The blackening on the lower side of avocados is mainly caused by fungal infections like anthracnose and sunburn. Anthracnose leads to dark, sunken spots, especially in humid weather, while sunburn affects exposed parts of the fruit. Poor pruning, lack of shade, or nutrient deficiencies (like calcium or potassium) can worsen it. Use copper-based fungicides, prune properly, and apply balanced fertilizers to control the issue.