Disease Severity Index Calculator
Compute foliar disease severity index from visual assessments using the standard 0-5 Horsfall-Barratt scale.
About this calculator
This calculator turns a field walk-through into a single, comparable number using the McKinney formula applied to the Horsfall-Barratt 0-5 rating scale. Instead of counting exact lesion percentages on every plant — impractical at scale — you sort scouted plants into six visual damage classes (0% affected up to 76-100% affected), and the calculator assigns each class a representative midpoint (0, 5.5, 18, 38, 63, and 88%). The Disease Severity Index is the sum of each class's count times its midpoint, divided by the maximum possible weighted sum (every plant at the top class), times 100. That gives a 0-100 score you can track week over week or compare across fields, even though no individual assessment was ever more precise than "roughly a quarter of the leaf." Alongside the DSI, the calculator reports disease incidence — simply the percentage of plants with any symptoms at all (everything above class 0) — which answers a different question: how far has the disease spread, versus how bad is it on the plants it has reached.
A field can have high incidence but low severity (widespread but mild) or the reverse (localized but devastating), and conflating the two is the most common misreading of this kind of data. The severity rating (Low/Moderate/High/Severe) is a simple threshold bucketing of the DSI to make results easy to communicate, not a formal action threshold — pair it with crop-specific economic injury levels before deciding on a fungicide application. Accuracy depends entirely on consistent, honest visual class assignment across raters.
Inputs
Results
Disease Severity Index (%)
20.57
Disease incidence (%)
60%
How to Use This Calculator
- Enter Class 0 plants (0% affected), Class 1 plants (1-10% affected), and Class 2 plants (11-25% affected).
- Set Class 3 plants (26-50% affected), Class 4 plants (51-75% affected), and Class 5 plants (76-100% affected).
- Review Disease Severity Index (%) and Disease incidence (%) (%).
- Use Mean severity (%) and Total plants assessed to inform your decision.
How the result changes with Class 0 plants (0% affected)
| Class 0 plants (0% affected) | Disease Severity Index (%) | Disease incidence (%) |
|---|---|---|
| 10 | 25.71 | 75% |
| 15 | 22.85 | 66.67% |
| 30 | 17.14 | 50% |
| 50 | 12.86 | 37.5% |
What each input means
- Class 0 plants (0% affected)
- Number of plants with no visible disease symptoms.
- Class 1 plants (1-10% affected)
- Plants with 1-10% tissue affected.
- Class 2 plants (11-25% affected)
- Plants with 11-25% tissue affected.
- Class 3 plants (26-50% affected)
- Plants with 26-50% tissue affected.
- Class 4 plants (51-75% affected)
- Plants with 51-75% tissue affected.
- Class 5 plants (76-100% affected)
- Plants with 76-100% tissue affected.
What each result means
- Disease Severity Index (%)
- Overall disease severity index on a 0-100 scale (McKinney formula).
- Mean severity (%)
- Average percentage of tissue affected per plant.
- Disease incidence (%)
- Percentage of plants showing any disease symptoms.
- Total plants assessed
- Sum of all plants across all severity classes.
- Diseased plants
- Number of plants with disease symptoms (class 1-5).
- Severity rating (1-4)
- 1 = Low (DSI ≤10), 2 = Moderate (11-30), 3 = High (31-50), 4 = Severe (>50).
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does this calculator use six damage classes with midpoints instead of asking for exact tissue percentages?
Precisely estimating the percent of diseased tissue on every scouted plant isn't practical in the field, especially across a large planting. The Horsfall-Barratt system lets you quickly sort each plant into one of six visual bands, and the calculator substitutes the class midpoint (0, 5.5, 18, 38, 63, or 88%) as a stand-in for the true average within that band. It trades a small amount of per-plant precision for scouting speed and rater consistency.
What's the real difference between the Disease Severity Index and disease incidence?
Incidence is just the percentage of plants with any symptoms at all — everything outside class 0 — so it measures how far the disease has spread. DSI is the count-weighted average severity across every plant, including the healthy ones, so it measures how bad the disease is where it has taken hold. A field can post high incidence with low DSI (widespread but mild) or the reverse (localized but severe), and the two numbers answer different questions.
Why might a field show high incidence but a low Disease Severity Index?
That pattern means the disease has reached a large share of plants but each affected plant only has minor tissue damage — most diseased plants are landing in the low midpoint classes (1 or 2) rather than the higher ones. Because DSI weights by both count and class midpoint, many plants sitting in the light-damage classes pull the overall index down even though incidence, which only counts presence or absence, stays high.
Is the severity rating (Low/Moderate/High/Severe) an actual treatment threshold?
No — it's a fixed bucketing of the DSI score into four bands purely to make the number easier to communicate, not a calibrated economic action threshold. Real spray or intervention decisions should compare your DSI trend against crop-specific economic injury levels from extension guidance, since the dollar impact of a given severity level varies enormously by crop and disease.
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