Late blight comes back from last year's potatoes
Late blight comes back from last year's potatoes. It does not come back from the soil. Tubers left in the ground carry it through winter as volunteers, so ground that grew potatoes last season needs its volunteers pulled before this year's tomatoes or potatoes go in.
Promising A person has checked it against its sources. If you skip this, it still grows, just less well. The planner suggests it.
How it works
Phytophthora infestans needs living tissue to overwinter. Infected tubers left in the ground, in a compost heap that does not freeze, or in a cull pile survive and sprout as volunteers that carry the blight into the new season; tomatoes do not carry it, because frost kills the whole plant and the seed is clean.
What to do about it
From April to June, walk last year's potato ground and any compost or cull pile, pull every volunteer potato and put it in the bin. Don't compost it. Then plant.
The evidence
Evidence: checked against 2 sources in the research-based literature.
The 2 sources
Potato tubers that are infected with late blight and don't freeze or decay during the winter can carry the pathogen over the winter to next spring
P. infestans can overwinter in Minnesota if protected in potato cull piles
Plants it applies to · 2
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How this rule has changed
Every change is dated, newest first. The full record, with each reason, is in the data download.
- Wording made clearer
- Added to the rules
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Milpa Gardens. “Late blight comes back from last year's potatoes. It does not come back from the soil. Tubers left in the ground carry it through winter as volunteers, so ground that grew potatoes last season needs its volunteers pulled before this year's tomatoes or potatoes go in.” Evidence grade: Promising. https://milpa.garden/rules/late-blight-comes-back-from-last-year/
A garden planner built on graded evidence. Every claim states its evidence grade and links its sources on the page. Cite it with the grade, and add the date you read it.
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How we grade evidence
- Well established
- The rule names its mechanism and is settled, by sources a person has read and checked or by plain arithmetic and geometry.
- Promising
- The mechanism is named and the literature backs it. A few questions are still open.
- A good hunch
- Plausible by its mechanism, but field studies are thin, so treat it as a lean.
- Contested
- The evidence points both ways, or no study has tested it, so the planner does not enforce it.
- Refuted
- The claim did not survive its sources. It is kept so the folklore can be answered.