• Definition

  Scab is a serious fungal disease caused by fungi of the genus Venturia. It attacks the leaves, flowers, fruits and young shoots of fruit trees, leading to dark spots and deformations. It is an obligate parasite, which only grows on living green tissue, which makes its control essential during the active phase of growth.

  Development Lifecycle

  1. Overwintering : the fungus overwinters in dead leaves on the ground, in the form of pseudothecia (structures containing ascospores).

  2. Release of spores : in spring, during rainfall, the ascospores are released and infect the young leaves.

  3. Primary infections : Ascospores cause the first visible spots on green organs.

  4. Secondary infections : the conidia produced on the lesions can lead to new contaminations throughout the wet season.

  5. Spread : Aided by rain, dew, and sprinkler irrigation.

  Symptoms

  • • Brown or blackish spots on the leaves (velvety appearance), sometimes halo-shaped.

    • Leaf deformities and premature leaf drop.

    • Fruit spotted, cracked, with deformations and stunted growth.

    • Lesions may be on young shoots or flower peduncles.

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  Contributing factors

  • • Wet and mild spring (optimal temperature between 15–20 °C).

    • Presence of dead leaves not decomposed on the ground.

    • Poor aeration of the foliage.

    • Moisture-promoting cultural practices (sprinkler irrigation, dense orchards).

Preventive control

• Collect and destroy dead leaves in the fall (limit sources of inoculum).
• Practice an aerated pruning to promote air circulation.
• Avoid overhead irrigation during critical periods.
• Choose partially resistant varieties when possible.

Chemical control

• Implementation of a strict program of preventive fungicide treatments :
  • Before and during the growth of the foliage and fruits (especially spring).
  • Fungicides: captan, dithianon, myclobutanil, difenoconazole, etc.
• Regular treatments in case of favorable weather for the disease.

Biological control

• Experimentation with antagonistic fungi (Aureobasidium pullulans, Bacillus subtilis).
• Improvement of microbial biodiversity in the orchard.

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