Red leaf spot of plum

Systematic position and pathogen type: The causal agent of the disease is the ascomycete fungus Polystigma ochraceum, which belongs to the order Phyllachorales. This pathogen is highly specific to the genus Prunus, primarily affecting plum trees and sloe, and is responsible for the condition commonly known as red leaf spot or red blotch.

Diseases and host crops: The fungus primarily infects the leaves of plum trees. The disease is identified by distinct, raised, shiny, orange-to-red spots on the leaf surface. As the infection progresses, the affected tissue undergoes necrosis, leading to premature leaf drop and negatively impacting the tree’s overall physiological health and vigor.

Biology and life cycle: The pathogen overwinters as perithecia within the dead, fallen leaves from the previous season. In the spring, coinciding with the flowering period, ascospores are discharged and dispersed by wind and splashing rain onto the newly emerged foliage. During the summer, the fungus produces conidia, allowing for multiple cycles of secondary infection during the growing season.

Conditions for development and economic impact: Development is highly dependent on humidity and temperature during the spring. High rainfall periods increase the severity of the outbreak. The economic impact includes reduced photosynthesis, which leads to smaller fruit size, lower sugar content, and reduced tree hardiness, potentially increasing susceptibility to winter injury or other secondary pathogens.

Protection and control measures: Effective management relies on an integrated approach focusing on sanitation and chemical intervention. The most critical step is the removal and destruction of fallen leaves to eliminate the primary inoculum source. Additionally, applying copper-based fungicides during the dormant season and systemic fungicides during the early leaf-development stage can significantly reduce infection rates.