Potato Review Group

Contents

Background

Soil test for black dot

Risk of black dot

Minimising the risk of black dot

Further information:

Black dot notes

 

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Background

  1. General.
    • Inoculum of the fungus causing black dot (Colletotrichum coccodes) may be seed or soil borne. More …
    • Infection of tubers reduces quality for pre-pack markets but crop performance may also be reduced. More …
    • There is some evidence of differences in cultivar susceptibility to black dot but this can also be affected by pathogenicity of different isolates of the fungus. (more …), which may be classified into vegetative compatibility groups (more …).
  2. Seed borne inoculum.
    • Seed certification does not include any threshold level of black dot infection.
    • Seed borne inoculum may be particularly important in the absence of soil borne inoculum. More …
    • Inoculum is carried as microsclerotia on the surface of seed tubers. More …
    • Spores are released and may be spread by movement of water, rain splash and wind. More …
  3. Soil borne inoculum.
    • Black dot is considered to occur on the majority of land where potatoes are grown (more …) and sclerotia have long persistence in soil (more …).
    • A soil test for black dot is available from SRUC.
    • There is evidence that other crops and weeds (such as Solanum and Brassica species) could be alternative hosts of black dot but that cereal crops are unlikely to be hosts. More …
  4. Disease development.
    • Infection of plants may occur early in the season, before any sclerotia are visible. More …
    • Standing water in soil (>10 hours) increases infection risk. More …
    • There is a potential for all parts of the plant to become infected but below ground infection is most common. More …
    • Hyphae may infect vascular and cortical tissue of stems, stolons and roots; microsclerotia may develop on below ground stems, stolons and roots, releasing further spores. More …
    • Infection of tubers can occur at any time but development of microsclerotia on tubers usually occurs late in the season. More …
    • Competition occurs between silver scurf and black dot, with the result that control of one disease may result in greater infection by the other. More …
    • The risk of black dot should be assessed in soil, in the growing crop, at harvest and during storage. More …

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Soil test for black dot

Sampling

  1. Sampling should be performed on 4 ha blocks of land.
  2. Sample using a PCN corer or similar (collecting c. 10 g of soil each time) using a grid or “W” pattern.
  3. The key to determining the level of contamination is to take as many samples as possible.  SRUC recommends at least 100 sampling points, resulting in a total sample weight of about 1 kg.
  4. The pathogen is usually distributed evenly through the soil profile so collecting from surface layers is sufficient.
  5. Label bag with details of farm, field, date, sender and contact details.

More …

Analysis

  1. Send samples to SRUC Crop Clinic, Ferguson Building, Craibstone Estate, Aberdeen AB21 9YA, Tel. 01224 711206 / 01224 711279.
  2. Cost: £168.60 + VAT (March 2021)
  3. A test for powdery scab is also available and a discount is possible if both tests are performed.

OR

  1. Send samples to FERA Plant Clinic.
  2. Cost: £195 + VAT for a single sample, discount possible for multiple samples (March 2021).
  3. Use these links for details of sample submission and for a sample submission form.

Interpretation

  • <100 pg DNA / g soil = low risk
  • Between 100 and 1000 pg DNA / g soil = moderate risk
  • >1000 pg DNA / g soil = high risk

More …

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Risk of black dot

  1. The risk of black dot is high in wet soil. More …
  2. Stress may increase susceptibility to infection. More …
  3. Black dot may be identified on stems and stolons early in the season, indicating a risk of tuber infection. More …
  4. Black dot on tubers may not become visible until after the start of canopy senescence. More …
  5. The disease increases during the season and the risk is greater at later dates of harvest. More …
  6. The more severe the infection and the beginning of storage, the greater the increase which may occur during storage. More …
  7. Crops in which black dot has been identified during the season should be harvested early and not stored.

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Minimising the risk of black dot

  1. Avoid fields where black dot has been a problem in the past (inoculum can be viable for many years, more …) or where a soil test indicates a high risk (more …).
  2. Use “clean” seed (to reduce the risk of seed borne infection). More …
  3. Apply azoxystrobin to soil at planting (use appropriate application technology). Even distribution through soil is required. Care is needed with application, to provide effective disease control and minimise risk to developing plants. More …
  4. The seed treatment fludioxonil may provide some suppression of black dot as part of an integrated control programme. More …
  5. Monitor underground plant parts for black dot:
    • sclerotia can be seen with a hand lens and resemble little black hedgehogs
    • sclerotia on stems or stolons indicate a risk of tuber infection
    • More …
  6. If there is any risk of black dot, harvest as early as possible and do not store (black dot can increase during the season and in storage). More …
  7. There is some evidence that disease increase during storage may be minimised by a rapid pull-down of store temperature, if appropriate. More …
  8. Follow a programme of risk assessment in: soil, the growing crop and the harvested tubers. More …

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Further information

See the manufacturers’ websites for details of agrochemical use.

 

Black dot notes

Transmission of seed borne diseases 2018 (Spread of disease from infected seed tubers)

Tuber blemishing diseases 2012 (Includes infection of stems with black dot and experiments to control these infections; vegetative compatibility groups)

Blemishing disease updates 2011 (Includes quantification of black dot in soil tests)

Black dot 2010 (Assessing the risk of infection from: soil tests, observations on growing plants, tubers before harvest)

Black dot 2009 (Development during storage; inoculum on seed tubers; soil test; alternative hosts)

Seed treatments 2009 (Fludioxonil experiment results: some suppression of black dot)

Recent research on diseases 2008 (Includes: incidence and severity of black dot in potato fields; soil test for black dot)

Tuber blemishing diseases 2006 (Includes black dot: development of the disease through the season)

Black dot 2005 (Use of azoxystrobin applied to soil; reducing the risk of fungicide resistance)

Black dot 2004 (Control with azoxystrobin applied to soil in replicated and farm scale experiments)

Amistar incorporation 2004 (Achieving appropriate application)

Black dot 2003 (Effects on crop performance; survival in soil; suppression with azoxystrobin applied to soil)

Black dot 2002 (2001 presentation and 2002 identification presentations combined)

Black dot identification 2002 (Inspecting plants to assess the risk of black dot after harvest)

Black dot 2001 (Patterns of development; relationship between black dot infection during growth and at harvest; isolates differ in pathogenicity; development requires wet conditions)

Tuber blemishing diseases 1999 (Includes black dot: cultivars differ in susceptibility)

Tuber blemishing diseases 1998 (Includes black dot: inoculum can be seed and soil borne; competes with silver scurf on the tuber surface)

 

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