AgronomyDisease Watch

Disease Watch

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[deck]The leading disease threats potato growers need to look for this season — and how to manage or prevent them.[/deck]

Late blight attacks both tubers and foliage. On tubers it causes dark external skin discolouration. 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. On foliage, dark, water-soaked lesions are observed on the upper surface of the leaf. Talc-like white spores appear on the lower surface of the leaf. The disease can also infect the stem, and eventually kill the whole plant.

How to manage late blight:

  • Plant disease-free seed potatoes.
  • Follow a regular spray program. Start your fungicide program early in order to obtain adequate coverage on the lower leaves. Begin your spray program before symptoms develop.
  • Reduce the likelihood of tuber infection by maintaining soil coverage of tubers through adequate hilling.
  • Check your crops regularly for any late blight symptoms, at least once a week. Scout more often in periods of wet weather.
  • Avoid frequent or night time overhead irrigation of potatoes. This practice maintains leaf wetness and high humidity in the plant canopy, which is favourable for the disease.
  • If there are known infected areas within a field, these areas should be flagged, and harvested last. The tubers from these areas should be stored separately at the front of the storage facility.
  • Kill vines at least two weeks before harvesting to reduce the possibility of tuber infection.
  • Destroy all cull piles and volunteer plants.
  • Use resistant cultivars where possible.
  • Grade out rotted tubers before storage. Turn on ventilation systems immediately to keep air moving through the pile in order to dry any rotting tubers. Keep the humidity low to prevent bacterial infection.
  • Tubers left on the surface of the field should be left to freeze and not disced or ploughed under.
  • A balanced fertilizer program must be reached. Petiole analysis for nitrate content can be helpful in regulating fertilizer rates.

Potato Virus Y

Potato virus Y (PVY) can be caused by a number of different strains (PVYo, PVYc and PVYn). Combined quality and yield losses make PVY the most damaging of the potato viruses. In the field, the virus is transmitted by aphids. It can also be tuber-borne and thus spread via seed as well. Infected volunteer plants are also a source of inoculum. Foliar symptoms of PVY can range from extremely mild mosaic, dwarfing, mottling of leaves, yellowing and leaf dropping, to severe foliar necrosis and the death of infected plants. PVYntn causes necrotic rings on the tuber surface. These rings might be isolated or coalesce to cover the whole tuber. Necrosis increases during storage and rings may become sunken, facilitating rotting.

How to manage PVY:

  • Plant disease-free seed potatoes. Field reading and post-harvest test results may be used as guides to select seed lots with low virus levels.
  • Plant resistant cultivars if possible.
  • Properly destroy cull piles according to established guidelines.
  • Rogue early in the season to remove infected plants from the field.
  • Use mineral oils.
  • Use insecticides to prevent the population of aphids from increasing within a field.
  • All cutting and planting equipment should be disinfected before coming in contact with seed.
  • Minimize mechanical damage of plants during cultivation and spraying.
  • Minimize visitor entry into potato fields.
  • Avoid planting seed potatoes downwind from commercial fields.
  • Control volunteer potato plants and weeds (wild rose, wild mustard, and wild radish are hosts for aphids on which large populations can develop).
  • 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.
  • Plant crop barriers. These consist of a non-PVY host crop (for example, cereals) planted around small early-generation seed lots to provide a buffer between the seed lot and the in-flight of aphids. Aphids usually land at the interface between fallow ground and green crop.

Fusarium Dry Rot

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.

How to manage fusarium dry rot:

