In citrus farming, fruit rot caused by Phytophthora citrophthora is one of those diseases that, although not immediately devastating, can steadily affect both fruit quality and commercial value. Its presence is linked to common conditions in many citrus-growing areas—humidity, rainfall, and poorly drained soils) which makes recognizing and managing it an essential part of fieldwork. Understanding how this pathogen develops and which practices help keep it under control is key to protecting profitability and ensuring healthier, longer-lasting production.

Understanding the Pathogen

Citrus fruit rot, also known as “brown rot” or aguado (in Spanish) is caused by Phytophthora citrophthora, a soilborne oomycete that can survive for long periods in the soil as spores for long-term persistence or as persistent mycelium in plant debris.

Under favorable conditions, high humidity and mild temperatures between 15 and 25 °C, the pathogen produces sporangia that release motile zoospores. These zoospores move through water and infect fruits that are either in contact with the soil or exposed to rain or irrigation splashes.

The pathogen can penetrate directly through the fruit cuticle or through small surface wounds. Once inside, it rapidly colonizes the outer peel (flavedo) and inner pith (albedo), leading to tissue necrosis and a soft, water-soaked decay. Although infections can occur at any stage of fruit development, the most severe cases typically appear on mature fruit close to harvest and during wet periods.

Recognizing Symptoms in the Field

Early detection is essential for effective management. Common symptoms include:

• Soft, water-soaked, sunken lesions at the fruit’s basal end or wherever it touches the soil.

• Dark brown discoloration that spreads quickly across the peel.

• White, cottony mycelium growth on the lesion surface under high humidity.

• A fermentative odor and loss of firmness.

• Premature fruit drop, which increases the amount of inoculum in the soil.

While these signs are characteristic, they can be mistaken for other rots. It’s important to consider environmental context—recent rainfall, high humidity, and fruits hanging low near the soil often point to Phytophthora activity.

Conditions that Favor Disease Development

Fruit rot rarely occurs in isolation; it’s usually the result of several contributing factors:

• Poor drainage and standing water, which promote zoospore movement and infection.

• Heavy rains or overhead irrigation, which cause splashing from soil to fruit.

• Low-hanging fruit, especially in short or dense trees, which increases contact with contaminated soil.

• High planting density and late harvests, which create humid microclimates ideal for the pathogen.

• In post-harvest stages, improper handling or lack of post-harvest treatment can allow the fungus to continue developing during storage and transport.

Each factor on its own might seem minor, but together they create the perfect conditions for Phytophthora citrophthora to thrive.

Integrated Management Strategies

Controlling Phytophthora fruit rot requires an integrated approach combining prevention, cultural practices, irrigation management, and chemical protection. No single method is sufficient, but applying several together can significantly reduce disease incidence.

a) Cultural Management

• Improve soil drainage to avoid water-logging; use vegetative ground covers to reduce splash dispersal.
• Conduct training and pruning to raise the fruiting height and minimize fruit-soil contact.
• Avoid wounding fruits during harvest and handling.
• Remove fallen or infected fruits to reduce primary inoculum in the orchard.

b) Irrigation Management

• Avoid overhead irrigation during critical or rainy periods.
• Adjust irrigation schedules and volumes to prevent excessive soil moisture.

c) Chemical Control

• Apply systemic and contact fungicides, such as those containing phosphonates, copper, or metalaxyl-M, preventively during rainy seasons or at the first signs of infection.
• Always follow technical recommendations, ensuring you're using authorized products with correct dosages, and rotating modes of action to prevent resistance.

Conclusion

Phytophthora citrophthora fruit rot is not just a chemical control issue—it’s a matter of integrated orchard management. Prevention, good drainage, and timely intervention are the most effective tools for protecting both fruit quality and profitability.

Each small action; monitoring soil moisture, pruning to lift fruit from the ground, or removing fallen fruit, adds up to create an effective defense against this persistent disease.

Is your orchard ready for the next rainy season? Prevention today can make the difference between a damaged harvest and a successful one.