From the symptoms shown by the leaf in your photo, your tree does NOT have citrus greening. Your tree's leaves look like that because you do not fertilize the tree. You do not say how old the tree is, but a citrus tree that is 4 years of age or older, and planted in the ground, should be fertilized three times a year during the growing season. Susanne is correct, the leaf your photo is showing is a manganese deficient leaf, also with a possibility of a zinc deficiency. Both deficiencies have pretty much the same leaf symptoms, with the exception that zinc deficient leaves are dwarfed by the deficiency. Never the less, when you treat for either deficiency you should treat for both at the same time. You can foliar spray the tree with any water soluble manganese/zinc trace mineral mix. You can also treat by fertilizing the tree through the soil with a fertilizer that contains NPK and all the trace minerals. The probable reason that your tree did not produce as much fruit this year as last year, is due to last years crop was a large crop, plus the fact that the tree is deficient.