tool a substitute for the product label and a disease model?+
No. The label is the legal authority for crop registration, rate, pre-harvest interval and use restrictions, and a validated disease-forecast model with local weather is the best infection trigger. This calculator distils FRAC mode-of-action timing and extension kick-back guidance to help you choose protectant versus curative, but confirm the infection period with your forecasting system and every figure against the label you hold.