Disease Watch

[deck]The major threats this potato storage season — and how to manage or prevent them.[/deck]

Potato diseases may originate in the field and spread to healthy tubers through natural or mechanical wounds, such as bruises, cracks, abrasions, or damages caused by diseases. Such wounds allow the pathogens to penetrate into healthy tubers, where they can proliferate and produce an abundance of spores or cells that can cause more infections, even under favourable storage conditions.

Storage rot is difficult to manage, but there are many strategic measures that can help limit the spread of pathogens from diseased to healthy tubers.

Leading Potato Storage Diseases

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. Growers should also be on the alert for late blight infection, which is common in years when in-crop foliar late blight disease is present, as it has been across the country this season.

Fusarium dry rot

Fusarium dry rot often begins from external tuber wounds and subsequently results in dry and crumbly decay that is tan to dark brown in colour. Rotted tissues often form internal cavities lined with fungal mycelia and spores of white to orange or yellowish-tan in colour.

Soft rot

Soft rot is a bacterial disease which often causes secondary breakdown of already infected tubers. Infected tissues are extremely soft and often appear as tan to brown-coloured, water-soaked areas or granular tissues. Although rot created by soft- rot bacterium is relatively odour free, secondary organisms usually cause a foul smell.

Under most storage conditions, tubers infected with any storage disease, except silver scurf, can be followed by soft rot, causing further breakdown of affected tissues. In a potato pile, soft rot spreads downward, potentially leading to the breakdown of the entire pile within a few days.

Late blight

Late blight causes dark external skin discoloration. Internal flesh tends to have reddish or tan brown granular internal dry rot. Depending on the length of infection, peeling of skin over the affected area and soft rot development due to secondary infections can be observed as well.

Pink rot

The outer skin on tubers infected with pink rot turns dark. Although infected tubers maintain a normal shape, the internal flesh has a rubbery texture and turns pink after 15 to 20 minutes of exposure to air. In addition, due to breakdown of tissues by the pathogen and secondary organisms, pink rot in storage is often accompanied by a distinctive ammonia odour.

Pythium leak

Pythium leak causes dark grey, brown or black rot or water soaked interior margined by a dark line. When infected tubers are cut and squeezed gently, a clear liquid comes out.

Silver scurf

Silver scurf causes external skin blemishes ranging between grey to silver in colour. Under humid storage conditions, these primary lesions produce secondary lesions and the problem becomes severe. Virtually the entire tuber surface can be covered, and eventually there is increased water loss from affected tubers, resulting in tuber shrinkage and deformation.

 Post-Harvest Potato Treatment Products

Source: Khalil Al-Mughrabi, Potato Research Centre, New Brunswick DAAF

Managing Diseases in Storage

Avoiding tuber damage during harvest can reduce disease problems in storage. Cultural practices and fungicide programs can be combined to give producers an edge in combatting potato storage diseases.

Fungicides

The table at right lists some fungicides and disinfectants that can be used to treat potatoes as they are being loaded into storage to protect them against diseases — consult your local potato specialist for up-to-date recommendations on registered and most effective post-harvest products.

Potato Handling

All conditions that influence storage disease development should be properly managed. Grading out any damaged or diseased potatoes before the tubers enter storage is important. Storage bins and farm machinery should always be thoroughly washed and disinfected before the new crop is harvested and stored.

Storage Environment

Harvested potato tubers are living organisms and hence interact with the surrounding environment. To maintain potato quality during storage, the storage environment must be regularly monitored and adjusted when necessary to minimize tuber deterioration. Temperature, humidity and air movement can always affect the keeping quality of stored potatoes. When potatoes are not properly stored, tuber losses due to fungal and bacterial infections can be high.

Temperature

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 (this depends on the variety — consult your local potato specialist for more information). At temperatures above 10 C, the growth and development of disease organisms increases dramatically, augmenting the risk of total breakdown of the pile. Risk of breakdown is greatest just after the storage has been filled, especially during hot weather. Temperatures above 10 C should be avoided during long-term storage. The hatching of flies is also inhibited below 10 C; therefore, the presence of flies indicates that the temperature is too high somewhere in the storage area and breakdown may become a problem.

Air Circulation

Air movement includes both through-the-pile ventilation and over-the-pile ventilation (recirculation). Through-the-pile ventilation is necessary to dry and cool the potatoes and supply fresh air, as well as remove carbon dioxide, volatiles and excess heat and moisture from the storage facility. Recirculation aids in maintaining uniform temperature conditions throughout the storage and also sweeps moisture from the walls and ceiling.

A storage facility with inadequate insulation or poor air circulation may experience excess moisture buildup. This can lead to water dripping on the pile, which must be avoided at all costs in order to minimize the danger of exposing the crop to rot. Adding extra insulation and placing fans above or on top of the potato pile will improve air circulation and help eliminate condensation problems.

Overfilling the storage facility hampers air flow and increases the chances of tuber breakdown from soft rot and pink rot. There should be at least 0.6 metres (two feet) between the top of the pile and the storage ceiling.

Daily Checks

A good storage management program should include daily checks of all storage bins. 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 to 100 centimetres below the top surface of the pile will give an indication of the highest temperature within the storage area.

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 infrared thermometers. These devices, which are similar in appearance to 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.

Daily records of all storage conditions should be maintained, so that if problems do arise there is some way of determining the cause.