Harvest Operations Tuber bulking vs vine kill timing Equipment operation Crop maturity Vine killing Bruise susceptibility

Potato Growth Stages

Physical Maturity (Skin set)

Methods to measure skin set Torque High Low High Low Skinning damage

Water loss from a potato in storage 98% through skin 2% through lenticels Increases water loss by 3 to 5 fold

Changes in tuber composition with maturity Physiological Maturity = 145 DAP Knowles et. al., WSU, Pullman

Physiological Maturity (chemical maturity) Defined as the average of date when: Maximum yield Maximum specific gravity Minimum sucrose Minimum reducing sugars

Physiological Maturity Dates Days after planting CultivarMax YieldMax SGMin SucroseMin SugarsDAP to PM Ranger Burbank Umatilla Alpine Clearwater Trailblazer * All cultivars reached maturity between 2800 and 3000 degree days

Sugar Ends can Increase With Increasing Maturity of Ranger Russet Courtesy Mel Martin, J.R. Simplot Co.

Crop Maturity Factors that influence crop maturity: Variety (Determinate vs indeterminate) Emergence date Nitrogen fertilization Irrigation Disease pressure

Crop Maturity Important reasons for correct maturity : Simplify vine killing Minimize harvest damage Maximize tuber solids Minimize tuber sugars Maximize storability

Vine Killing Reasons: Regulate maturity Prepare tubers for handling Control tuber size Coordinate harvest with appropriate season (need to balance equipment capabilities with climatic conditions)

One main focus of harvest management is bruise reduction - Estimated to cost the US potato industry $298 million annually (1996) - Processing contract incentives - Rejections at market (5% internal defects) - Storage losses (Shrink and rot) - Losses in productivity (processing and fresh pack)

Black spot bruise (a chemical reaction) occurs just beneath the vascular ring (not visible), and skin is not broken. Shatter bruise breaks the skin, and is a physical damage.

Chemistry of Black Spot Bruise Brook, R.C Potato Bruising. National Potato Anti-bruise Committee Tyrosine (a substrate) and polyphenol oxidase (PPO - an enzyme) mix within a cell when cell membranes are damaged Oxidation (addition of oxygen) occurs forming intermediate compounds End result is melanin (black, gray, or brown color) Process takes 12 to 72 hours, depending on temperature

Source: Thornton et al., 1973 Effect of tuber susceptibility and harvester operation on bruise damage

Tuber Physical Properties Affecting Bruise Tuber Size o Large tubers > damage small tubers o Small radius (end) > damage large radius Cell Size o Large cells > damage than small cells o Growth conditions play large roll in cell size Age o Blackspot increases with vine maturity o Tubers less susceptible after one month curing – tubers can withstand 30-80% more impact Brook, R.C Potato Bruising. National Potato Anti-bruise Committee

Tuber Susceptibility to Black Spot Bruise Brook, R.C Potato Bruising. National Potato Anti-bruise Committee Mineral Nutrition  N and P do not directly affect susceptibility.  Indirectly – deficiency results in earlier senescence and more mature tubers.  Indirectly – high specific gravity > blackspot than low specific gravity (N and P both affect specific gravity).

Research on Potassium and Black Spot Bruise  Black spot bruise potential decreased with increased potassium application in excess of that needed for optimum yield.  Shatter bruise increased with more potassium.  For each 100 pounds potassium, specific gravity decreased 2.5 to 3.5 points. Porter, Greg and P.C. Ocaya. Bruise susceptibility and potassium uptake of Russet Burbank potatoes in response to varied potash rate and timing. University of Maine. Presentation at 2005 PAA meeting. Calgary.

Two requirements for blackspot bruise: -Susceptibility to impact damage - Potential to darken after impact 0 = No Damage 5 = Most Damage

Field Maturity as it Relates to Blackspot Bruising of Russet Burbank Potatoes Blackspot Severity Group 1 Field Maturity (%) 2 Resistant (<2.5) Mod. Susceptible (2.5 – 3.0) Susceptible (3.1 – 3.5) Very Susceptible (>3.5) Corsini, D., J. Stark, and M. Thornton American J of Potato Research. P Abrasive peel test ratings where 0 = no blackspot and 5 = most damage. 2. Percent dead vines in fields in late August. 3. R-square values: 1993 = 0.37; 1994 = Both significant at P = Mean

Specific Gravity as it Relates to Blackspot Bruising of Russet Burbank Potatoes Blackspot Severity Group 1 Specific Gravity 2 Resistant (<2.5) Mod. Susceptible (2.5 – 3.0) Susceptible (3.1 – 3.5) Very Susceptible (>3.5) Corsini, D., J. Stark, and M. Thornton American J of Potato Research. P Abrasive peel test ratings where 0 = no blackspot and 5 = most damage. 2. Average for all fields in the survey within a blackspot severity group, No relationship between specific gravity and bruise in 1993.

Soil Moisture as it Relates to Blackspot Bruising of Russet Burbank Potatoes Blackspot Severity Group 1 Soil Moisture (%) 2 Resistant (<2.5) Mod. Susceptible (2.5 – 3.0) Susceptible (3.1 – 3.5) Very Susceptible (>3.5) Corsini, D., J. Stark, and M. Thornton American J of Potato Research. P Abrasive peel test ratings where 0 = no blackspot and 5 = most damage. 2. Available soil water at time of tuber sampling; average for all fields within a severity group. Data for 1994 only. Little to no differences in percent soil moisture in 1993.

Factors Increasing Black Spot Bruise Susceptibility  More mature vines Russet Burbank - 70% dead vines Ranger Russet - 20% dead vines  Higher specific gravity Russet Burbank - Above Ranger Russet - Above  Available soil moisture less than 60%

Management Through Harvest -Tuber hydration -Drop height -Pulp temperature -Drop surface

Tuber Hydration Dehydrated (limp) Hydrated (crisp) % Damage Tuber Hydration Level Effect on Black Spot and Shatter Bruise of Russet Burbank at 42ºF Smittle, D.A., et al Harvesting Potatoes with Minimum Damage. Am. Potato J. 51:

Influence of Post-Vine-Kill Irrigation on Blackspot Bruise of Lemhi Russet Potatoes Irrigation Treatment Percent Blackspot No irrigation 1 4 days before harvest 2 8 days before harvest Stark, J. C. January University of Idaho Potato School Proceedings. P Soil moisture at 50% or less at vine kill. 2. No effect of irrigation if soil moisture kept at 65% or above after vine kill.

Impact of temperature and drop height on bruise incidence Hyde, G., R. Bajema and R. Thornton, 1993

Impact of temperature and drop height on bruise incidence 50 oF on coated chain Steel surface

Relationship Between Soil Temperature and Tuber Bruising During a 24-Hour PeriodMoreLess % Bruise MN 4 am 8 am Noon 4 pm 8 pm MN Time of Day Smittle, D.A., et al Harvesting Potatoes with Minimum Damage. Am. Potato J. 51: Temperature (°F) % Bruise Soil Temperature