Diseases of Fruit Plantation Medicinal and Aromatic Crops

Apple scab

causal organism: Venturia inaequalis (Cooke) Wint.

Symptoms:

• Infections are most obvious on leaves and fruit.
• When infection first occurs on leaves, it is visible as a lighter shade of green when compared to the rest of the leaf surface.
• The inner portion of the lesion may become grey - brown as the fungus leaf tissue in that area die.
• Under severe conditions, the whole leaf can give a velvet appearance, a phenomenon known as scab and this condition can lead to premature defoliation.
• Foliar lesions which are formed in the autumn (late season scab) may appear on either surface of the leaf as a small, round, tan or black spot that closely resembles other fungal lesions which can affect apple leaves.
• Early infection of the fruit often results in large lesions which deform the fruit and may cause its dropping.
  

Disease cycle:

• The fungus primarily over winters on infected fallen leaves in the orchard floor.
• Ascospores are released in the spring at about bud break stage and disseminated by wind during rainy season.
• Moisture is required for the spores to germinate.
• The time it takes for infection to occur is a function of the number of hours of leaf wetness and the temperature Several secondary cycles of infection, arising from spores produced in primary lesions, may occur during the growing season
• During the primary stage of disease development, when ascospores are released from leaves on the orchard floor, begin the degree day model in order to determine ascospore maturity.
• Orchards should be scouted on a weekly basis during the primary stage.
• Approximately seventeen days after predicted depletion of ascospores a final monitoring should be conducted for primary scab lesions.
• Management of scab for the remainder of the season should be based on the incidence of primary scab lesions.
• Late season determination of inoculum levels in the orchard may be done after harvest.
Dissemination:
Ascospores are disseminated by wind during rainy periods in the spring, and conidia are disseminated by wind and rain once infection is established on the tree
Primary source of inoculum: Infected fallen leaves
Secondary source of inoculum: Airborne conidia

Cultural Practices:

While establishing new orchard, cultivars resistant to the disease should be planted. Applications of 5% urea to leaf litter as spray in the fall in order to hasten leaf decomposition and reduce primary inoculum.