Answer: When leaf color is pale, it is almost always due to overwatering. Although you have a line of trees and most of them are doing well, the two affected trees must be planted in ground that doesn't drain as well as it should. Overly wet soils can suffocate roots because the oxygen is driven out by the water. You can water those two trees less often than the others or you can wait until they are dormant, dig them up, amend the soil so it drains faster and then replant the trees.
Best wishes with your pear trees!
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