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Harvest and Storage of Potato Crops Exposed to High Moisture Levels

Storage management strategies for growers for high moisture conditions.

Frequent and excessive rain received during the growing season is potentially conducive, especially in low spots and/or in poorly drained areas, to the development of some diseases, such as pink rot and Pythium leak (water rots).

Growers with potato crops exposed to high moisture levels should follow the management practices below.

  1. Top kill at least two weeks prior to harvest to allow time for infected tubers to rot and to promote tuber maturity and thicker skin at harvest.
  2. Stop irrigation well in advance of harvest.
  3. Avoid harvesting infested fields when soils are especially wet, or soil temperatures are below 50 F (10 C) or above 65 F (18 C).
  4. Windrow potatoes to allow tuber surfaces to dry before harvest.
  5. During harvest, adjusting equipment properly by keeping harvester chains fully loaded and minimizing drops to six inches or less will help avoid skinning or bruising tubers, which provides direct entry points for diseases.
  6. Identify low spots with flags prior to harvest; these spots should be left in the field unharvested if they have been waterlogged and rot is present.
  7. Grade out infected tubers as much as possible before placing tubers in storage.
  8. Keep tubers cool and as dry as possible during harvest, loading and transit. In storage, the crop should be ventilated with high volumes of air at low humidity until it is dry.
  9. Store lots of harvested tubers containing many infected tubers separately from healthy lots and place them in the front of the storage.
  10. Lots with significant amounts of disease should be marketed as soon as possible as they will not store well.

Along with weather conditions and forecasts, soil and tuber pulp temperatures are key elements affecting your harvest schedule and the storability of your crop.

Managing Potato Crops in Storage

In preparation for the potato storage season, growers must be aware of the potential risk and consequences of storing potatoes infected with pink rot, soft rot, and other diseases if not properly stored. Although late blight was not an issue this year in most Canadian provinces, other diseases can pose a significant threat to healthy potatoes going into storage. Therefore, a good storage management program including daily monitoring and checking for potential threats should be implemented, while maintaining proper temperature and humidity conditions.

To measure the temperature in storage, a thermometer or temperature probe should be employed 50 – 100 cm below top of surface of the pile. By checking daily, soft rot can be detected early using an infrared thermometer. Areas of breakdown will show up as hot spots as often as three weeks before other symptoms are noticeable. Maintaining detailed daily records of all storage conditions and activities is essential. If unexpected problems arise, there is some way of determining the cause and resolving it.

Ensure the functionality of the ventilation control system and dampers, especially during very cold weather when the danger of ice buildup is greatest. In addition, the relative humidity (RH) in storage should be high enough to prevent shrinkage losses and pressure bruising.

Generally, about 92 – 97 per cent of RH for dry healthy potatoes and 85 – 90 per cent for wet, leaky potatoes are recommended — these conditions in ventilated storages can only be reached using a humidifier. To achieve the above conditions, air movement in storage is essential not only over-the-pile but also through-the-pile to maintain recommended constant optimal temperatures. Any storage holding over 160 tonnes should have a forced air circulation system that can blend outside air with inside air.

 Storing Healthy Potatoes

Wound healing and curing are essential for the development of a good skin to reduce water loss and minimize the danger of rots. If the potatoes are dry and in good condition, curing can begin soon after the storage is full. Adjust the pile temperature to 13 – 15.5 C and maintain for 10 to 14 days along with 92 – 97 per cent RH and one- to two-hour daily ventilation. This curing period for processing potatoes may need to be extended up to four to five weeks to improve colour. Colour should be tested weekly during curing of processing potatoes. After the curing process, healthy potatoes should be kept at a specific holding temperature according to the end use.

Storing Problem Potatoes

When storing problem potatoes, growers may want to consider one of the following optional storage conditions. In most cases, when potato lots are mixed with infected tubers, such potatoes should initially be dried in storage as quickly as possible using extensive air circulation. When the temperature of tubers brought to storage is greater than 20 C or less than 7 C, it should be adjusted at a rate of 2 – 3 C per day until the curing temperature is reached.

Meanwhile, drying of potatoes should be the first priority when the danger of soft rot exists due to blight, frost, pink eye, blackleg, or excess moisture. Until the risk of breakdown is over, the pile should be ventilated with dry air (humidifier off) which may take several weeks or even months.

In cases of severe frost or disease damage, maintain pile temperature at 8 – 9 C for three to four weeks before lowering further to 3 – 4 C. Low temperatures will keep disease organisms in check while enhancing the drying process. However, if processing potatoes are involved, the possibility of losing colour due to low temperatures should be taken into account.

Tubers harvested under wet conditions late in the season may be subject to pink rot and Pythium leak infections. Since these diseases can hasten the spread of soft rot in storage through watery ooze from infected tubers, good ventilation in storage is essential to manage them.

With adequate air circulation, infected tubers will become mummified and not undergo a wet rot. As such, you can minimize the risk of soft rot by drying the tubers quickly by continuously circulating air (minimum 2 cfm/barrel or 1.2 cfm/cwt) during the first 30 days of storage. Never wet the tubers that are going into storage.

Finally, it is important to keep in mind that in most years there will be a combination of potential problems. For that reason, it may be a good idea to ventilate continuously for a few days prior to curing just to evaluate the quality of your crop while creating uniform pile conditions. If the crop is in reasonably good condition, the humidifier can be turned on during ventilation, otherwise it should be off. 

Cooling and Holding Periods

Seed stock should be cooled at 1 C per one to two days up to 3 – 4 C for holding since rapid temperature fluctuations may shorten dormancy. Table stock should be cooled at 1 C per three to four days up to 4.5 – 5.5 C for holding.

Rapid cooling can affect cooking and eating quality. Processing quality should be maintained when storing processing stock. Cool at 1 C per week since rapid cooling tends to cause sugar accumulation. Holding temperature should be 7.2 C. Stock for French fries before Christmas can be held at 10 C while chip stock can be held at 10 – 12 C. Either stock can be held at 8 – 10 C up until early May. Summer processing stocks should not be stored above 10 C. For better quality, it may be beneficial to maintain at 4 – 5 C until early May, and then warm gradually to 13 – 15.5 C, four to six weeks prior to shipping.

It’s important to remember the following:

  • Adjust the pile with problem potatoes to the holding temperature at a rate of 2 – 3 C per day.
  • Never overfill the storage because this hampers air flow and increases the chances of tuber breakdown due to rots. Therefore, the gap between the top of the pile and the ceiling should be at least 0.6 metres (2 feet). If the potatoes are otherwise dry and disease free, the humidifier can be turned on during this period especially during hot, dry conditions.

Khalil Al-Mughrabi is a pathologist for the Potato Development Centre at New Brunswick’s Department of Agriculture, Aquaculture and Fisheries, and an adjunct professor at Dalhousie University.

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