Red leaf spot of plum

Red leaf spot of plum, caused by the fungus Polystigma graminis (often referred to as Polystigma rubrum), is a common fungal disease affecting stone fruit trees, primarily plums. It belongs to the Polystigmataceae family and is characterized by the formation of distinct, fleshy, red-orange spots on the leaves, which significantly interfere with the plant's photosynthetic capabilities.

The primary symptoms appear on the upper surface of the leaves during the early summer. These spots are initially small and yellow-orange, eventually turning a vivid red or deep orange, becoming thickened and leathery. As the season progresses, the affected leaf tissue often dies or dries out, leading to premature leaf drop and overall stress to the host plant.

The life cycle of Polystigma graminis is dependent on the survival of the pathogen in fallen leaves over the winter. During the spring bloom period, ascospores are released from the perithecia present on last year's leaf debris. These spores are spread by wind and splashing rain to the fresh, emerging foliage, initiating the primary infection cycle for the new growing season.

The development and spread of the disease are heavily influenced by environmental conditions, particularly humidity and temperature. Extended periods of rainfall during the spring months create optimal conditions for the germination of spores and the establishment of the infection. Dense, poorly ventilated canopies often exacerbate the spread of the fungus within an orchard.

Economic damage is primarily caused by premature defoliation, which reduces the tree's ability to store energy for the following year. This leads to weakened trees, lower yields, and increased susceptibility to winter damage and other secondary infections. Management and control strategies include:

• Sanitation: Raking and destroying or composting leaf litter in autumn to remove the overwintering inoculum.
• Fungicide applications: Spraying trees with protective fungicides during the bud-break and post-bloom periods.
• Orchard management: Pruning to improve air circulation within the canopy, which helps to keep foliage dry.
• Soil cultivation: Turning over the soil under trees to bury leaf debris, preventing spore release in the spring.