  • Plant disease-free seed potatoes.
  • Only harvest tubers after top-kill when vines are completely dead, to ensure skin maturity.
  • Careful handling at harvest and in storage is required in order to avoid bruising and wounding; bruises and cuts can act as entry points for the fungus.
  • Clean and sanitize storage bins and handling equipment.
  • Newly harvested potatoes should be held at between 12 to 15 C and 90 to 95 per cent relative humidity for the first one to two weeks in storage to promote wound healing. Following the curing period, temperatures should be lowered to typical levels used for long-term storage. Dry rot does not spread in storage if the tubers are properly healed.
  • Warm tubers before cutting and planting (12 to 15 C) for one to two weeks.
  • Knives on the cutter should be sharp to make a smooth cut that heals easily.
  • Cut seed should go through the wound healing process prior to planting or a seed piece treatment should be applied and the seed pieces planted immediately.
  • Plant seed in warm, moist soils to promote prompt sprout growth.
  • Protect the seed from wind and sunlight during planting because dehydration weakens the seed piece.
  • If possible, avoid irrigation before emergence.
  • Applications of nitrogen in excess of soil test recommendation should be avoided. Too much nitrogen delays maturity of tubers and results in excessive skinning and bruising.
  • Post-harvest fungicides may be applied to tubers going into storage if storage problems are anticipated.

Pink Rot

Most often seen at harvest, pink rot 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.

How to manage pink rot:

  • Plant clean, disease-free seed.
  • Carry out a regular three-year crop rotation.
  • Plant potatoes in well-drained soil.
  • Prior to harvesting, check low areas in the field and rogue diseased plants and tubers.
  • Avoid harvesting during wet conditions and kill vines at least two weeks before harvesting. Delay harvest until the pulp temperature is between 7 and 10 C.
  • Tubers with significant pink rot should be harvested and stored separately for immediate grading and marketing.
  • Grade out rotted tubers before placing them in storage. In storage, turn on ventilation systems immediately to keep air moving through the pile in order to dry any rotting tubers. Cool tubers as quickly as possible. Keep the humidity low to prevent secondary bacterial infection from developing. The fungus is usually inactive at temperatures below 4.4 C.
  • Fungicides applied in-furrow at planting or applied to the foliage during early tuberization can reduce pink rot losses.

Soft Rot

Soft rot is a bacterial disease that 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 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.

How to manage soft rot:

  • Plant disease-free seed potatoes.
  • Avoid excessive soil moisture before harvest to reduce lenticel infection.
  • Harvest tubers only when mature and only when soil temperatures are less than 20 C.
  • Minimize mechanical damage to tubers during harvesting and handling.
  • Protect harvested tubers from solar irradiation and desiccation.
  • Cool tubers to 10 C or lower as soon as possible after harvest and store at temperatures as low as possible (preferably 1.6 to 4.5 C).
  • Provide good ventilation to keep tubers cool and to prevent accumulation of carbon dioxide and moisture films.
  • Do not wash tubers before storage, and when washing them before marketing, dry them as soon as possible and package them in well-aerated containers.
  • Use only clean water to wash potatoes. Contaminated holding tanks used for soaking potatoes almost assure soft rot infection. Treat wash water with chlorine to reduce the amount of soft rot inoculum.
  • Grade out rotted tubers before storage. In storage, turn on ventilation systems immediately to keep air moving through the pile in order to dry any rotting tubers. Keep humidity low to prevent secondary bacterial infection.
  • Plant seed in well-drained soils.
  • When possible, plant small whole seed.
  • Split applications of water-soluble calcium applied at 100 to 200 pounds per acre during bulking have been shown to reduce infection and severity of soft rot.
  • Storage floor soil from previous season severely affected with bacterial soft rot should be removed and thoroughly cleaned.
  • If only some loads are suspected with soft rot infection, place these potatoes closer to the access doors so that they can be removed quickly if they begin to deteriorate.
  • Clean and disinfect seed cutters, planters, harvesters and other handling equipment frequently to prevent the spread of bacterial and fungal pathogens.
  • Reduce pile depth to approximately three to 3.5 metres to facilitate drying.
  • Check the storage regularly for soft rot symptoms.

Pythium Leak

Pythium leak is a common storage-related disease that 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.

