Blue mold of citrus

Pathogen and disease type. The causative agent of the disease is the microscopic fungus Penicillium italicum. It is a highly destructive pathogen responsible for blue mold rot in citrus fruits. This disease is classified as a postharvest disorder, as the pathogen primarily attacks harvested fruit during shipping, packing, and long-term storage in warehouses.

Host range and infection process. The fungus affects various citrus species, including oranges, lemons, mandarins, and grapefruits. Infection typically occurs through mechanical wounds on the fruit peel, such as punctures, cuts, or abrasions sustained during harvesting or sorting. Once inside, the fungus colonizes the fruit tissue rapidly, utilizing the sugars and nutrients available in the rind and pulp.

Symptoms and signs. Symptoms begin as soft, water-soaked spots on the surface of the peel. Shortly after the initial infection, a dense fungal growth appears. Initially white, this mold quickly develops a distinctive blue-green or grey-blue color due to the massive production of conidial spores. A characteristic feature is a wide, white margin of healthy mycelium surrounding the sporulating area.

Development conditions and impact. The development of P. italicum is favored by high relative humidity (above 90%) and temperatures ranging from 15°C to 25°C. The fungus is highly contagious and spreads rapidly by contact ("nesting") and through airborne spores within storage containers. Its economic impact is devastating, as it can ruin entire shipments of citrus within a very short timeframe if conditions are not managed.

Protection and prevention measures. Effective management of blue mold requires strict postharvest sanitation and logistical care.

• Careful handling of fruit to minimize wounds and peel damage during harvesting.
• Sanitation of packing facilities and storage containers to reduce spore load.
• Maintaining proper cold chain temperatures to slow fungal growth.
• Controlled atmosphere storage and adequate ventilation to manage humidity levels.
• Application of approved postharvest fungicides to fruit to inhibit infection during the transport phase.