Marssonina blotch

Marssonina blotch, caused by the fungus Diplocarpon mali, is a significant fungal disease affecting apple trees worldwide. It primarily targets the foliage, often leading to severe defoliation in late summer if not managed correctly, which negatively impacts the tree's physiological status and overall productivity.

The pathogen overwinters primarily in infected fallen leaves on the orchard floor. As spring weather warms and rainfall increases, the fungal spores are released and disseminated by wind and splashing water onto new leaves. High humidity and moderate temperatures create an ideal environment for the rapid colonization and sporulation of the fungus.

Initial symptoms include the appearance of small, round, greyish-brown to dark brown spots on the leaf blades. As the infection progresses, these spots expand and may coalesce, covering a large portion of the leaf surface. Small black structures, which are the reproductive spores, develop within the lesions, eventually leading to chlorosis and premature leaf drop.

The economic impact of Marssonina blotch is substantial due to premature defoliation, which interrupts photosynthesis. Trees losing their leaves early fail to accumulate sufficient carbohydrate reserves, resulting in poor flower bud development, reduced fruit quality, and increased susceptibility to winter damage in subsequent seasons.

Effective management requires a combination of sanitation and chemical interventions:

• Removing or deep-burying infected leaf litter in the autumn to eliminate the primary inoculum source.
• Pruning trees to improve light penetration and canopy aeration, which helps keep the foliage dry.
• Applying timely fungicide treatments during periods of high disease pressure and wet weather.
• Using a robust fungicide rotation program to prevent the development of resistant fungal strains.