The major threats facing potato producers this season — and how to manage or prevent them.
Potato growers are familiar with the expression ‘a potato storage is not a hospital’, which reflects the simple truth that diseased potatoes going into storage are not going to get better. It is essential that growers carefully monitor potatoes going into storage and keep a close eye on storage conditions to keep infections from spreading to maintain the value of their crop. It’s also important to understand that managing potato diseases in storage is both an art and a science.
As Steven Johnson, crop specialist with the University of Maine, puts it: “Storage management is not just following general guidelines and recommendations. Every storage manager will be faced with situations requiring special management techniques and corrective measures. This is the art of potato storage management.”
The top five storage diseases that require continuous vigilance during the storage period are:
- late blight
- pink rot
- Pythium leak
- fusarium dry rot
- soft rot
Late Blight
External tuber symptoms appear as slightly sunken, brown to purplish areas of variable size. The flesh has a reddish or tan/brown, dry, granular rot that extends from the skin of the tuber inward an inch or more. Tissues with late blight are firm to the touch, not wet or mushy.
Pink Rot
Infected areas of the tuber surface are purplish-black. The flesh first appears cream coloured, when sliced open, a salmon-pink coloration develops after 15 to 20 minutes at room temperature. The affected flesh has a rubbery, “boiled potato” consistency. A distinct line separates healthy and infected tissue. Pink rot in storage is usually accompanied by a distinctive ammonia odour.
Pythium Leak
External tuber lesions are grey with a water-soaked appearance. The flesh is brown/grey to black; with time it turns inky black. When affected tubers are squeezed, a clear liquid is readily released by the rotted tissue.
Fusarium Dry Rot
Dark depressions develop on the surface of the tuber. The skin becomes wrinkled in concentric rings as the underlying dead tissue desiccates. Internal symptoms are characterized by dry, necrotic tissue shaded from light to dark brown or black. Rotted cavities are often lined with mycelia and spores of various colours from yellow to white to pink. Infected tubers eventually shrivel and mummify.
Soft Rot
Tuber flesh decays and becomes mushy and cream to tan coloured. Often there is a black border between healthy and rotting tissue. Soft-rotted tubers tend to break down easily, spreading bacteria to surrounding tubers. This results in localized pockets of rot or ‘hot spots’.
Correct Identification of each of these diseases is critical to apply post-harvest treatments and to implement specific storage management practices. Harvesting, handling and storing problem potatoes require continuous attention.
Pre-Harvest
Growing conditions and crop management will have an impact on the potato storage period. The storage requirements of the crop can be assessed before harvest by doing several dig tests. Dig tests will allow you to determine if disease is present and the distribution and level of tuber infection.
If there are risky areas in a field such as low spots that have blighted potatoes, do not harvest these problem areas. If up to two per cent of the crop contains blighted potatoes that are scattered throughout the field, try to market the crop straight from the field. If this is not feasible, store the crop near the storage door and move it at the earliest opportunity.
Harvest
Harvest should start as soon as the tuber skin is set. Do not harvest under wet conditions, and avoid bruising. Wounded and bruised tubers are readily attacked by soft rot bacteria. Tuber pulp temperature should be between 10 C and 15 C when lifting the crop.
Post-Harvest Treatment
Phosphorous acid products such as Rampart, Phostrol and Confine applied as post-harvest treatments reduce the incidence of late blight and pink rot in storage (Confine is also registered as a post-harvest treatment to reduce the incidence of silver scurf in storage). Tubers should be rolling when they pass under the spray bar to ensure uniform coverage.
Storage Management
A clean, sanitized storage is a must before storing potatoes. Grade out suspicious tubers and remove debris, clods and dirt before putting the potatoes into the bin.
The three basic storage management tools available are temperature, humidity and airflow. One of the toughest situations potato storage managers face is when they realize that a potato crop in storage is at risk of deteriorating due to diseases. The critical point is to limit pathogen spread from diseased to healthy potatoes and to keep the problem from getting worse.
Curing Period
Proper curing is necessary to heal cuts and bruises produced during harvest, to reduce pathogen spread, and to keep shrinkage losses at a minimum. The recommended storage temperature for curing potatoes at risk of wet rots is 10 C for two to three weeks with continuous ventilation to dry out wet tubers.
The pile should be ventilated with dry air (humidifier off) until there is no further risk of breakdown. In some cases, this may take several weeks. If ‘hot spots’ begin to develop during curing, supply higher airflow to the area to help prevent additional pathogen spread. Supplemental ventilation can be added by auxiliary fans on top of the pile or in the ducts below trouble spots.
Holding Period
After the required curing period, adjust the storage temperature to the holding temperature. Lower holding temperatures decrease the rate of disease progression. Any storage decision involving temperature adjustments must take into consideration the end use of the potatoes. For example, the option of low storage temperatures is not available for processing potatoes, which need to be stored at higher temperatures than the seed and fresh market crop.
Depending upon the nature and percentage of wet rot in the storage, the pile may need additional drying ventilation with reduced-humidity air during this period. However, reduced humidity results in additional shrinkage and also delays wound healing, which can increase the incidence of fusarium dry rot. Decreasing relative humidity in storage to 85 per cent or less can also decrease the secondary spread of the silver scurf pathogen. Still, growers should consider and assess other management strategies before deciding on reducing relative humidity.
Monitor the storages daily. Thermometers suspended at various depths in the pile provide a good indication of the average temperature. Infrared guns are helpful in locating hot spots before they begin to sink and spread.
Processing growers may want to measure carbon dioxide levels. If the amount of CO2 builds up in storage, fry colour can be variable. Carbon dioxide sensors can help to maintain desired CO2 levels.
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