Red leaf spot of plum

The causative agent of red leaf spot of plum is the ascomycete fungus Polystigma fulvum (also known as Polystigma rubrum). This pathogen is highly host-specific, primarily infecting trees of the genus Prunus, such as European plums and damsons. It is a common fungal disease that significantly impacts the foliage health of stone fruit trees during the growing season.

Susceptible cultivars are found in orchards where humidity levels are high, especially during the spring bloom period. The fungus overwinters in infected leaf litter, producing fruiting bodies called perithecia. During spring, ascospores are released into the air and spread by splashing rain or wind onto young, susceptible leaves, initiating the primary infection cycle.

Symptoms are distinct and easily recognizable: they appear as circular, slightly thickened spots that turn bright red or orange as the season progresses. These spots have a glossy, leathery texture and are visible on both sides of the leaf. In severe cases, the infected leaf tissue dies, leading to leaf curling, premature yellowing, and heavy defoliation by mid-summer.

The economic impact of this disease is primarily caused by early defoliation, which severely weakens the trees. Because the foliage drops prematurely, the tree fails to accumulate sufficient carbohydrate reserves, leading to poor bud development, reduced cold hardiness, and lower yield potential in the following years. Fruit size and sugar content may also be negatively affected.

Management strategies focus on breaking the disease cycle by removing primary inoculum sources. Growers should collect and burn all fallen leaves to prevent fungal spores from spreading the following spring. Additionally, preventative fungicide applications, particularly during the dormant season and immediately after petal fall, are essential for maintaining a healthy orchard environment.

• Sanitation by removing and destroying fallen leaves.
• Soil cultivation around the base of the trees.
• Application of copper-based fungicides.
• Pruning to improve canopy air circulation.