Start Thinking About Harvest Now To Ensure a Bruise-Free Crop
Producers need to attend to numerous details during the growing season to raise a quality potato crop. It begins by properly working the soil followed by paying attention to planting accuracy, and then protecting the crop from diseases and insects during the growing season, while also maintaining proper plant nutrition. The final step in obtaining a quality crop is harvesting tubers with minimal damage. Minimizing bruising is actually a season-long process, but a large percentage of keeping bruise damage in check occurs at harvest.
In areas where vines are mechanically or chemically killed, the time of vine kill in relation to maturity can play a role in potential blackspot bruising. Research has indicated that for certain varieties, more mature vines have greater potential to blackspot bruise. For Russet Burbank, a field should not have more than 40 percent dead vines when killed. However, vine maturity has no effect on varieties such as Russet Norkotah that are not as susceptible to blackspot bruise.
To establish a vine maturity rating for a field, view a field as a whole to estimate the amount of vines that are senescing due to natural vine death, or sample representative areas within a field and count senescing versus green plants to estimate percent maturity. Keep in mind, though, that vine maturity is only one aspect of minimizing blackspot bruising.
Another factor linked with bruising is soil condition. Unfortunately, clods that pose a problem at harvest are those formed from working the soil too wet in the spring. There’s not much that can be done to address this problem for the current crop. Spring clod elimination needs to be kept in mind for the next potato crop. Where irrigation is an option, a light irrigation before harvest may soften some of the clods, helping to break them up during harvest rather than hitting and damaging the tubers.
Anyone in the potato industry has likely heard that warm, dehydrated tubers will blackspot bruise, and cold, hydrated tubers will shatter bruise. This is true, but there’s more. When it comes to temperature, the colder the pulp, the more susceptible a tuber is to damage, regardless of tuber hydration. A cold, dehydrated tuber will incur more blackspot bruise damage than a warmer one. Likewise, a cold, hydrated tuber will incur more shatter bruise damage. The bottom line is cold tubers bruise more easily.
Ideally, tuber pulp temperature should be 50˚ F to 60˚ F, with a tuber hydration level midway between very dehydrated and completely hydrated. It’s simple to check tuber pulp temperature with a thermometer, but there’s no instrument available for determining hydration level. The person managing the harvest operation has to rely on experience to evaluate tuber hydration level.
Before using a pulp thermometer to check tuber pulp temperature, be sure it is giving an accurate temperature reading. To check thermometer accuracy, fill a small container with crushed ice and add just enough water to get good contact between the thermometer and the cold water. If the thermometer isn’t reading 32˚ F or very close to that, adjust it to read that temperature. If it’s not adjustable, invest in a new one.
If tuber hydration level is too low, an irrigation application should be made about one week before harvest to rehydrate tubers. Irrigations applied less than a week before harvest to condition the soil so it readily separates from the tubers during harvest will not allow adequate time for the tubers to absorb the moisture.
Back to tuber temperature, the question may arise: If warmer pulp temperatures result in less bruise, why is the upper limit for desired tuber temperature during harvest 60º F? It’s not that warmer temperatures are negatively associated with bruising, it’s because warm tubers placed into storage have an increased risk of rotting. If tubers arrive at storage with pulp temperature above 60º F, be certain the storage facility has a ventilation system with the capacity to rapidly cool the tubers. To accomplish this, the ventilation system needs to be operating before the storage is completely filled.
As mentioned above, bruise management is mostly a harvest management issue. The one piece of equipment likely to cause the most damage is the harvester. It’s important to maintain the harvester and correctly operate it to minimize tuber damage. All conveyors on the harvester need to be filled to capacity with tubers to reduce bruising. Tubers striking other tubers are less likely to be damaged than tubers hitting a conveyor chain link. The conveyors on the harvester should be set to speeds in relation to harvesting ground speed.
Whether conveyors on the harvester are filled to capacity can be visually estimated. To do this, operate the harvester in the field and have someone observe the amount of tubers on each conveyor. The conveyor that is more likely to cause tuber damage is the rear cross. If there is more than one layer of tubers on the rear cross, it is likely operating at full capacity. If there is a single layer of tubers on the rear cross or there are empty spaces, the conveyor is under-loaded. One option to correct this is to slow the conveyor to a speed that allows its speed to be synchronized with the speed of the secondary conveyor. Another option is to increase the volume of tubers from the secondary by increasing the forward speed of the harvester and the speed of the primary and secondary conveyors. A properly timed harvester should separate most of the soil from the tubers by the time the tubers reach the end of the secondary conveyor. The remaining conveyors should be carrying mostly tubers.
Many potato operations use numerous pieces of equipment to deliver tubers from a truck into the storage facility. Every piece of equipment used to move tubers has the potential to cause damage, including pilers, conveyors, dirt eliminators or other pieces of equipment. Keep all drop heights to a minimum, cushion all impact points and operate conveyors at full capacity.
One last point to keep in mind, in order to harvest a potato crop with minimum bruise, is the human factor. A manager can’t be everywhere all the time, so he or she must rely on the workers to help ensure the crop is being handled carefully to reduce bruising. Providing training to assure that workers understand and accept their role in minimizing bruising is an essential part of a safe and successful harvest. Be sure each employee understands the consequences of excessive drop heights, the role hard surfaces that are not padded play in damaging tubers, and especially what a properly loaded conveyor looks like which can be accomplished by providing visual examples.
Efforts to educate employees about bruising provide an excellent opportunity to conduct a training session about safety. Even seasoned employees should be reminded about potential hazards during potato harvest and what they can do to keep safe.
This is also an appropriate time to educate potato-harvest employees about keeping foreign material out of the potatoes. Inform them about the importance of not bringing glass containers to work, using trash bins and keeping a sharp lookout for foreign material in the crop.
William H. Bohl is Extension educator-potatoes with the University of Idaho in Blackfoot. He can be reached via e-mail at firstname.lastname@example.org. Robert E. Thornton is Extension specialist/horticulturist emeritus, Washington State University, Pullman, Wash.