Late blight comes back from last year's potatoes, not 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.

UMass Extension, 'Late Blight Management for Fall, Winter and Spring', umass.edu/agriculture-food-environment/home-lawn-garden/fact-sheets/late-blight-management-for-fall-winter-spring (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): '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' - 'Left in the ground at harvest, down several inches in the soil'; 'Disposed of in a compost pile that does not fully decompose and does not freeze'; 'Tomatoes will not carry late blight over the winter, because freezing kills the whole plant'; 'Tomato seed, even from fruit that was infected with late blight, will not carry the pathogen'; 'In the spring (April - June), inspect last year's potato plot and any compost or cull piles for volunteer potato plants that might come up. If you find potato plants, pull them out and put them in the trash or destroy them.'

UMN Extension, 'Late blight of tomato and potato', extension.umn.edu/disease-management/late-blight (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'P. infestans can overwinter in Minnesota if protected in potato cull piles'; 'The host range is typically limited to potato and tomato, but hairy nightshade (Solanum physalifolium) is a closely related weed that can readily become infected'; 'Control volunteer potato plants, as infected plants can grow from infected tubers.'