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Sooty Blotch and Fly Speck

 

Sooty blotch and flyspeck on apple fruit

Sooty blotch and fly speck are two distinct diseases of apple but usually occur together. The diseases occur sporadically in conventionally sprayed orchards but are prevalent and can cause serious losses in wet seasons in organic orchards or where fungicide use is reduced.

Both diseases are caused by fungal disease complexes rather than individual fungal species and cause characteristic symptoms of a sooty-like discolouration (sooty blotch) or black shiny dot blemishes (fly speck) on near mature fruit and, although superficial, this causes fruit to be downgraded and reduced in value.

 

The life cycle and epidemiology of these fungi involves overwintering on apple twigs and on many hedgerow and wild tree species.  In spring spores produced on apple twigs and hedgerow hosts spread in wind and rain to infect apple twigs and subsequently infect fruit from early summer to harvest. Secondary spread from these early colonies occurs throughout the summer. Growth of these fungi is favoured by high humidity and temperatures from 18-27 degrees C. However, visual symptoms on fruit do not appear until 3-6 weeks after initial colonisation.

Control

At present control is dependent on an integrated approach combining monitoring,  and cultural measures with fungicide use. In well managed conventional orchards, which receive frequent fungicide sprays for scab control, specific control measures are not usually required.

  • Trim hedgerows to limit inoculum.
  • Prune apple trees to ensure good light penetration and air circulation so that fruit dries rapidly.
  • Maintain good weed control to ensure good air circulation.
  • As disease symptoms do not appear until 3-6 weeks after initial colonisation, orchard monitoring  as a basis for decision makers is not practical. However, identifying the problem in one year can be used as an alert for treatment in later years.
  • Where the disease has been a problem the previous season, apply sprays of an effective fungicide to fruit in early summer (mid-late June) and in July and August.
  • Mancozeb (Karamate) is the most effective fungicide.
  • DMI fungicides such as penconazole (Topas, Topenco) are ineffective and captan has only limited effectiveness.
  • Some disease warning systems have been developed such as RIMpro, which include a model based on leaf wetness, rain and temperature. This allows sprays to be more targeted.

Fungicides approved for use on apple which offer some incidental control of sooty blotch when applied to control other diseases – efficacy

Active ingredient Trade name Fungicide group Typh safety Efficacy
boscalid + pyraclostrobin Bellis strobylurin (QoI) safe effective ?
captan Captan 80 WDGPP, Captan 80, Clayton Core phthalimide safe partial
mancozeb Karamate Dry Flo dithiocarbamate harmful effective

Fungicides approved for use on apple which offer some incidental control of sooty blotch when applied to control other diseases – safety factors

  Hazards     Buffer zone
Active ingredient human fish + aquatic life bees Harvest interval (days)  Max. no. of sprays Width (m)
boscalid + pyraclostrobin h t u 7 4 40
captan h, ir, c t u 31 10 30
mancozeb ir t u 28 Varies with product 40

d = dangerous; h = harmful; ir = irritating, a = may cause allergic reaction, t = toxic

PH = post harvest; Pre bb = pre-bud burst, sm=statutory minimum of 5 m for broadcast air assisted sprayers

u=uncategorised/unclassified/unspecified, c=closed cab required for air assisted sprayers

Control in organic orchards

  • Control is dependent on cultural measures.
  • Sulphur sprays appear to have only limited efficacy against the diseases. Potassium bicarbonate may also give some control.
  • Use of the warning system allows the sprays to be more targeted.