Does a cold wet harvest increase the risk of any particular storage concerns? What should producers keep in mind when preparing for storage in a difficult harvest year? Khalil Al-Mughrabi, pathologist at the Potato Development Centre in New Brunswick, shares his answers to these questions below.
Frequent and excessive rain received during the growing season is potentially conducive, especially in the low spots and/or in poorly drained areas, to the development of some diseases such as pink rot and Pythium leak (water rots).
To avoid the spread of these diseases, growers are advised to:
- 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.
- Stop irrigation well in advance of harvest.
- Avoid harvesting infested fields when soils are especially wet, or soil temperatures are below 50 F or above 65 F.
- Windrow the potatoes to allow the surface of tubers to dry before harvest.
- During harvest, adjusting equipment properly by keeping harvester chains fully loaded and minimizing drop to six inches or less will help avoid skinning or bruising tubers which provides direct entry points for diseases.
- 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.
- Grade out infected tubers as much as possible before placing tubers in storage.
- 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.
- Store lots of harvested tubers containing many infected tubers separately from healthy lots and place them in the front of the storage.
- Lots with significant amounts of disease should be marketed as soon as possible as they will not store well.
The most common potato storage diseases include late blight, pink rot, Pythium leak, Fusarium dry rot, silver scurf, black dot, and bacterial soft rot. The most prevalent storage diseases in Canada are Fusarium dry rot, bacterial soft rot and silver scurf. Late blight is common in years with foliar late blight disease. Temperature is the single most important factor in the keeping quality of stored potatoes. The majority of storage diseases are partially or completely inhibited by storage temperatures below 7.2 C (depending on the variety – consult your local potato specialist). At temperatures above 10.0 C the growth and development of disease organisms increases dramatically, augmenting the risk of total breakdown.
Risk of breakdown is greatest just after the storage has been filled, especially during hot weather. Temperatures above 10.0 C should be avoided during long-term storage. The hatching of flies is also inhibited below 10.0 C, thus the presence of flies indicates that the temperature is too high somewhere in the storage and breakdown may become a problem.
Throughout the storage season, in order to minimize storage loss due to disease, the following points are recommended:
- Do not overfill the storage. There should be at least 4 feet between the top of the pile and the storage ceiling.
- Diseased tubers are subject to secondary infections. Ventilate thoroughly during the curing/suberizing period to dry tubers and prevent secondary infections.
- Avoid free moisture in or on the pile. Adjust ventilation and humidity controls to avoid/prevent condensation.
- Monitor the pile closely for any signs of hot spots.
- Be prepared to move the tubers quickly if necessary.
A good storage management program should include daily checks of the storage. Make sure that the ventilation controls and dampers are functioning correctly especially during very cold weather when the danger of ice buildup is greatest. Use an accurate thermometer to check the air and tuber temperatures at several locations in the storage. A thermometer or temperature probe located 50-100 cm below the top surface of the pile will give an indication of the highest temperature in the storage. Relative humidity can be checked at the same time with a humidity gauge or psychrometer. Be alert for the signs of soft rot development - a pungent smell, depressions in the pile, water in the ventilation ducts and hot spots in the pile. Early detection of soft rot is now possible with the use of infra-red thermometers. These devices, which look like radar guns, can be used to measure temperatures at the top of the pile. Areas of potential breakdown will show up as "hot spots", often as much as three weeks before other symptoms are noticeable. Records of all storage conditions should be kept daily so that if problems arise there is some way of determining the cause.