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Apple Scab

 

Scab lesions on fruit

Apple scab is the most economically important disease of apple in the world.  All parts of the tree are attacked. Scab infection of fruit is most obvious. Most commercial UK apple varieties are susceptible to scab and Gala is the most susceptible.

The lifecycle and epidemiology are straightforward.

  • The scab fungus overwinters mainly as the sexual state (pseudothecia) on leaves on the orchard floor.
  • It can also survive as mycelium in lesions on the tree as wood scab, bud scale scab or shoot base scab.  The importance of this source of inoculum varies with variety, region and season.
  • In spring spores either ascospores released in rain from overwintering leaves or conidia from tree sources infect developing leaves and fruit to initiate the new scab epidemic.

Scab lesions on a leaf

  • Wet leaves or fruit are essential for infection.
  • Once new infections are established, conidia are produced and these spread in wind and rain to infect other new leaves and fruit.
  • Infection of fruit near harvest can lead to the development of storage scab on fruit in store.
  • In late summer/autumn older leaves which had become resistant to scab with maturity become susceptible to scab again as the cuticle cracks with age.
  • This diffuse late scab developing on leaves contributes most to the overwintering scab on the orchard floor.

Apple scab infects most parts of the tree including leaves, petioles, blossoms, sepals, fruits, pedicels, shoots, bud scales.  Symptoms are most easily observed on leaves and fruit.

Control

Successful control of apple scab combines orchard monitoring and disease forecasting systems in an integrated approach using cultural control to minimise fungicide inputs to achieve high quality scab-free fruit.

  • Before bud burst inspect orchards and estimate likely scab inoculum carryover, based on the scab incidence the previous autumn and the remaining leaf litter.
  • Where leaf litter is excessive, macerate with the mower to encourage breakdown.
  • Where scab levels the previous season were high, a pre-bud burst spray of a copper fungicide such as Cuprokylt will give some control of scab overwintering on the tree.
  • Apply the first spray promptly at bud burst using a protectant fungicide which is also active against Neonectria canker, e.g., dodine (Radspor) or dithianon (Dithianon).
  • If possible make use of scab warnings (e.g. ADEM or RIMpro) to rationalise fungicide use.
  • Follow a key-stage strategy applying routine fungicide at the keystages of bud burst and petal fall and at other times basing spray decisions on scab risk from ADEM or RIMpro. Also take into account the need for sprays for other diseases, pests and nutrients and practical considerations such as holidays and weekends.
  • Information on ascospore release (e.g. from RIMpro), if available, can also assist in spray decisions particularly on the start and end of ascospore release but do not rely on this information only as it does not include scab inoculum overwintering on the trees.
  • Where fungicides are applied in response to a scab warning, choose a DMI fungicide such as myclobutanil (Systhane) which has curative or kick-back action of up to 4 days.
  • Use in combination with a protectant such as captan or dithianon to enhance protectant action, particularly on fruit, as DMIs have only limited protectant action.
  • Ensure scab fungicides from different chemical groups are used to avoid the development of fungicide resistance.
  • In addition to scab warnings, monitor scab incidence in the orchard, pre-bloom on whole trees, post bloom on rosette leaves and shoots.
  • This provides information on inoculum levels for input to ADEM or other scab models and also on the success of the scab programme and whether adjustments to the scab programme need to be made.
  • Generally scab sprays are applied between bud burst and June.  The need for later scab sprays depends on the success of the earlier programme.
  • Spray 5% urea post harvest and before leaf fall.  This will prevent formation of the scab sexual state and encourage leaf rotting to make leaves more palatable to earthworms.
  • During winter pruning, check for and cut out any wood scab found on trees.

Organic production

Scab control in organic production requires careful planning.

  • The emphasis is on use of scab-resistant varieties.
  • Where susceptible varieties are used, cultural control is most important, particularly eradication of overwintering scab and maintaining trees with good air circulation to encourage rapid drying of leaves and fruit after rain and dew.
  • Where permitted, spray a copper fungicide pre-bud burst.
  • Currently in the UK only sulphur sprays are available for organic scab control during the growing season.
  • Sulphur is only partially effective against apple scab.

Fungicides approved for use on apple which are recommended to control apple scab or offer some incidental control when applied for other diseases – Efficacy factors – Download

 

Fungicides approved for use on apple which are recommended to control apple scab or offer some incidental control when applied for other diseases – Safety factors

  Active ingredient Hazards Harvest interval (days) Max. no sprays Buffer zone
Human Fish + aquatic life Bees width(m) 
boscalid + pyraclostrobin h d u 7 4 40
captan h, ir, c t u 31 10 30
copper oxychloride h d u Before bud burst 3 (max. conc. 5 litres/1,000 L) sm
cyprodinil + fludioxonil a, c d u 3 3 30
dithianon h, ir, c d u 28 Varies with product sm
dithianon + pyraclostrobin h, ir, c d u 35 4 40
dodine h, ir d u Dessert: Pre blossom, Culinary: early July None stipulated sm
fenbuconazole ir t u 28 10 (when applied at 1.4 l/ha) sm
kresoxim-methyl h t u 35 4 sm
mancozeb ir d u 28 Varies with product sm
myclobutanil h d u 14 10 (when applied at 0.45 l/ha) sm
penconazole ir d u Varies with product Varies with product sm
pyrimethanil u h u 56 4 30
sulphur u u u Varies with product Varies with product sm

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

 

Further reading