Disease Severity Index & Score the Field, Track the Spread
Scores leaf blight
Enter plant disease ratings on a 0–9 (or 0–5) scale, number of plants and maximum grade to get the Percent Disease Index (PDI %)and severity class — to track spread over time and compare treatments objectively.
Score disease severity (PDI)
Next: at 26.7% (high) decide whether to spray now and re-score the same plants after each cycle to track whether the disease is advancing.
PDI = (Σ ratings ÷ (plants × max grade)) × 100. Always rate the same tagged plants on a fixed 0–max scale so scores are comparable over time.
Disease severity index — key facts
- PDI formula
- (Σ ratings ÷ (plants × max grade)) × 100
- Max possible score
- plants × maximum grade
- Common scales
- 0–9 or 0–5
- Grade 0
- healthy plant
- Tracks
- spread over time
- Compares
- fungicides & varieties
- Sample size
- ≈ 10–25 plants/plot
- Privacy
- Runs in your browser; nothing uploaded
Turn eyeballed disease into one comparable number
Judging disease by eye is unreliable: "looks bad" means different things on different days and to different people. The Percent Disease Index fixes that by scoring each plant on a fixed scale and rolling the ratings into a single 0–100% figure — PDI = (sum of plant disease ratings ÷ (plants × maximum grade)) × 100. Once disease is a number, you can track whether it is spreading, holding or retreating, and compare plots side by side.
This tool gives the PDI percent, the maximum possible score and a severity class from your plant ratings, plant count and maximum grade. Use it to monitor epidemics across the season, judge whether a fungicide or resistant variety is working, and report results in a standardised, defensible way. Pair it with the Economic Threshold, Pest Degree-Day and Fungicide Spray Interval tools to plan when and how to act.
Score objectively
One 0–100% number instead of a guess.
Track the spread
Re-score over time to see disease progress.
Compare treatments
Lowest PDI shows what's working best.
Report cleanly
Standardised index anyone can reproduce.
Frequently Asked Questions
What is the Percent Disease Index (PDI)?+
The Percent Disease Index quantifies how much disease is present in a field on a single 0–100% scale. It is calculated from individual plant disease ratings: PDI = (sum of all plant disease ratings ÷ (number of plants × maximum grade)) × 100. It lets you track disease spread over time and compare treatments objectively rather than by eye.
How is PDI calculated?+
Rate each plant on a fixed scale (commonly 0–9 or 0–5), add up all the ratings, then divide by the maximum possible score — the number of plants multiplied by the maximum grade. Multiply by 100 to express it as a percent. For example 20 plants rated against a maximum grade of 9 give a maximum possible score of 180; if the ratings sum to 54, the PDI is 30%.
What disease rating scale should I use?+
Use a standard ordinal scale where each grade describes a band of leaf or plant area affected — 0–9 and 0–5 scales are the most common in plant pathology. Grade 0 is healthy and the maximum grade is the most severely diseased. Keep the same scale across all assessments so the PDI values are comparable.
What is the difference between disease incidence and severity?+
Incidence is the share of plants showing any symptoms (a yes/no count), while severity — what PDI measures — captures how badly each affected plant is hit. A field can have high incidence but low severity (many plants, light symptoms) or the reverse, so PDI gives a fuller picture than incidence alone.
Why use PDI instead of just counting diseased plants?+
Counting diseased plants only tells you how many are affected, not how severe the disease is on each. PDI weights every plant by its severity grade, so a few heavily diseased plants register differently from many lightly affected ones. That makes it far better for tracking progression and judging treatment effects.
How does PDI help compare treatments?+
Score plots under different fungicides, varieties or schedules on the same scale and calculate PDI for each. The treatment with the lowest PDI is suppressing disease best. Because PDI is a standardised percent, you can fairly compare plots, dates and even trials run by different people, as long as the rating scale is the same.
What does the severity class tell me?+
The severity class translates the PDI percent into a plain-language band — for example healthy, low, moderate, high or severe. It is a quick read of where the field stands so you can decide whether to spray, change product, or keep monitoring, without having to interpret the raw percent every time.
How many plants should I rate?+
Rate a representative sample — typically 10 to 25 plants per plot taken at random or along a diagonal — so the PDI reflects the whole field rather than a hotspot. The more plants you score, the more stable the index. Keep the sampling method consistent between assessments.
Are the figures precise?+
PDI is a solid, repeatable index, but it depends on consistent scoring. Different assessors can rate the same plant slightly differently, and the ordinal scale groups severity into bands. Use the same scale, train assessors, and re-score at regular intervals — PDI is about tracking trends and comparing treatments, not exact biology.