Phytophthora brown rot of citrus

The causal agent of this disease is the oomycete Phytophthora hibernalis. This pathogen is distinct for its preference for cooler temperatures compared to other Phytophthora species, often leading to outbreaks during damp winter or spring months.

This disease primarily impacts citrus species, including lemons, oranges, and grapefruit. The pathogen can attack all parts of the tree, but it is most economically devastating when it causes fruit rot, often referred to as brown rot, which can lead to significant post-harvest losses.

Symptoms initially appear as firm, dark, leathery, and somewhat translucent patches on the fruit surface. As the disease progresses, the rot becomes soft and watery. Affected trees may also show symptoms of leaf blight and dark lesions on the bark of young twigs near the soil line.

The spread of the disease is highly dependent on environmental conditions, specifically splashing rain or irrigation water that transfers spores from the soil to the lower branches of the trees. High humidity and prolonged leaf wetness provide ideal conditions for the rapid multiplication of the pathogen.

Effective management requires a combination of cultural and chemical practices. Growers should maintain good orchard hygiene, perform regular copper-based fungicide applications, and ensure adequate canopy pruning to improve airflow. Preventing soil from splashing onto the lower foliage is a critical factor in reducing infection rates.