When I first began working for University of Idaho, one major concern of the potato industry was tuber bruising. Some 20-plus years later, tuber bruising still seems to raise its ugly head from time to time. Not all factors relating to tuber bruising are within the control of a potato producer but several very important factors are. Producers need to know what these factors are in order to do all they can to minimize tuber damage.
1) Soil condition - Soil moisture should be 60-80 percent of field capacity. Wet soil is difficult to separate from tubers during harvest while soil that is too dry separates too quickly. This subjects the tubers to bare harvester conveyor chain, resulting in potentially higher levels of tuber damage.
Ideally, soil should be carried only to the end of the secondary conveyor on the harvester. A well-timed irrigation prior to harvest should be planned to make sure soil is at the proper moisture level. Granted, potato producers may need to contend with an unwelcome rain during harvest, making it challenging at times to have the desired soil moisture.
2) Tuber hydration - Fully hydrated (crisp) tubers are more susceptible to shatter bruise, while dehydrated (limp) tubers are more likely to manifest physical damage as black spot bruise. Consequently, the tuber hydration level should be midway between these two extremes. If tuber hydration is too low, irrigate at least one week prior to the projected harvest date to allow adequate time for tubers to absorb water and become more hydrated. If rain occurs, making the tubers too hydrated, harvest may need to be delayed to allow the soil to dry.
3) Tuber temperature - During potato harvest, temperature is one of the most important factors. Producers need to particularly pay attention to temperature because cold tuber pulp temperatures dramatically increase bruising potential. Whether there is a greater percentage of shatter bruise or higher levels of black spot bruise depends on tuber hydration. Regardless, there is greater potential for increased tuber bruise damage of all types with tuber pulp temperatures below 50 degrees.
Ideally, all potatoes should be harvested with a pulp temperature of 50-60 degrees. Harvesting tubers with pulp temperatures warmer than 60 degrees becomes a problem if tubers are put into storage, resulting in increased rot potential, especially if tubers cannot be rapidly cooled.
Again, it is realized that producers work in the real world, and management decisions may need to be made that preclude harvesting all potatoes within the ideal temperature range. With cold conditions, producers need to especially pay attention to minimizing drop heights-ideally, no more than 6 inches-and keeping conveyors loaded to capacity. The only way to know if tuber pulp temperatures are in the desired range is to monitor the pulp temperature of every truckload. This task could be assigned to each truck driver or to one individual. Be sure the pulp thermometer is accurate. To check, simply fill a small container with ice and just enough water so the thermometer makes good contact with the ice. The thermometer should read at or near 32 degrees F.
4) Harvester operation - Most tuber damage will occur on a harvester, so make sure the harvest conveyors are filled to capacity, which may require adjusting the conveyor speeds. Detailed information for adjusting conveyor speeds developed at Washington State University can be found by logging onto PotatoGrower.com, clicking on "Extras" and selecting "Potato Harvest Management by William H. Bohl." Also, be sure the handling equipment and conveyors used at the storage facility are not damaging tubers. PG