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Western Canadian potato producers face numerous disease threats

March 17, 2008  By Myron Love

The potato is susceptible to more
than 60 diseases, according to plant pathologist Gary Secor. While he
notes many of these diseases may not be a threat to northern plains
growers, the potential for infection is still there.

Potatoes infected with late blight are shrunken on the outside, corky and rotted inside. Photo courtesy of USDA-ARS

The potato is susceptible to more than 60 diseases, according to plant pathologist Gary Secor. While he notes many of these diseases may not be a threat to northern plains growers, the potential for infection is still there.

“We are facing new challenges as changes in production practices and climate potentially lead to new strains,” explains Secor, a plant pathology professor at North Dakota State University since 1978.


“We need to increase our awareness of diseases that are spread by seed potatoes,” he says, adding biogenetics is likely the most effective way to prevent the spread of disease in potato production.

The issues

Silver scurf is a minor skin disease that can spoil the appearance of the tuber, adversely affecting the crop’s value in markets where appearance is an important factor. It can be a problem for both table stock and processing potato varieties and spreads in stored potatoes, Secor explains. There is no simple control, he notes, but a combination of strategies seems to be working. These include:
•    Planting clean, disease-free seed.
•    Practising a three-year crop rotation.
•    Allowing tubers to mature fully and harvesting as soon as possible       after that.
•    Ventilating storage with dry, warm air to remove moisture from the       surfaces of tubers.
•    Storing tubers at as low a temperature as possible (3 to 5ºC).
•    Applying seed-piece treatments.

Black dot has been around for a long time, says Secor, and is generally thought of a secondary disease, especially in irrigated potato fields. Infection rates are currently on the increase and “it looks like Black Dot is replacing Silver scurf as the major blemish disease,” he says.

Secor says most fungicides work against black rot but researchers haven’t yet figured out how to control seed-borne transmissions.
Some other recommendations include:
•    Using long rotations (three to four years) between potato crops.
•    Planting clean potato seed for planting.
•    Deep plowing to bury infected debris and encourage decomposition.
•    Not using solanaceous crops in the rotation scheme and ensuring solanaceous weeds are controlled.
•    Remembering that late-maturing varieties are more vulnerable to some yield reduction.
•    Avoiding plant stress and maintaining adequate fertility.
•    Selecting well-drained land if possible.

Bacterial Ring Rot is a tuber-borne disease that can survive for up to two years as dried slime on cutting knives, storage bins, burlap bags and harvesting equipment. Under seed potato regulations in Canada, once bacterial ring rot is discovered, all lots under cultivation by the grower are rejected for certification.  Even so, Secor says the disease is re-emerging. Why? “Perhaps because growers are missing it,” he suggests, adding testing may not be sensitive enough to detect the disease at low levels.

Control recommendations include:
•    Planting only certified disease-free seed tubers. Certified seed potatoes are produced under regulations mandating zero tolerance for ring rot. Although use of certified seed tubers does not guarantee total freedom from ring rot bacteria, it is the best assurance.
•    Discontinuing use of any lot of seed tubers in which ring rot is found. Seed lots believed to be contaminated with ring-rot bacteria should never be planted.
•    Before handling seed tubers, all containers, tools, knives and mechanical cutters, planters, and other equipment should be thoroughly washed with a detergent solution, rinsed, and then sanitized with a disinfectant.
•    When cutting seed tubers, the cutting tool should be periodically washed and sanitized. It is essential that this be done before cutting seed tubers from a different source. To be effective, disinfectants must be present for a minimum of 10 minutes (preferably 20 to 30 minutes) on any surface being treated. It is much easier to disinfect metal surfaces than wood or burlap.
•    If ring rot is confirmed to be present, a thorough cleanup must be undertaken. Dispose of all infected tubers away from potato production areas. Clean all surfaces of storages and equipment to remove all mud, dirt and debris and then wash with a strong detergent in hot water applied by a high-pressure washer. After cleaning, sanitize all storages and equipment with a disinfectant. Do not plant potatoes for two seasons in any field in which ring rot has been found.

One of the new diseases appearing in potato fields is Tobacco Rattle Virus  (TRV), which is spread by stem and bulb ground nematodes. TRV infection is characterized by a mottled yellow appearance and necrotic areas on the potato plants leaves, says Secor, adding the disease has a wide range and moves with seed or soil. Infection of the tubers results in corky ring spot, blemishes (or curved patterns) that are corky brown in the tuber flesh, sometimes without any external symptom. These necrotic rings generally form a concentric pattern, but these are
much less regular than those due to the
Mop Top virus.

TRV is essentially a soil disease and control methods are limited. They include:
•    Choosing healthy plots for potato crops.
•    Choosing resistant or fairly insensitive varieties (Viola, Bintje, etc.) for contaminated plots.
•    Treating infected soil with a nematicide (very expensive and toxic).

Potato Virus Y (PVY) is the most important virus infecting potatoes in Ontario. It was first described in Hungary in the 1970s and was identified in Canada in 1996.  It is transmitted by aphids, whose numbers are increasing through the increased production of aphid-friendly crops – such as canola – adjacent to potato fields. Insecticide sprays don’t control the problem, explains Secor, since the aphids usually transmit the disease before the spray kills them.

Control recommendations include:
•    Using insecticides to prevent the population of aphids from increasing within a field. Follow label recommendations for application.
•    Planting seed certified through a recognized seed potato inspection program. Field readings and post-harvest test results may be used as guides to select seed lots with the lowest virus levels.
•    Rouging as early as possible.
•    Avoid planting seed potatoes downwind from commercial fields.
•    Controlling weeds and volunteer potato plants.
•    Top-kill seed fields early to prevent late-season virus infection.
•    Avoid planting susceptible varieties in close proximity to fields with varieties that have poor symptom expression.