How to manage Pythium leak:

  • Plant disease-free seed potatoes.
  • Rotate crops out of potatoes for a minimum of four years.
  • Avoid planting potatoes in low, poorly drained soils.
  • Avoid mechanical injury to the potatoes during harvest.
  • Delay planting for at least two weeks after ploughing down green vegetation as this may temporarily stimulate populations of Pythium fungi.
  • The fungus survives in the soil and enters tubers at harvest through wounds and bruises. Therefore, allow skin to mature before harvest so that bruising and wounding during harvest can be reduced.
  • Consider marketing early and place problem potatoes closer to the access doors for easy removal.
  • Nitrogen fertility should be monitored closely to prevent heavy top growth, as soils tend to remain wetter under a heavy canopy.
  • Avoid exposing potatoes to the sun as it increases their susceptibility to Pythium leak.
  • Do not harvest during very warm weather. Wait until soil temperatures are below 15 C. If tubers are windrowed on moist, warm soil (around 21 C) they should be removed from the field immediately.
  • Grade out rotted tubers before placing them in storage. In storage, turn on ventilation systems immediately to keep air moving through the pile so drying any rotting tubers. Keep the humidity low to prevent secondary bacterial infection.

Silver Scurf

Silver scurf is a storage disease that causes external skin blemishes ranging from grey to silver in colour. 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.

silverscurf

How to manage silver scurf:

  • Plant disease-free seed potatoes. However, this may not always be possible since the disease can be found in most storages.
  • Use seed treatment fungicides that are capable of inhibiting more than one biochemical reaction site in the metabolism of the fungus.
  • Remove all volunteer potato plants.
  • Harvest the tubers as soon as they are mature.
  • Ventilate storage areas with warm air for drying, and store tubers at a low temperature (3 to 5 C) to heal wounds and to avoid other storage diseases.
  • Clear field of all tubers left after harvest.
  • Use a post-harvest fungicide after soil has been removed from the harvested tubers.
  • Practice a three-year crop rotation.
  • Remove any soil or organic debris from potatoes when placing in storage. Disinfect all machinery and storages before placing potatoes inside.

Rhizoctonia Canker and Black Scurf

Rhizoctonia canker and black scurf may cause poor emergence, brownish root, stem or stolon cankers, swollen stems, aerial tuber formation, leaf rolling, purpling of upper leaves, premature death of vines and tuber malformations. Signs of the fungus are evidenced by sclerotia (black scurf) on potato tuber surfaces and a white felt-like fungal growth covering on stems at the soil surface.

How to manage rhizoctonia canker and black scurf:

  • Plant disease-free seed potatoes.
  • Seed treatments can sometimes help facilitate emergence but in general do not have much effect in heavily infested soils or heavily infected seed. Fungicidal seed-treatments are no substitute for clean seed.
  • Long rotations out of potatoes may be useful in reducing soil levels of the aggressive strains of the fungus.
  • Practice rotation with cereals and grasses.
  • Avoid a rotation with buckwheat before potatoes as rhizoctonia colonizes mature buckwheat stems.
  • Some varieties with moderate resistance are now available.
  • Try to avoid planting in cold wet soil or covering seed pieces with more than five centimetres of soil.
  • Harvesting quickly after vine desiccation and death limits the development of black scurf.
  • Post-harvest applications of certain fungicides may help control this fungus.

Blackleg

Blackleg is a tuber-born bacterial disease that is characterized by stunted, yellowish foliage that has a stiff, upright habit. The lower part of the below ground stem is dark brown to black in color and extensively decayed. The pith region of the stem is particularly susceptible to decay and in blackleg-infected plants the decay may extend upward in the stem far beyond the tissue with externally visible symptoms. The typical blackening and decay of the lower stem portion is the origin of the “blackleg” designation for this disease. Young plants affected by blackleg fail to develop further and typically die.

How to manage blackleg:

  • Plant disease-free seed potatoes.
  • There are no chemical controls available for blackleg.
  • Remove diseased plants from the field and rotting tubers during harvest.
  • Harvest plots containing diseased plants last, preferably under dry conditions.
  • Clean all machines, means of transportation and storage equipment afterwards.

Potato growers are encouraged to consult their extension specialists and their local potato management guides for the latest information on pesticides registered for the management of potato diseases. Read and follow label recommendations. If you are unsure what disease you are dealing with, send a sample to your local diagnostics laboratory for positive identification of the disease. Accurate diagnosis will help you choose the appropriate pesticide and saves you time and money.

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