Plum red leaf spot

Plum red leaf spot, caused by the fungus Polystigma rubrum, is a common disease affecting stone fruit trees, particularly plums. It belongs to the Ascomycota phylum and is known for causing significant aesthetic and physiological damage to orchards.

The symptoms appear during the spring and summer. Initial signs include small yellowish-orange spots on the leaves. As the infection progresses, these spots expand, change color to a vibrant red or reddish-brown, and the leaf tissue becomes thickened and leathery, leading to localized leaf deformation.

The pathogen overwinters primarily in fallen leaves on the ground. When spring arrives and environmental conditions become favorable — specifically with sufficient moisture and mild temperatures — the fungus produces ascospores. These spores are spread to the fresh foliage by rain splashes and wind currents, initiating the primary cycle of infection.

The impact of this disease on the host plant is considerable. Severe infestation causes premature leaf drop, which disrupts the tree’s ability to perform photosynthesis efficiently. This depletion weakens the tree over time, reduces winter hardiness, and results in a significant decline in fruit quantity and quality.

Management strategies focus on sanitation and chemical intervention. The most critical step is the removal and destruction of leaf litter in the autumn to eliminate the primary inoculum source. Additionally, applying fungicides at the critical stages of bud break and post-blossom helps maintain the health of the trees throughout the growing season.