Potato Mop Top Virus is a soil-borne disease spread to potatoes through the powdery scab fungus. The virus lives inside the resting spores of the fungus that can remain viable in the soil for as long as 18 years. In potato, the virus can cause a wide range of symptoms including dwarfing of shoots (mop top), pale green V-shaped markings (chevrons) on young leaves, bright yellow blotches, mottling, rings and V-shapes on foliage, cracking of tubers and internal necrotic rust-coloured spots, arches and rings within tubers.

Mop top is a common occurrence in Europe potato production, says Secor, adding the disease is not yet a big concern in North America.
Control measures include:
•    Avoiding the introduction of powdery scab and PMTV into uninfested
soils by using disease-free seed and sanitation practices.
•    Using sanitation measures aimed at preventing the movement of powdery scab infested soil on seed, equipment, and in irrigation water.
•    Controlling powdery scab fungus.

Secor also groups Potato Leaf Roll Virus as a “new” disease. It is a highly destructive virus spread mostly by green peach aphids. Foliar symptoms of leaf roll infection include a pale green plant and rolled leaves. Tubers from infected plants exhibit net necrosis – which appears as little brown flecks around the vascular ring in the tuber. Severe potato leaf roll virus infection will cause seed fields to be downgraded and processors will reject potatoes with net necrosis as it causes brown spots in the rench fries. The virus is almost impossible to control, says Secor. “In my view, GMO is the best solution.”

Control measures include:
•    Planting seed free of potato leaf roll virus.
•    Spraying to control aphids.
•    Not planting close to any other potato operations, especially commercial ones.

In the very new and rare category of diseases, Secor mentions the phytoplasmas, which can result in plants with stunted growth, chorotic or reddened leaves, leaf roll, reduced numbers of subterranean tubers, and the production of aerial tubers. Some plants exhibit witches’ broom symptoms, i.e., the proliferation of many small shoots and leaves. Symptoms may be confused with damage caused by psyllid insects and, in some cases, with leaf roll virus.

Although these diseases are rare, they are often found in fields with weedy edges or with weedy areas nearby. Controlling weeds near potato fields helps to prevent development of these diseases.

A disease that has long been associated with dry rot in potato is fusarium. But it is also a disease that is changing, explains Secor. In recent years, losses to fusarium dry rot have been increasing as the disease becomes resistant to the benzimidazole fungicides commonly used to control dry rot of potato. About “40 per cent of dry rot in our area has been identified only since 2004,” says Secor. “That may be due to agronomic practices.”

Management techniques include:
•    Purchasing seed that has as little dry rot as possible. Seed should be inspected, preferably during the last months of storage.
•    Warm seed tubers to at least 50°F before handling and cutting to minimize injury and promote rapid growth.
•    Cleaning and disinfecting the seed storage area before receiving seed. Disinfect seed cutting and handling equipment often and clean up well between seed lots.
•    Treating seed pieces with fungicide helps control decay and other diseases caused by seedborne pathogens.
•    Protecting the seed from wind and sunlight during planting because dehydration greatly weakens the seed piece. Do not precut seed – it is best to cut only as much as can be planted within 24 hours.
•    Planting seed that has a fusarium problem in warm soil and cover with as
little soil as is practical.
•    Harvesting tubers after skins are set and when pulp temperature is greater than 50°F.
•    Providing adequate coverage of the tuber surface when using a postharvest fungicide.
•    Allowing a period for wounds to heal before dropping the temperature in
•    Dropping the storage temperature slowly to prevent condensation on the tubers.
•    Monitoring storages often for dry rot.

A section of a potato exhibiting advanced symptoms of dry rot caused by the fungus Fusarium sambucinum. Photo courtesy of USDA-ARS

Fungal diseases such as dry rot, pink rot, and late blight continue to be serious diseases for potato producers and can cause serious losses, Secor notes. The main chemical controls for these diseases usually contain mefanoxam or metalaxyl, two ingredients which are losing effectiveness due to widespread resistance, which has cropped up in Maine, he adds. Of course, their alternatives, – Ranman and Phostrol – cost four times as much, says Secor.
Control measures include:
•    Managing water (avoid overly wet conditions).
•    Keeping tuber pulp temperatures below 65ºF.
•    Handling problem areas differently.
•    Managing bruising during harvest and handling.
•    Applying mefanoxam.
•    Rapidly cool known problem lots.
•    Providing continuous ventilation.
•    Avoiding humidity for some situations.
•    Moving away from long-term storage as an option.
•    Monitoring problem storages often and carefully.

For every country, except Australia, Potato Wart can be an issue. Back in 2000 and again in 2002, the fungal parasite reared its head in P.E.I. after 50 years, leading to the U.S. placing an embargo on P.E.I. potatoes. “The good news was it wasn’t widespread,” recalls Secor.
Ways to avoid an outbreak include:
•    Planting only certified potato seed in wart-free soil.
•    Planting potato wart resistant cultivars.
•    Cleaning and sterilizing all potato equipment.

The Potato Tuber Moth is an important parasite in hot regions, but has recently migrated from California to Washington state, according to Secor. The adult moth is small (10- to 15-mm wingspan) and grey with fringed wings. It lays its eggs in the stems, sprouts and tubers of potatoes and in the soil of potato fields. The hatched larvae can then attach the foliage and the tuber. Secor says he hopes the pest won’t be able to move too far north due to the cold climate.
Control measures include:
•     Destroying contaminated tubers.
•     Using long crop rotations.
•     Avoiding very broken up or cracked soil, etc., since it’s a great atmosphere for the laying and hatching of eggs.
•     Harvesting early.
•     Disinfecting the premises.
•     Treating the crop with insecticides.

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