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Apple scab lesions on leaves and fruit Photo: Samuele Pellegrino · Pexels License · source ↗

Apple scab (Venturia inaequalis)

Apple scab is caused by the ascomycete fungus Venturia inaequalis (anamorph Spilocaea pomi) and is the single most damaging disease of apple in Kashmir Valley and the wetter mid-hill belts of Himachal Pradesh and Uttarakhand. The disease attacks leaves, fruit, twigs and buds. Heavy outbreaks following prolonged spring rain can cause 70-90% fruit-marketability loss. The 1973 scab epidemic in Kashmir Valley remains a benchmark for the disease's destructive potential.

Identification and symptoms

  • Leaves: small olive-green velvety spots on the upper surface that turn dark brown to black and scabby; severely infected leaves drop prematurely.
  • Fruit: rough, corky, brown-to-black scabs; cracks develop on heavily scabbed fruit; immature infection causes deformed fruit; late infection ("storage scab") produces small pin-point lesions that appear only after pre-cooling.
  • Twigs: cracked, scaly lesions on young shoots; small overwintering pustules on bud scales.

Royal Delicious, Red Delicious, Golden Delicious and McIntosh are highly susceptible; Ambri, Granny Smith, Florina and Prima are moderately resistant; Vf-gene cultivars (Florina, Prima, Priscilla, Liberty) carry race-1 resistance.

Hosts and lifecycle

The fungus overwinters as pseudothecia (sexual fruit-bodies) in fallen leaves on the orchard floor. Ascospores mature with spring rain and are forcibly discharged into the canopy in March-May, coinciding with green-tip and pink-bud stages — these primary infections are the foundation of the season's epidemic. Secondary inoculum (conidia from primary lesions) drives summer cycles. Optimum infection conditions: 16-24 °C and continuous leaf wetness of 9-12 hours; the Mills table relates wetness duration and temperature to infection risk.

Management

ICAR-CITH and HRRS Mashobra recommend an integrated programme:

  • Sanitation: rake and burn or compost fallen leaves in autumn; urea (5%) spray on fallen leaves accelerates pseudothecia decomposition and is the most cost-effective single intervention.
  • Resistant cultivars: prefer Vf-gene cultivars or moderately resistant cultivars in newly established blocks.
  • Canopy management: open-centre training, summer light-management pruning and wide row spacing reduce leaf-wetness duration; this is harder to achieve in dense HDP blocks and so disciplined fungicide programmes are essential there.
  • Pre-bloom protectant sprays: dodine (0.075%) or mancozeb (0.25%) or captan (0.3%) at green-tip and pink-bud; repeat at petal-fall and 7-14 day intervals through May-June.
  • Curative / systemic sprays: dodine + mancozeb tank-mix, or triazole (difenoconazole 0.03%, hexaconazole 0.05%) or strobilurin (kresoxim-methyl 0.05%) within 72-96 hours of an Mills-table infection event. Rotate FRAC groups (M, 3, 11) to manage resistance.

Forecasting and decision support

J&K and HP departments operate Mills-table-based scab forecasting using orchard weather stations; SMS alerts to growers signal high-risk infection windows so curative sprays can be deployed within the 72-hour window after rain.

See also: Royal Delicious, Red Delicious, Ambri, HDP, codling moth, fire blight.

Sources

  1. Apple scab management. ICAR-CITH Srinagar.
  2. Apple scab. HRRS Mashobra, Dr YS Parmar University.
  3. Apple Scab. SKUAST-Kashmir Plant Pathology.