Greasy spot of citrus

Greasy spot is a significant fungal disease affecting various citrus species, caused by the pathogen Amycosphaerella africana. The disease is known for its dual impact on both foliage and fruit rind, frequently causing premature defoliation and severe cosmetic degradation of the marketable harvest.

The pathogen primarily attacks trees within the Citrus genus, including popular commercial varieties such as oranges, lemons, and grapefruits. While it can exist in many regions, the disease is most destructive in warm, humid climates where prolonged leaf wetness provides ideal conditions for fungal proliferation.

Symptoms typically manifest as small, yellow spots on the upper leaf surface, while the underside develops characteristic raised, oily, brown or black lesions. On the rind, the condition is often referred to as greasy spot rind blotch, characterized by dark, sunken or oily spots that render the fruit visually unappealing for commercial sale.

The disease cycle is heavily dependent on moisture. Fungal ascospores are produced in decaying leaves on the ground and are released during rainy or humid weather. These spores are dispersed by wind and water splash to the underside of young leaves, where infection is initiated through the stomata.

To effectively manage the disease, growers should focus on the following strategies:

• Promote good air circulation through proper pruning to reduce humidity within the canopy.
• Sanitize the orchard floor by removing or destroying fallen leaves that harbor the fungus.
• Apply fungicides, particularly copper-based or systemic products, during peak infection periods.