Potato scab and a soil that leans alkaline
Scab likes a higher pH. Don’t lime a potato ground by habit.
- Scab is on the tuber skin, not the soil surface.
- Worse on alkaline, dry, sandy ground.
- Machine damage is broken skin; scab is corky spots.
- Cracked skin tracks water; scab is a disease.
- Don’t lime a scab field; don’t spray and ignore pH. pH: Follow the production-area target. Don’t go alkali. Test the soil, then talk
- Name what you see before you feed.
- If leaves, roots and the surface disagree, stop.
What you see in the field
Scab is on the tuber skin, not the soil surface. Worse on alkaline, dry, sandy ground.Squat on the whitest, worst strip and scrape the crust. Compare with a neighbour that is still green. White crust plus scorched margins count; wilt without a crust may just be drought. Note whether the ditch actually runs.
Tap to enlarge. The caption says what to photograph. Missing field photos are marked as to-be-added.
Why it looks like this
The field looks like this because the root cannot take water or nutrients the usual way. Fix the soil condition first — drainage, salt, acidity, compaction or a salt of fertiliser — then a thin feed. Rates live in the range on this page; confirm with a local station and the label.
Easy to mix up
Looks like “Machine damage”
Machine damage is broken skin; scab is corky spots.
Looks like “Nutrient-shortage cracked skin”
Cracked skin tracks water; scab is a disease.
Do this / don’t do that
- 1Compare the worst plants with a neighbour ridge. Photograph whole plant, close-up and soil surface before you add fertiliser.
- 2Match roots, leaves and the soil surface to this guide. If they do not line up, do not copy a bag count.
- 3Use the rate range on this page only as a range. Confirm with a local station and the product label.
Don’t do this first
- Don’t lime a scab field; don’t spray and ignore pH.
Rate range
pH
Follow the production-area target. Don’t go alkali
Test the soil, then talk.
Water
Even through tuber set
Wet-dry swings also hurt.
These are ranges. Confirm with your local extension station and the product label.
How soon after a spray or a feed it showed
A 1–3 day burst looks more like spray injury / fertiliser burn.
When you see Scab is on the tuber skin, test. Walk an S and mix — one hole is not the field.
Lesson video
2026-03-14 · Li Zeyu · For learning. Serious damage: local agronomist or a soil test.
For learning only. Fertiliser and pesticide decisions need a local agronomist and a soil test.
Not sure? Photograph the field to identify the problem