Marssonina blotch

Early symptoms appear as small, chlorotic spots on the upper surface of the leaves, which gradually enlarge over time.

The spots eventually turn dark brown or black with irregular margins and can sometimes coalesce to cover larger areas of the leaf.

Tiny black fruiting bodies, known as acervuli, become visible as small dots within the centers of these necrotic lesions.

Severe infections lead to significant yellowing of the leaves, followed by premature defoliation that can occur as early as mid-summer.

Fruit symptoms, although less common, manifest as small, dark, depressed spots that diminish the aesthetic and market value of the harvest.

The causal agent of this disease is the fungus Diplocarpon coronariae (anamorph Marssonina coronaria), a significant pathogen of apple trees.

The fungus produces conidia within acervuli, which develop under the leaf epidermis and emerge as the disease progresses.

It overwinters primarily in infected fallen leaves on the orchard floor, surviving as dormant mycelium or as mature fruiting bodies.

In spring, when conditions become suitable, these fruiting bodies release spores that are dispersed by wind and splashing rain to healthy foliage.

The pathogen is highly efficient at establishing infections throughout the growing season, especially in humid and temperate environments.

Prolonged periods of leaf wetness and high relative humidity are essential for the germination of fungal spores and infection of host tissues.

Optimal temperatures for the development of Marssonina blotch typically range between +18°C and +25°C, providing perfect conditions for the pathogen.

Orchards with dense tree canopies and poor airflow retain moisture longer, creating a microclimate that accelerates the spread of the disease.

Frequent rainfall during the summer months is a major driver for the secondary spread of spores throughout the orchard canopy.

Inadequate sanitation practices, such as leaving leaf debris under trees, ensure a high inoculum load for the following growing season.

The primary impact of the disease is the reduction of total photosynthetic area due to the development of numerous lesions on leaves.

Premature leaf drop depletes the tree's energy reserves, leaving it weakened and unable to produce adequate flower buds for the next year.

Stressed trees are significantly more susceptible to winter damage and cold injury due to their reduced ability to harden off properly.

Overall orchard productivity declines over time as the trees become chronically exhausted and susceptible to secondary stressors.

In extreme cases, persistent defoliation over several consecutive years can lead to branch dieback and even the loss of entire trees.

The most effective management strategy is rigorous sanitation, which includes removing and destroying all fallen leaves to reduce the primary inoculum.

Implementing regular pruning practices helps to open the canopy, improving airflow and ensuring that leaves dry quickly after rainfall.

Preventive fungicide applications during critical infection periods are essential to protect developing foliage from initial pathogen colonization.

Balanced fertilization and irrigation management help maintain tree vigor, allowing the plant to better withstand the pressure of fungal diseases.

• Removal of leaf litter from the orchard floor in autumn.
• Timely application of systemic fungicides like strobilurins or triazoles.
• Selecting and planting resistant apple cultivars where possible.