StorageDisease PreventionKnow Your Enemies

Know Your Enemies

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[deck]Storage diseases can be headache-inducing, but with the right strategies, growers can keep on top of potential problems this fall.[/deck]

When preparing for the storage season, loss of potatoes is one of the main problems that growers have to contend with annually. There are a number of causes for loss of potatoes during storage, including respiration, transpiration and—very often—disease.

Spud Smart consulted with Khalil Al-Mughrabi, pathologist at the Potato Development Centre of the New Brunswick Department of Agriculture, Aquaculture and Fisheries, about the most common storage-related diseases growers need to take particular note of this coming season, and information on preventing and managing these diseases.

According to Al-Mughrabi, the most prevalent storage diseases in Canada are fusarium dry rot, bacterial soft rot and silver scurf. Late blight of stored potatoes is common in years when there have been incidences of foliar late blight disease. He points out that tuber diseases in storage may often originate in the field and then spread to healthy tubers through natural or mechanical wounds, such as bruises, cracks, abrasions or damage caused by the disease itself. Such wounds allow the pathogens to penetrate healthy tubers, proliferate and produce an abundance of spores or cells that can cause more infections under favourable storage conditions.

The Line-up

The term storage rot refers to a common group of diseases that in most cases is difficult to manage. There are several strategic measures that can help growers limit the spread of pathogens from diseased to healthy tubers. But first, the disease line-up:

Fusarium dry rot is a disease that affects tubers in storage and seed pieces after planting. It often begins from external wounds and subsequently results in dry and crumbly, tan to dark brown decay. Rotted tissue often forms internal cavities lined with fungal mycelia and spores of white to orange or yellowish-tan in colour.

Soft rot is a bacterial disease which often causes secondary breakdown of already infected tubers, says Al-Mughrabi. “Infected tissues are extremely soft and often appear as tan- to brown-coloured, water-soaked areas or granular tissues,” he points out. “Although rot caused by the 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 susceptible to 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.”

Bacterial soft rot causes secondary breakdown of already infected tubers.
Bacterial soft rot causes secondary breakdown of already infected tubers.

Silver scurf is a storage disease which is causing increasing concern for storage operators in Canada. This disease causes external skin blemishes ranging from grey to silver in colour. Al-Mughrabi points out that under humid storage conditions, the primary lesions produce secondary lesions and the problem becomes severe, sometimes within a short period of time. “Virtually the entire tuber surface can be covered, and eventually water loss from affected tubers increases, resulting in shrinkage and deformation of the affected tubers,” he says.

Late blight attacks both tubers and foliage at any stage of development and causes dark external skin discoloration. The internal flesh of tubers tends to show 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 also be observed.

Pink rot is a fungal disease most often seen at harvest which results in dark outer skins in infected tubers.  Diseased tubers can affect healthy tubers at harvest or at bin filling. “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,” says Al-Mughrabi.

Pink rot-infected tubers have dark outer skins.
Pink rot-infected tubers have dark outer skins.

Pythium leak is another common storage-related disease. This disease causes dark grey, brown or black rot or water-soaked interiors of tubers, often margined by a dark line. When infected tubers are cut and squeezed gently, a clear liquid comes out.

Black dot is most observed on tubers but can affect all parts of the plant. The disease causes lesions on tubers very similar to those of silver scurf, says Al-Mughrabi. With the aid of a magnifying lens, growers will notice small black sclerotia visible within the lesions.

Growers should always keep in mind that harvested potato tubers are living organisms and hence interact with the surrounding environment.

– Khalil Al-Mughrabi

Living Organisms

According to Al-Mughrabi, prevention of potato storage diseases requires a combination of both cultural practices and fungicide programs. “Avoiding tuber damage during harvest can reduce disease problems in storage,” he says. “Potatoes can be treated with specific fungicides and disinfectants 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 effective post-harvest products.”

All conditions that influence storage disease development should be properly managed. Grading out damaged or diseased tubers before they enter storage is important, and in most cases essential, to prevent the spread of disease within the storage facility. Storage bins and farm machinery should always be thoroughly washed and disinfected before the new crop is harvested and stored.

“Growers should always keep in mind that harvested potato tubers are living organisms and hence interact with the surrounding environment,” Al-Mughrabi points out. “To maintain healthy potatoes during storage, the storage environment must also be managed effectively 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 is the single most important factor in maintaining the quality of stored potatoes, according to Al-Mughrabi. The majority of storage diseases are partially or completely inhibited by storage temperatures below 7.2 C, depending on the variety. At temperatures above 10 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,” says Al-Mughrabi. Temperatures above 10 C should be avoided during long-term storage of potatoes. The hatching of flies is also inhibited below 10 C—in other words, the presence of flies indicates that the temperature is too high somewhere in the storage facility, and breakdown may become a problem sooner rather than later.

Proper air movement in a storage facility is another essential element to ensure successful storage management. This includes both “through-the-pile” ventilation and “over-the-pile” ventilation, or recirculation. Through-the-pile ventilation is necessary to dry and cool the potatoes, supply fresh air, and remove carbon dioxide, volatiles and excess heat and moisture from the storage. Recirculation aids in maintaining uniform temperature conditions throughout the storage and sweeps moisture from the walls and ceiling.

A storage facility with inadequate insulation or poor air circulation may experience excess moisture build-up. Al-Mughrabi points out that this is a problem which can lead to water dripping on the pile—and this must be avoided at all costs in order to minimize the danger of rot. “Adding extra insulation and placing fans above or on top of the potato pile will improve air circulation and help eliminate condensation problems,” he says. “Overfilling the storage hampers air flow and increases the chances of tuber breakdown from soft rot and pink rot. There should be at least 0.6 metres, or two feet, between the top of the pile and the storage ceiling.”

A good storage management program should include daily checks of the storage, Al-Mughrabi advises. Storage operators must make sure that the ventilation controls and dampers are functioning correctly, especially during very cold weather when the danger of ice build-up is greatest.

“Use an accurate thermometer to check the air and tuber temperatures at several locations in the storage,” he recommends. “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 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.”

According to Al-Mughrabi, 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. He recommends that growers make sure to keep records of all storage conditions on a daily basis so that if problems arise there is some way of determining the cause.

One of the key principles of successful potato storage has always been prevention rather than cure. Growers should keep in mind that a diseased potato going into storage cannot be cured, but a healthy potato can maintain its quality with proper management.

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