Disease Progress Rate & See How Fast the Epidemic Builds

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.

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.

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.

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.

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.

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.

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.

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.