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Sooty blotch – additional information

Disease status

Sooty blotch and fly speck are two distinct diseases of apple, but usually occur together.

  • The fungi cause a discolouration or blemish on near-mature fruit in the orchard.
  • The blemish is superficial and downgrades the fruit and hence reduces the market value or saleability of the fruit.
  • In the UK the diseases occur sporadically in conventionally sprayed orchards, usually associated with wet summers and shady orchards.
  • However, the diseases are becoming of greater significance with the increased production of fruit with reduced fungicide inputs in summer and of organic production, where losses can be high.

Other hosts

  • Both sooty blotch and fly speck have a wide host range of forest and hedgerow trees and shrubs, especially blackberry.
  • The diseases also occur on pear and on plum.

Varietal susceptibility

  • All apple varieties are susceptible to infection, but the diseases are usually more severe on yellow or light-coloured varieties such as Golden Delicious, Cox and Fiesta
  • Several of the scab-resistant varieties, such as Edward Seventh, can be badly affected.

Distribution

  • Sooty blotch and fly speck are two of the most common diseases of apple in many temperate areas of the world.
  • In the UK they probably occur unnoticed in most apple orchards and hedgerows.

Symptoms and recognition

In the orchard sooty blotch and fly speck are most commonly found on fruit on trees near windbreaks/hedgerows and in the shadiest part of the tree or orchard.

Sooty blotch

Sooty blotch on apple fruit

  • Olive green to dull black sooty blotches appear on near mature fruit in the orchard, usually from late July/August onwards.
  • The blotches may coalesce to cover practically the entire fruit.
  • The infection is superficial and in many cases blotches can be rubbed off.
  • However, if infection occurs early in the season, then the blotches are much more difficult to remove and may need bleaching.

 

Fly speck

Fly speck on apple fruit

  • These can usually be found on the same fruits as sooty blotch and occur as groups of 6-50 or more black shiny round dots that resemble the fly excreta often found on fruits.
  • Individual fly specks are clearly separated and easily distinguishable from the more diffuse blotches of sooty blotch.
  • Fly speck like sooty blotch is superficial but much harder to rub off.

 

 

 

 

Other problems that may be confused with sooty blotch and fly speck

The two diseases are usually quite distinct, but sooty blotch can be confused with sooty moulds.

  • The latter are caused by saprophytic fungi such as Alternaria, Cladosporium which colonise insect honey dew or other sticky deposits on fruit surfaces.
  • Sooty moulds are more obvious as fluffy growth and very easily rubbed off.

Disease cycle and epidemiology

Sooty blotch

  • The fungus overwinters on twigs of various woody plants in hedgerows and windbreaks and on apple twigs.
  • In spring, pycnidia (fruiting bodies) on wild plants and apple twigs produce large numbers of spores (conidia) that ooze out and are spread by rain splash or wind blown mist in orchards in spring and early summer to autumn.
  • The fungus first infects apple twigs and from these infections fruit are colonised from late June/early July onwards.
  • Cool, humid weather (optimum 18oC) is essential for disease development.
  • Free water on the fruit surface is required for infection.
  • The disease does not develop at high temperatures or if conditions are dry in late summer.
  • The incubation time from fruit infection to symptom appearance can be as short as 5 days under optimum conditions.
  • In the orchard the incubation period usually lasts 3-4 weeks on fruits that are 42-45 days old.
  • The blotch symptom on the fruit consists of hundreds of minute pycnidia interconnected by fungal mycelium.
  • The fungus can also continue to develop in store.

Fly speck

  • Fly speck similarly overwinters on wild hosts and on apple twigs as sexual fruiting bodies (pseudothecia).
  • In spring these release ascospores around blossom time which infect apple twigs.
  • Conidia produced on these infections spread to infect apple fruits.
  • Optimum temperature for conidial production is around 17oC at high humidity.
  • Conidia are spread by wind to infect fruit.
  • Free water on the fruit surface is required for the fungus to infect.
  • In the orchard there is usually three weeks between fruit infection and symptom appearance on apple fruits.
  • On the fruit the individual fly specks are the fungal fruiting bodies.

