Black pod disease of cocoa is controlled by the application of fungicide. This is because black pod disease is caused by a fungus that attacks the pods of cocoa trees, resulting in loss of yield and quality of the crop. The application of fungicide helps to prevent or reduce the spread of the disease by killing or inhibiting the growth of the fungus. Other methods like farm sanitation and fumigation of soil may also help to control the disease indirectly by reducing the spread of spores and creating a healthier growing environment, but they are not direct methods of controlling the disease like the application of fungicide.