Scab of apple

Host: Apple (Malus sylvestris (L.) Mill.)
Pathogen: Venturia inaequalis (Cke.) Wint.

Distribution

Apple is grown in the temperate region of the world. Scab is a very common disease of apple in all the apple growing countries. In India, the disease was first reported in 1935 in the native cultivar Ambri from the Kashmir valley. During early 1980s and mid-1990s, it caused epiphytotic in Himachal Pradesh of India. Infection in the leaves reduces the photosynthetic area and reduces the yield. Infected apple fruits have low marketability. In severe cases, the disease destroys whole the blossom and hence, the fruit production.


Symptoms

V. inaequalis infects the young and soft tissues of leaves, twigs and fruits. Mature leaves are resistant to the diseases. Early symptoms on leaves appear as dark green, small, irregular spots on lower surfaces. The mature scabs on leaves become circular, olive-green color, velvety and finally become black and raised. Heavily infected leaves become deformed and defoliate.

scab of apple
Fruits of apple showing scab
Infection in the young fruits case premature drop. Symptoms on the fruits are more less similar to that on the leaves, but the mature scabs on the fruits are cracked.

Apple scab disease
Apple fruit showing scab with crack. (Photograph was provided by Dr. Imtiyaz Hussain from an apple orchard of Laddakh, India.)


Pathogen

Venturia inaequalis is a facultative saprophyte growing between the cuticle and epidermis (subcuticularly).

Disease cycle

The pathogen overwinters in the infected leaves fallen on the substratum. In early spring, it germinate at the time when apple plant produces young leaves and blossom. Ascospores produced during the overwintering cause primary infection. Within 9 to 17 days of primary infection, fungus produces numerous conidia, which cause secondary infection and spread the disease in the whole orchard during the same season.


Control measures

  • Field sanitation should be maintained in apple orchard.
  • Varieties resistant to scab should be planted.
  • Infected leaves should be burned and destroyed.
  • Avoid overcrowed plantation.
  • Spray protective broad-spectrum fungicides (e.g., captan).

References



Content first created on 09-10-2020
last updated on 22-04-2021