Recent US research on the sooty blotch  fly speck complex

The sooty blotch fungus (Gloeodes pomigena) was previously believed to exist in different mycelial forms.  Research in the USA in the 1980s has shown that in fact sooty blotch is associated with at least three unrelated fungi: Peltaster fructicola, Geastrumia polystigmatis and Leptodontium elatius which correspond to the different mycelial types of Gloeodes pomigena.

Dominance of one particular species of the three fungi associated with sooty blotch varies according to its response to the environment, sensitivity to fungicides and competitive abilities on the fruit surface.

  • However, the most recent research in the USA, combining molecular techniques with morphological characterisation has revealed that the sooty blotch fly speck complex (SBFS) is far more diverse than previously realised.
  • Surveys of orchards in 14 eastern U. S. States in 2000 and 2005 uncovered 62 SBFS species in five taxonomic orders.
  • Conventionally sprayed orchards had lower diversity in the SBFS complex than unsprayed orchards.
  • Some SBFS species occurred in almost all orchards, whereas other species were regional in distribution or were found in only one or two orchards.
  • Similar patterns patterns of SBFS diversity have been found in Germany, Serbis and Montenegro, Brazil, China, Florida and Costa Rica.
  • It is likely that similar diversity in the SBFS complex exists in the UK but until this can be evaluated the disease will be referred to as sooty blotch caused by Gloeodes pomigena.
  • In the USA the fungal species still originate from wild hedgerow trees and shrubs, so that the overall epidemiology is essentially unchanged, although fungicide efficacy may be affected.

Disease monitoring and forecasting

Monitoring sooty blotch and fly speck based on visual symptoms is probably ineffective as a management tool because of the long interval between infection and symptom appearance.

  • Where the diseases were a problem the previous year, probably specific control measures need to be applied in the current year to protect fruit.
  • In the USA a simple model has been developed to predict the first symptoms of sooty blotch and fly speck.
  • This is based on the hours of leaf wetness of four hours duration or greater accumulated from the first rain that occurred 10 days after petal fall.
  • Fungicides are applied at a threshold value of 200-250 hours of leaf wetting.
  • The threshold value of leaf wetting was shown to vary with region in the USA, which was probably associated with the fungus complex responsible for sooty blotch.
  • In the Midwest of USA, cumulative hours of relative humidity greater than 97% was more accurate in forecasting SBFS than cumulative hours of leaf wetness.
  • Whether this model ca be applied to the UK is not known.
  • A model for sooty blotch is also included in the RIMpro system and has been tested in several locations in Europe but not in the UK.

Cultural control

  • Select an orchard site that has good sunlight, good air circulation and soil drainage.
  • Prune trees to reduce shading and encourage good air circulation.
  • Both sooty blotch and fly speck are encouraged by shading and wet conditions.
  • Therefore, any pruning that opens up the trees and encourages good air circulation and rapid drying will reduce the diseases.
  • Fruit thinning to aid air circulation around fruit will also reduce disease.
  • Good weed control under the trees to promote air circulation will also help.
  • Ideally, elimination of wild reservoir hosts to reduce inoculum is required.
  • However, this is impractical and not conducive to good environmental practices.
  • Mowing banks and keeping hedgerows well trimmed may help.
  • Some sooty blotch on fruit can be rubbed off during grading.

Biological control

  • No biocontrol methods have been developed.

Chemical control

Use of protectant fungicide sprays is the main means of controlling the diseases.  In the UK in conventionally sprayed orchards, specific fungicide sprays are not normally required.

  • However, if the diseases have been a problem and in the following season spring conditions are wet, then protectant sprays may be needed for fruit in summer.
  • Current information on spray timing indicates treatment should be applied from mid-late June as first fungal colonisation generally occurs in early July.
  • A treatment programme based on several different fungicides at 14 day intervals is the best approach.
  • Sprays applied in conjunction with a warning system should improve control and reduce fungicide inputs.
  • Dithiocarbonate fungicides such as mancozeb (Karamate Dry Flo) are most effective with good protection on fruits.
  • Captan will also give some protection but is less effective and, under favourable conditions, will only give limited protection.
  • Strobilurin fungicides such as kresoxim-methyl (Stroby) or boscalid + pyraclostrobin (Bellis) will also give control of sooty blotch and fly speck.
  • The DMI fungicides such as Systhane (myclobutanil) are ineffective.

Avoiding fungicide resistance

  • The risk of fungicide resistance is very low.