Disease Progress Rate & See How Fast the Epidemic Builds
Tracks rust
Most leaf diseases compound — each new lesion seeds more. Enter two severity readings and the days between to get the logistic infection rate r, the severity change and the percent-per-day rate, so you know how fast it is moving and when to act.
Track epidemic progress
Next: the epidemic is growing at 0.3/day; tighten your spray interval and switch to a curative fungicide before severity passes the action threshold.
The logistic infection rate r (van der Plank) describes how fast a polycyclic disease compounds; the per-day percent change is a simpler linear read of the same two assessments.
Disease progress — key facts
- Logit
- ln(x ÷ (1 − x))
- Infection rate r
- [logit(x₂) − logit(x₁)] ÷ days
- Severity change
- reading 2 − reading 1
- Rate per day
- severity change ÷ days
- Steady build
- r ≈ 0.1–0.2 per day
- Explosive
- r above ≈ 0.3–0.4 per day
- Best fit
- polycyclic foliar diseases
- Privacy
- Runs in your browser; nothing uploaded
A rate, not a snapshot, tells you when to act
A single severity reading tells you where the disease is today; two readings tell you where it is heading. Polycyclic diseases — rusts, blights, mildews, leaf spots — compound because every lesion makes spores that start more lesions, so the right measure of speed is a logistic rate, not a flat percentage. The tool converts each reading to a logit and divides the difference by the days between, giving the apparent infection rate r that plant pathologists use to compare epidemics and judge whether conditions are turning against the crop.
This tool reports the infection rate r, the severity change and the percent-per-day rate from two field assessments. Use it to time fungicides before severity passes the yield-loss threshold, to compare varieties or treatments, and to spot when weather is accelerating spread. Pair it with the Weed Control Efficiency and Insecticide MoA Rotation tools for a full scouting-to-spray plan.
Catch it early
A rising r flags an epidemic before symptoms cover the canopy.
Time the spray
Act while a protectant can still hold the disease back.
Compare treatments
Lower r under one programme proves it is working.
Works any foliar disease
Same logistic rate for rust, blight or mildew.
Frequently Asked Questions
What is the logistic infection rate r?+
It is the speed at which a polycyclic disease compounds in your field. The tool converts each severity reading to a proportion x, takes the logit ln(x ÷ (1 − x)) of each, and divides the difference by the days between them: r = [logit(x₂) − logit(x₁)] ÷ days. A higher r means the epidemic is multiplying faster, the same way a higher interest rate grows a balance faster.
Why use a logistic rate instead of a simple percentage change?+
Most leaf and stem diseases are polycyclic — every new lesion makes spores that start more lesions, so spread compounds rather than adding a fixed amount each day. The logistic rate r accounts for that compounding and for the ceiling at 100% severity, giving a rate that stays comparable as severity climbs. A plain percent-per-day figure looks slower near the start and faster in the middle, which can mislead.
What counts as a fast infection rate?+
It depends on the pathogen, but for many foliar fungal diseases an r of roughly 0.1–0.2 per day is a steady build, while values above about 0.3–0.4 per day signal an explosive epidemic that warrants immediate action. Use your first reading as a baseline and watch whether r rises between assessments; a rising r means conditions are favouring the disease.
How do I measure severity for the two readings?+
Walk a fixed pattern through the field and rate the percent of leaf or plant area showing symptoms, ideally using a standard disease-assessment key for that crop so the numbers are repeatable. Take the second reading after a known number of days from the same areas. Consistency between the two assessments matters more than perfect accuracy — the rate depends on the change.
What is the percent-per-day rate the tool also shows?+
Alongside the logistic r, the tool reports the plain severity change (second reading minus first) and that change divided by the days between, giving an easy percent-per-day figure. It is less rigorous than r but very intuitive — for example, severity rising from 5% to 20% over 10 days is +15 points, or 1.5 percentage points per day. Use r for comparing epidemics and the percent-per-day for a quick gut check.
Can I use this to decide whether to spray?+
Yes — a high or rising infection rate is a strong signal that a protectant or curative fungicide is needed before severity passes the point where yield is lost. Combine the rate with the crop's growth stage, the weather forecast and any economic threshold for that disease. The tool tells you how fast the disease is moving; the spray decision also weighs cost, residue limits and resistance management.
Does the tool work for any crop or disease?+
The logistic model fits most polycyclic foliar and stem diseases — rusts, blights, mildews, leaf spots and the like — across cereals, vegetables, fruit and pulses. Monocyclic soil-borne diseases that progress in a single cycle fit it less well. Enter your two severity readings for any disease and the rate follows from the arithmetic.
Why does the rate look strange at very low or very high severity?+
The logit transform stretches towards infinity as severity approaches 0% or 100%, so a reading of exactly 0 or 100 cannot be converted and very extreme readings produce large swings in r. Use realistic field readings — a few percent up to the high nineties — and avoid entering 0 or 100. Near those edges the percent-per-day figure is the safer guide.