Geotrichum citrus rot

The causative agent of the disease is the fungus Geotrichum citri-aurantii, which belongs to the order Saccharomycetales. It is widely recognized in agriculture as the primary pathogen responsible for sour rot in various citrus species.

The fungus develops as a white or grayish mycelium that rapidly colonizes the tissues of the fruit. Its conidia are airborne and can spread easily within storage facilities, making it a highly contagious pathogen for packed produce.

Taxonomically, it functions as a facultative parasite capable of surviving in soil or on decaying organic matter. Its ability to grow under various environmental conditions allows it to persist in orchard environments for extended periods.

The optimal temperature range for fungal development is between 20 °C and 25 °C. High relative humidity is a major limiting factor for its spread; moisture on the fruit surface significantly accelerates the germination of conidia.

As a warehouse pathogen, it is particularly dangerous because it thrives in the microclimate of shipping containers and packing houses. Effective management requires a combination of sanitation and careful handling to prevent the initial infection from spreading.

This pathogen affects a broad range of citrus fruits including lemons, oranges, mandarins, and grapefruits. It is primarily a postharvest disease, resulting in significant financial losses during the storage and distribution stages.

In the field, infection occurs primarily through wounds in the peel caused by mechanical harvesting damage, insect feeding, or adverse weather conditions. Healthy fruits with intact rinds are generally resistant, but any rupture creates a pathway for the fungus.

The fungus causes rapid tissue disintegration, turning the fruit interior into a soft, watery mass. The associated sour odor and the rapid loss of structural integrity make the affected fruit completely unmarketable.

In storage conditions, the infection spreads via contact between neighboring fruits. A single infected orange in a carton can lead to the rotting of the entire batch if the humidity levels allow for mycelial growth.

Secondary damage includes the contamination of machinery and packing equipment. If not addressed, the fungus can build up in the packing environment, increasing the risk of infection for subsequent batches of harvested fruit.

The first external sign is the appearance of soft, water-soaked spots on the peel. These spots grow rapidly in size and are eventually covered by a dense, white, fungal mycelium that lacks the bright color of other common molds.

The pulp inside the infected area quickly turns into a mushy, watery liquid. When pressure is applied, the fruit releases this fluid, which is characterized by a strong, offensive, acidic smell, giving the disease its name "sour rot".

The rind loses its firmness and becomes slimy, making it easily tearable. As the fungus consumes the fruit, it moves deeper into the internal segments, effectively destroying the entire contents within a very short timeframe.

There is usually no clear demarcation line between infected and healthy tissue, as the mycelium spreads aggressively through the fruit. In later stages, the entire fruit loses its shape and becomes a sodden mass covered in white fungal threads.

• Watery, discolored lesions on the rind.
• White, fluffy mycelial growth on the surface.
• Strong, sour acidic odor from decaying tissues.
• Rapid softening and loss of fruit structure.
• Infection spread to adjacent fruits in storage containers.

The most important preventive measure is to minimize physical injuries during harvesting and handling. Since the fungus is a wound pathogen, keeping the fruit peel intact is the first line of defense against Geotrichum citri-aurantii.

Maintaining high sanitary standards in packing houses is essential. This includes the regular cleaning and disinfection of sorting lines, crates, and storage facilities to eliminate fungal spores that accumulate in the environment.

Applying postharvest fungicides during the washing process can help protect fruits during transportation. It is crucial to follow the recommended application rates for approved chemical agents to ensure effective control and consumer safety.

Orchard management practices, such as controlling insect pests that cause fruit damage and the timely removal of fallen fruit, help reduce the initial inoculum level before the harvest season begins.

Managing the storage environment by maintaining adequate ventilation and controlling temperature is vital. Preventing condensation on the fruit surface significantly reduces the likelihood of the spores germinating and initiating an infection outbreak.