February is the period of the year that a lot of storages are still filled with potatoes. These crops are maintained at their storage temperature and if there are no problems with rot, being a store house operator doesn’t take more efforts than a daily check of potato temperatures and a weekly inspection of the potatoes themselves in the storage.

In this expert view article written by Jan van Maldegem, product manager from potato storage supplier Tolsma-Grisnich, information is presented on most commonly occurring storage diseases and how to handle this when they are faced in storage. In winter periods the insulation of the storage and the climate control computer takes care of an optimal inside climate to keep moisture inside the potatoes and respiration at a minimal level. Because of that there is more time available in the working schedule of a store house operator to gain new knowledge or to update knowledge. Storage diseases cannot be seen apart from the growing period prior to storage. As potatoes are planted in different soil types in different climates, often under irrigation, many organisms are trying to profit from the tubers as a food source. This can be done by wild animals who really dig up the tubers to eat them, but most of time the organisms attacking the planted seed potatoes are not even visible and their number is overwhelming: bacteria, fungi, viruses, insects and nematodes. When optimal climate conditions are there, bacteria and fungi can cause serious harm to potato tubers which can result in rot or extra moisture loss during the storage period. Most of time it will not really cause big problems during storage when there is just a small percentage of affected tubers. But during specific phases of storage, climate conditions may be very supportive for a specific bacteria or fungus to expand. In this article we’ll focus on a number of potato diseases which can cause serious problems in the storage period. Some problems will occur more often in seed potatoes and others in processing potatoes. This distinction has to be made and has two reasons. The quality requirements for seed or processing are different and the storage conditions are different. So, although we do talk about potatoes in both situations different storage diseases need to be discussed.

Late blight – potato disease

The world’s best known potato disease late blight caused by the fungus phytophthora infestans causes tuber infections both in the field and during harvest/storage. There is a higher risk of tuber infection in wet and heavy soils. Tuber infection is first being recognized by change of color on some spots on the potato skin to blue. After some time, the spots dry out and the surface becomes rough and knobbly. The color of the affected tissue changes to rust-colored. Late blight tuber affection is often succeeded by other types of rot (fungi and bacteria) which cause the leakage of moisture. Due to weight/pressure of above laying tubers in storage moisture may start leaking from deteriorating tubers. This free moisture is risky because bacteria and fungi can easily spread and affect other healthy tubers. The fungi will start sporulating on the affected and wet spots and by ventilation this is easily spread over the storage. The different stages are not always easy to see, so when there has been an infection during the field period a thorough inspection of the tubers before (during) harvest is necessary to know exactly what percentage of the tubers is affected. If this is too high maybe the best decision is not storing such a crop or at least only for a very short period. Depending on how many affected tubers come into storage late blight can be kept under control by regularly drying away the leaking moisture from the tubers. Tuber deteriorating is a slow process with late blight and can last up to 20 weeks. This means continuous drying is not necessary and undesirable because it will also dehydrate the healthy tubers. Regular drying e.g., two times per day for one hour can easily keep the surrounding tubers dry and will avoid new sporulation. But this must be done for 20 weeks, which means that potato temperature should be kept at such level that outside air can always be used for this. So, don’t cool down to quick. Cold potatoes are always difficult to dry. When there is no possibility to dry with outside air and the storage is just equipped with a refrigeration system one should at least consider decreasing the humidity by stopping the humidification system. Together with ventilation/circulation this will at least support the evaporation of free moisture. Potato temperature should be kept below 15 °C because optimal fungus growth is between 15 – 20 °C.

Potato tuber rot – bacteria

Potato tuber rot or wet rot is caused by bacteria. In potato the genus of Erwinia bacteria is a well-known name causing rot in potatoes. The problem is that these Erwinia bacteria are highly contagious and can be latently present for a long time. The best way to avoid problems with wet rot is the use of healthy, certified seed and by taking care of cultivation in such a way that excessive rain is removed quickly so that lack of oxygen in the potato ridge doesn’t take place.  Wet and anaerobic conditions are ideal for this group of Erwinia bacteria. Potatoes will start rotting very quickly and can start leaking moisture in such a way that ponds can be found on the floor or in the underfloor air ducts. Once again, inspection before harvest of the potatoes in the ridge is the best advice to avoid storage problems like this later one in the season. However, if there are a limited number of rotting tubers found during harvest the advice is to switch on the fans without any delay! The aim should be to dry the moisture (80% of the tuber) as quickly as possible. In fact, this can only be done if outside conditions are suitable to heat up cold air with a heater to create a high drying rate. Drying can only be done quickly by using cold outside air which is heated up by the heat of the potatoes and by this a large moisture deficit is created to take up moisture from the rotting tubers. The disadvantage is that by using cold air the potatoes will cool down and the drying rate goes down. So additional heat is necessary to maintain the potatoes at such a temperature that there are enough possibilities during the day to dry. This temperature should be around 15 °C, at least below 18 °C for that being the optimal temperature of Erwinia bacteria to expand. One should take care of inspecting the drying process twice a day and tuning the climate control computer to maximum drying, otherwise the situation will not stabilize. When the situation is stable regular inspection remains necessary, because when there is a period of less drying/ventilation rot can start expanding very suddenly again. This means potatoes like these are not suitable for long-term storage.

You can read the rest of this article in your complimentary e-copy of the January/February Issue of Potato Processing International magazine, which you can access by clicking here.