Hi, Sara! Thank you for contacting  AskExtension/Ask a Master Gardener with your questions about potato gardening. You asked about soil preparation after potato harvest for fall gardening and you asked about the cause of larger potatoes rotting while smaller potatoes were okay. I’m going to answer in reverse order.

The two most common reasons for potatoes rotting before harvest are overly wet soil, including soil that does not drain well, and pathogens including bacteria and fungus. It is not unusual for larger potatoes to exhibit rot, as they have been exposed to conditions for a longer period, while smaller potatoes are still viable. This article from the Ohio State University describes a similar scenario and discusses methods to improve soil drainage. If wet conditions and/or soil drainage don’t appear to be a problem in your garden, pathogens might be your issue. This article from University of Massachusetts Extension or this article from University of Illinois Extension, can help you diagnose and treat.

Your other question was about soil preparation. Some suggestions: 1) If you suspect that a pathogen affected your potato crop, you might want to solarize your soil to help reduce bacteria and fungus. This article from Oklahoma State University Extension describes the process, which takes four to six weeks and is best done July through September. 2) Adding organic material, such as compost, is beneficial. 3) Completing a soil test will help you determine what nutrients to add to your soil. You can find more information about soil testing here. 4) And finally, a three-year crop rotation is recommended for gardens where potatoes are grown. (See here for that and other great advice about growing potatoes.) Additionally, if you suspect a pathogen, avoid planting other members of the Solanaceae family of plants including peppers, tomatoes and eggplant in the same location, as they can be susceptible to the pathogens.

In preparing to answer your question, I referred to several research-based sites to ensure that my response was accurate and current. I have included links to the best of those sites, embedded in the response and listed below. I hope you find them interesting and helpful and that you have great success with your gardening. If you need further information or clarification, you can reach me directly by replying to this email. I would love to hear from you!

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Resources for you:

Ohio State University What Happened to My Potatoes? https://u.osu.edu/bhanr/2015/06/30/what-happened-to-my-potatoes/ Description of overly wet conditions causing large potatoes to rot in ground.

University of Massachusetts Extension Potato, Identifying Diseases https://ag.umass.edu/vegetable/fact-sheets/potato-identifying-diseases

University of Illinois Bacterial Soft Rot https://bpb-us-w2.wpmucdn.com/u.osu.edu/dist/9/34289/files/2017/07/Bacterial-Soft-Rot-of-Vegetables-Fruits-and-Ornamentals-1g3ql7k.pdf Information about diagnosis and treatment of soft rot

Oklahoma State University Soil Solarization for Control of Soilborne Diseases https://extension.okstate.edu/fact-sheets/print-publications/epp-entomology-and-plant-pathologhy/soil-solarization-for-control-of-soilborne-diseases-epp-7640.pdf Describes process for soil solarization.

The Ohio State University Extension Soil Testing for Ohio Lawns, Landscapes, Fruit Crops and Vegetable Gardens https://ohioline.osu.edu/factsheet/hyg-1132

Seed Savers Exchange Growing Guide: Potatoes https://seedsavers.org/grow-potato/?gad_source=1&gclid=Cj0KCQjwzby1BhCQARIsAJ_0t5NOpIFSD7u5ySKfqUATTpkE9lkvwVmc0B6-fdzN3jkrsrM4gryVujEaAp5jEALw_wcB Recommends crop rotation.

Britanica List of Plants in the Family Solanaceae https://www.britannica.com/topic/list-of-plants-in-the-family-Solanaceae-2026039 tomatoes, peppers, eggplant

Ohio State University Crop Profile for Potatoes in Ohio https://ipm.osu.edu/sites/ipm/files/imce/OHpotato.pdf Descriptions and treatments for Late Blight and Pink Rot.

Utah State Extension How to Grow Potatoes in your Garden https://extension.usu.edu/yardandgarden/research/potatoes-in-the-garden Videos on potato gardening. Use of hills for drainage and tuber development.

ND State Extension Potatoes Garden to Plate https://www.ndsu.edu/agriculture/extension/publications/potatoes-garden-table ; Pink Rot, Late Blight (Tuber Rot)

University of Minnesota Extension Growing Potatoes in Home Gardens https://extension.umn.edu/vegetables/growing-potatoes Information about Hollow Heart disorder.

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