Global: Tubers Bruise Far More Easily When Pulp Temperature Falls Below 7 °C
Harvest advice from Canadian specialists sets a pulp window of about 10–16 °C. Cold tubers, low potassium and long drops all raise blackspot and shatter bruise risk.
Bruising is one of the costliest quality defects in potatoes. Much of it is decided on the day of harvest. Tuber temperature, crop nutrition and handling all play a part.
Temperature sets the risk
Potato specialist Eugenia Banks set out the key thresholds in Spud Smart on 3 September 2024. Tubers are very sensitive to bruising when pulp temperature is below 45 °F, or about 7 °C. The optimal range for harvest is 50 to 60 °F, or about 10 to 16 °C. Above 65 °F, or about 18 °C, the risk of soft rot and Pythium leak rises.
Ryan Barrett of the PEI Potato Board gave the same window of 10 to 16 °C in Spud Smart on 17 September 2026. He wrote that cold potatoes are much more susceptible to blackspot bruise, shatter bruise and cracking.
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Varieties differ. Mark VanOostrum of WD Potato in Ontario told Spud Smart in 2019 that Atlantic is prone to white knot bruise below 12 °C. He advised harvesting Lamoka at 12 to 13 °C, three to four weeks after vine kill.
Nutrition and skin
Crop nutrition also matters. Emmerson McMillan of Yara Canada told Spud Smart in 2019 that low potassium, boron and calcium all contribute to bruising. He said potassium below 10% in the first tissue sample needs immediate action. Calcium uptake varies by variety.
Skin set protects tubers too. Banks advised waiting at least 14 days after vine death before harvesting potatoes for long storage. She named 60 to 65% available soil moisture as optimal at harvest. Dry soil produces more clods and more impacts.
Handling
Drop height is the third factor. Scott Graham wrote in Spud Smart in 2022 that all drops should be six inches, about 15 cm, or less, including the loading boom. Electronic impact recorders can find the worst points on a harvester or grading line. High specific gravity crops bruise more easily, so dense processing potatoes need extra care.
Why it matters
Blackspot often shows only weeks later, after peeling or cutting. Processors and packers then reject or discount the lot. A cold snap during harvest can therefore cost growers more than a short delay. Monitoring pulp temperature each morning is a simple and cheap safeguard.
Photo: Mechanised potato harvest in Brittany, France (illustrative). Thesupermat / Wikimedia Commons, CC BY-SA 2.5
