Use a high-nitrogen formula. The tree may suffer from iron chlorosis or zinc deficiency. Treat with chelated iron or iron sulfate. Zinc deficiency shows up as a yellowish blotch or mottle between leaf veins. Control with zinc foliar sprays.
Commercial products are available as sprays containing both iron chelates and zinc.
Aphids, mites, scale insects and mealybugs may be a problem. If these pests' natural enemies fail to handle the infestations, and if jets of water fail to keep the pests in check, spray with appropriate chemicals. If scale remains troublesome, spray with light oil in early spring. Bait or spray for snails and slugs whenever necessary, especially during warm-night spells of winter and spring. With an appropriate maintenance routine, you should have more grapefruit than you can eat.