Curative vs Protectant & Kick-Back Timing

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.