Per the Medallion WDG Fungicide label: For indoor drench applications, use up to 30 oz of Medallion WDG per 1000 sq ft. (80 lb. product/A) per year or crop cycle. Applications to pre-potting mix can be made up to 4 oz of Medallion WDG per cu. yd. For field grown and landscape ornamentals, apply up to a maximum of 1.5 oz/1000 sq. ft/year (4 lb. product/A/year). For outdoor container growth ornamentals, apply up to a maximum of 3 oz/1000 sq. ft/year (8 lb. product/A/year). Generally, 1 gallon will cover 1000 sq ft.
The Medallion WDG Fungicide does not list the Kentia Palm on the label and would not be recommended without consulting your local master gardener.
http://npic.orst.edu/mlrDetail.html?state=HI