Control of Potato Storage Disease in Theory and Practice
During the summer period, we often remember well how the previous storage season went. Meanwhile, the new harvest is growing in the field and we can still make small adjustments for the coming storage period that will start again in a few months. In addition to the right storage technique, the right use is at least as important. When and how long to dry in which situation. How quickly can the temperature be brought down? And if diseased tubers are present, how should we handle drying and cooling? In this article, information is given on the most common storage diseases and how to deal with them if they are spotted in storage.
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 the 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 the 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 the 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 effect 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.
SILVER SCURF – FUNGUS
In seed potatoes other diseases play an important role. This is caused by the fact that the seed potatoes are not full-grown and sometimes the skin is not set so well when harvest is already started. One of the fungi to pay attention to is silver scurf (Helminthosporium solani) which isn’t hardly visible on fresh harvested potatoes but after a short period in storage in can become visible by a silvery glistening shine on the potato skin. The skin becomes permeable and isn’t protective anymore against evaporation from moisture, resulting in high weight loss and shrinking of the tubers and vitality loss. Storage measures to avoid spreading of the disease is keeping free moisture away from the skin as the fungus can multiply very quickly. Quick drying (1-2 days) of the adherent moisture on the potato skin is the best start of the storage season and avoiding condensation during the complete storage period. Silver scurf doesn’t spread at low temperatures 3-5 °C without free moisture (r.h. < 90%) so this is advised in case of susceptible varieties.
DRY ROT (PHOMA – FUSARIUM)
In contrast to rot caused by bacteria where tubers deteriorate really quick and start leaking, fungi can cause dry rot, without leaking moisture. On the one hand this is an advantage, on the other hand this potato disease can be a disappointing surprise when grading and inspection of the crop starts. Two typical fungi causing dry rot are Fusarium and Phoma (Gangrene). Dry rot starts as small brown spots on the skin, which develop into dry rot lesions and in the end results in a dried out and mummified tuber. Spreading of these fungi can be minimized by harvesting gently and taking care of good and quick wound healing at a minimum temperature of 15 °C for 3 weeks. An undamaged skin avoids fungi (and bacteria) getting into the tuber and moisture getting out of the tuber. In storage Fusarium will spread at higher temperatures (15-20°C), where Phoma multiplies easily at temperatures < 7-8 °C e.g. in cold stores. This is also one of the reasons this storage disease is seen in seed potatoes more often.
ASSISTANCE OF THE STORAGE SPECIALIST
As there are many different diseases which are not so easy to distinguish from each other a storage specialist should be asked to inspect on the situation and identify the typical disease, always together with the farmer who is aware of the growing conditions. As all storage diseases originate somewhere from the seed potato or during the field period, the aim in storage should be to keep the not yet affected tubers healthy till the end of the storage period. With modern climate control computers and ventilation, heating and refrigeration systems, the tools are there to win the battle. Keeping calm and converting theory into practice is the challenge!