your point is valid but what factor can be behind this. Is it some particular change in environment like reduction in humidity? Well light penetration? or some defense mechanism related to better develop architecture espacially in case of open architecture. My personal observation is that even fungicide spray at flowering, fruit setting, even few weeks before harvesting cant stop postharvest diseases like anthracnose in mango. But the literature is lacking for testing the efficacy of different architectural system against postharvest disease incidence in case of mango

There are a few known instances of pruning trees to reduce postharvest disease occurrence (brown rot of citrus, cluster decay of grapes), which involve altering various environmental factors, such as humidity, wind, irradiation etc., probably different for each specific case. Although I don't know of any method that affects postharvest mango diseases, I doubt that they could be prevented by manipulation of the tree canopy, but they could possibly be reduced if the fruit could be protected from wounding and extreme climatic conditions.