Scab

Venturia spp. and Spilocaea spp.

1. Identification

Scab is a fungal disease that mainly affects pome fruits, characterised by olive‑green to brownish spots, of a velvety appearance, on leaves, fruits and young shoots. The fungi develop subcuticularly, giving rise to lesions that reduce photosynthesis, cause deformations and compromise the commercial quality of the crop. The sexual stage belongs to the genus Venturia and the asexual stage to the genus Spilocaea.

2. Causal agent

The genus Venturia was described by Saccardo (1882), while Spilocaea was established by Fries (1832) as a conidial form associated with Venturia.

• Ascomycete fungi of the family Venturiaceae, mainly Venturia inaequalis (apple tree) and Venturia pyrina (pear tree).
• Conidial stage represented by species of Spilocaea.
• Subcuticular mycelium and abundant production of conidia under humid conditions.
• Need for free water for germination and infection.
• Formation of pseudothecia on fallen leaves and fruits, ensuring the primary inoculum.
• Dissemination by wind, rain and water splashes.

3. Main hosts

• Apple tree (Malus domestica).
• Pear tree (Pyrus communis).
• Other ornamental pome-fruit rosaceous plants.
• Young tissues and vigorous varieties show greater susceptibility.

4. Symptoms

• Olive‑green to brown spots on the leaves, with a velvety appearance, which may develop into necroses and premature drop.
• Circular or irregular lesions on the fruits, developing into scabs, cracks and deformations.
• Shoots with dark lesions and superficial necroses.
• Black pseudothecia visible on fallen leaves and fruits at the end of autumn/winter.

5. Disease cycle

• Overwintering survival in pseudothecia present on fallen leaves and fruits.
• Release of ascospores in spring, synchronised with bud burst.
• Production of conidia in the lesions, responsible for secondary infections.
• Dissemination favoured by rain and periods of wetness.
• Formation of new pseudothecia at the end of the season.

6. Favourable conditions

• Temperatures between 10–24 °C, with optima close to 16–20 °C.
• Presence of free water, such as rain or dew.
• Humid and mild springs.
• Dense canopies, poor air circulation and shading.
• Excessive nitrogen fertilisation increases susceptibility.

7. Disease management

• Removal or shredding of fallen leaves and fruits to reduce the inoculum.
• Aeration pruning to improve air circulation.
• Avoid sprinkler irrigation over the canopy.
• Maintain balanced fertilisation, avoiding excess nitrogen.
• Select less susceptible varieties when possible.
• Monitor climatic and phenological conditions to position preventive measures.

Bibliographic references

• Khajuria, Y. P., Akhoon, B. A., Kaul, S., & Dhar, M. K. (2023). Secretomic insights into the pathophysiology of Venturia inaequalis. Pathogens, 12(1), 66.
• Mubashir, S. S., Khan, N. A., Padder, B. A., Bhat, Z. A., & Bhat, S. N. (2024). Morphological differentiation of Venturia species infecting pome and stone fruits. Indian Phytopathology, 77, 335–343.
• Le Cam, B., Sargent, D., Gouzy, J., Amselem, J., et al. (2019). Population genome sequencing of scab fungal species (Venturia inaequalis, V. pirina, V. aucupariae, V. asperata). G3: Genes, Genomes, Genetics, 9, 2405–2414.
• Belete, T., & Boyraz, N. (2017). Review on apple scab (Venturia inaequalis) biology and management. Journal of Plant Physiology & Pathology, 5(2).
• Rancāne, R., Valiuškaitė, A. V., & Stensvand, A. S. (2023). Primary inoculum of Venturia inaequalis in its asexual form. Frontiers in Horticulture, 2.