Polystigma leaf spot

Polystigma leaf spot, commonly known as red spot disease, is a fungal pathology primarily affecting stone fruit trees, such as plums and cherries. The causal agent, the fungus Polystigma rubrum, overwinter in the fallen leaves of infected trees. During the spring, the fungus develops sexual fruiting bodies, which release spores that are disseminated by rain and wind to the new foliage.

The first clinical signs typically appear in early summer. The disease manifests as distinct, bright red to orange spots on the leaves. These spots are often slightly thickened and shiny on the upper leaf surface. As the season progresses, the infected tissue may turn brown, and the foliage eventually becomes brittle and drops prematurely, leading to significant defoliation of the tree.

Environmental conditions play a critical role in the development of the disease. High humidity, frequent rainfall, and moderate temperatures provide the optimal environment for spore germination and infection. Orchards with poor air circulation, dense canopies, and unmanaged debris on the ground are significantly more prone to severe outbreaks of the disease.

The economic impact of Polystigma leaf spot is centered on tree vitality. The premature loss of leaves impairs the tree's photosynthetic capability, which prevents the accumulation of enough energy reserves for winter. This weakening effect leads to reduced fruit yields in subsequent years, stunted growth, and a higher susceptibility to secondary infections or severe frost damage.

Effective management requires a combination of good sanitation practices and chemical controls:

• Removing and destroying all fallen leaves in autumn to eliminate the primary inoculum source.
• Tilling the soil under the tree canopy to bury infected debris.
• Pruning trees annually to improve airflow and increase sunlight exposure within the canopy.
• Applying protective copper-based fungicides at the bud-burst stage and immediately post-bloom.
• Monitoring the orchard regularly to identify early signs of infestation and prevent spread.