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Preventing potato storage diseases


November 7, 2008
By Lukie Pieterse

Topics

Most potato growers are aware of the fact that storage diseases of
potatoes are seldom curable, but there are ways to help prevent – or at
least limit – disease spread to healthy potatoes and to keep a problem
with diseased tubers in storage from getting worse.

Most potato growers are aware of the fact that storage diseases of potatoes are seldom curable, but there are ways to help prevent – or at least limit – disease spread to healthy potatoes and to keep a problem with diseased tubers in storage from getting worse.

When a crop is produced and harvested, most growers have only reached the halfway mark of the crop cycle – storing the crop for several months is a crucial and potentially risky phase when growers can encounter numerous problems if management of the stored crop is not done right. But, similar to crop cultivation, adherence to certain ground rules can help to ensure that a crop is marketed successfully at the end of the storage phase. What are these ground rules?

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Identify poential diseases
It is of great importance that growers identify risky lots that go into storage, and also pinpoint the nature of a potential diseased batch. “Knowing what tuber disease is causing the problem will also help in field management decisions for the following growing seasons,” says Dr. Nora Olsen, extension potato specialist at the University of Idaho.

The major diseases that plague potato storages include: soft rot, pink rot, Pythium leak, late blight, dry rot, soft rot, silver scurf, and black dot.  It is important to properly identify the major storage disease problem either before loading the storage or while the potatoes are in storage prior to making any changes to typical storage management procedures, according to Dr. Jeff Miller, assistant professor of plant pathology at the University of Idaho.

“Disease management decisions can impact other potato quality attributes,” he said. Dr. Miller is of the opinion that there are three basic tools of storage management: temperature, humidity, and airflow. “Balanced use of these tools is the key to managing potato storage diseases.”

Be mindful of pulp temperature at harvest time
If potatoes are harvested with pulp temperatures above 60 F, effort should be made to cool potatoes to at least 60 F within the first two to three days after harvest. Dr. Olsen and Dr. Miller advise that this is especially important for tubers infected with late blight, pink rot, Pythium leak and soft rot since warmer temperatures promote a faster rate of decay. According to Dr. Olsen, the storage manager should supply a high volume of air to the potatoes to decrease the temperature and to help dry out rotted or wet potatoes. The ventilation air system should be initiated as soon as the first two to three ducts are covered in the loading process. Then the storage manager should continue to decrease storage temperatures to the desired curing temperature.

Wound healing important
Proper management of the storage facility during the wound healing or “curing period” is critical, noted Dr. Phil Nolte, extension seed potato specialist at the University of Idaho. “The curing period is important for the proper healing of cuts and bruises, the reduction of disease spread, and to keep shrinkage losses at a minimum,” he said. “Wound healing requires oxygen, high humidity and favourable temperatures. The recommended storage regime for wound healing and curing is typically 50 to 55 F for two to three weeks with good ventilation and a high relative humidity of 90 to 95 per cent.”

Dr. Nolte noted that curing temperatures might need to be lowered to 50 F if disease incidence and severity are high. But storage managers should be aware that curing temperatures below 50 F may delay wound healing and are not generally recommended unless they are necessary and the curing time can be extended. “Providing storage conditions for proper wound healing is important for dry rot disease control,” Dr. Nolte says.

Dr. Olsen recommends continuous ventilation to dry out wet potatoes. She cautioned that if “hot spots” begin to develop during the curing time, the storage manager should supply high airflow to the area to help prevent additional disease spread. It is of great importance that sufficient air supply be provided within the storage environment to remove water given off by the decaying tubers.

The storage manager should reduce the storage temperature from the curing to holding temperature as quickly as possible (at around 0.5 F per day). According to Dr. Miller, limiting the amount of time the disease-infected tubers are held at warmer temperatures will decrease the rate of disease progression. A fast “ramping rate” may not be appropriate for potatoes to be processed where sugar concentrations and fry color are a concern. 

Holding temperatures
Lower holding temperatures typically decrease the rate of disease progress and can be used to the storage manager’s advantage, although any storage decisions involving temperature adjustments must take into consideration the end-use of the potato. Dr. Nolte noted that processing potatoes typically need to be stored at higher storage temperatures (44 to 55 F), and therefore the luxury of low storage temperatures are not an option compared to seed and fresh market potatoes, which can be stored at 37 to 45 F. Continuous fan operation and high airflow may still be necessary to dry out wet or problem potatoes, especially those infected with soft rot, pink rot or pythium leak.

Depending upon the nature and the percentage of rot that might be present in the storage, the pile may need additional drying time by reducing the humidity in the storage facility, although this management procedure comes at a cost: additional shrinkage of tubers will most likely be the result. “Lowered relative humidity also delays wound healing, which can increase the incidence of dry rot. Decreasing relative humidity in storage (85 per cent relative humidity or less) can also decrease the secondary spread of silver scurf,” Dr. Olsen said.
The University of Idaho specialists cautioned that storage managers should keep in mind that any deviation from normal storage practices can potentially have a negative effect on the quality of stored potatoes. Under some conditions, early marketing may be the only option to preserve the value of a stored crop or a particular batch of potatoes in storage.