Some banana pests are soil borne. Once you see the signs of infection in a banana plant, the best strategy is to uproot it and burn it. Then get Calcium oxide powder and and sprinkle it generously in the hole and then mix the powder with water. A vigorous exothermic reaction will take place and generates alot of heat which will kill the disease causing organisms. The residue, slaked lime is highly basic and would improve the soil fertility and mitigate against the recurrence of pests and diseases.