Agricultural Field Calculators
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Potato Bruise Risk Calculator

Assess blackspot and shatter bruise risk at harvest based on pulp temperature, soil moisture, specific gravity, drop height, vine kill timing, and cultivar.

Temperature
Optimal: 50–60°F. Below 45°F = high shatter risk. Above 65°F = high blackspot + rot risk.
Tuber & Field Conditions
Dry soil → dehydrated tubers → high blackspot risk. Wet soil → turgid tubers → high shatter risk.
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Measure with a hydrometer. Columbia Basin thresholds (recalibrated to WSU trial distribution): <1.074 Low · 1.074–1.080 Med · 1.080–1.087 High · >1.087 Very High. Median for CB process varieties = 1.080. Leave blank to skip.
Harvest Operation
Harvesting before 2 weeks significantly increases skinning injury risk. Late harvest also substantially raises shatter bruise risk.
WSU data: shatter bruise increases with tuber size — <6oz mean 21%, 6–8oz 34%, 8–10oz 40%, 10–12oz 48%. Larger tubers carry more impact energy.

Fill in the harvest conditions and press Assess Bruise Risk to see your risk scores and recommendations.

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Disclaimer — Bruise risk ratings are general guidelines based on published research. Actual bruise outcomes depend on many variables including equipment condition, operator technique, and variety. Always conduct physical bruise sampling to confirm results. This tool does not replace qualified agronomic advice.

About Potato Bruise Risk

What is blackspot bruise and how does it differ from shatter bruise?

Blackspot bruise results from impact crushing cells beneath the intact skin. The bruise is invisible externally but appears as dark discoloration when peeled — typically 24–48 hours after impact. Shatter bruise produces visible cracks or fissures in the skin and is caused by cold, brittle tuber tissue tearing under impact. Both reduce marketable yield, but shatter is immediately visible while blackspot is discovered at the packer or processor.

How is this risk score calculated?

V6: Each factor contributes weighted points to two separate scores — one for blackspot risk and one for shatter risk. Cultivar sensitivity scores are recalibrated against WSU Columbia Basin trial data from 2008–2019 (679 records across 13 seasons, 4 trial types). A key V6 change: harvest timing now interacts directly with cultivar shatter susceptibility — Russet Burbank, for example, shows a 4.5× increase in shatter from early to late harvest in the WSU dataset (13% early vs 59% late). Specific gravity thresholds are recalibrated to the Columbia Basin distribution (median 1.080). Tuber size is added as a shatter modifier based on the WSU size-vs-shatter gradient. Additional sources: UI BUL966, CSU Extension Fact Sheet 5.621, Hendricks & Thornton 2022 AJPR, Mosley et al. 2000.

What other resources exist for potato bruise management?

University of Idaho Extension Bulletin 966 (Monitoring Tools for a Potato Bruise Prevention Program) is the most comprehensive free resource. WSU Extension Bulletin EB1080 covers reducing harvest bruise. Colorado State Extension Fact Sheet 5.621 covers blackspot specifically. For on-farm assessment, the iodine dip method and instrumented sphere (Smart Spud) are the most practical tools. Related calculators: Potato Storage Calculator.

Frequently Asked Questions

The optimal range is 50–60°F (10–15°C). Below 45°F (7°C), shatter bruise risk increases sharply as cell walls become brittle. Above 65°F (18°C), blackspot bruise and storage rot risk increase significantly. Pulp temperature is the single most important harvest factor.
Blackspot bruise is subsurface — cells are crushed beneath intact skin, oxidizing phenolic compounds into dark melanin visible only after peeling, 2–48 hours after impact. Shatter bruise produces visible surface cracks when cold, brittle tissue tears under impact force.
Based on WSU/UI impact trials (Hendricks and Thornton 2022): Clearwater Russet and Ranger Russet had the highest blackspot incidence. Russet Burbank and Umatilla Russet showed moderate-high susceptibility. Teton Russet and Targhee Russet had the lowest blackspot incidence but highest shatter bruise risk.
Higher specific gravity means more dry matter and larger starch granules that rupture cell membranes more severely on impact. SG above 1.085 is elevated risk; above 1.095 is very high risk. Chipping varieties with SG above 1.090 require extra care during harvest.
Wait at least 2 weeks for basic skin set; 3+ weeks for fully set skin. Harvesting before 2 weeks significantly increases skinning injury risk. Maintain adequate soil moisture (60–80% of field capacity) after vine kill to prevent tuber dehydration.
Under 6 inches is optimal. Drops of 6–12 inches create moderate risk, especially at cold temperatures. Drops over 12 inches onto hard surfaces substantially increase both bruise types. Cushioned or rubber belting reduces impact force 40–60% vs bare metal.
A 5-level composite: Low / Low-Moderate / Moderate / Moderate-High / High. Calculated from all seven input factors, weighted 60% toward whichever of the two bruise scores (blackspot or shatter) is worse, since the highest risk drives operational decisions.
Collect 15–25 tubers per location. Warm to 65–70°F for 2–3 hours to accelerate melanin development. Submerge in 1–2% iodine solution for 5 minutes and rinse. Blackspot bruises stain dark blue-black. Count bruised tubers ÷ total for percent incidence.