possibly a nitrogen deficiency. I planted some pears four years ago. In the years following I figured out a bit more what I was doing and ammended the soil to plant other trees nearby. Original trees leaves turn yellow. It is hard to ammend the soil when you can't turn it over, so have to do it over time. In the meantime I spray with a foliar fertilizer with a higher N than the P & K.

You don't say where you live. If the soil pH is too high or too low (outside 6.5+/-0.3) the ability of the trees to take up nutrients is greatly reduced. My notes on yellowing leaves:

yellowing leaves (chlorosis)

salt burn (tip scorched, under watering, not watering deeply) Causes: Salt accumulation from irrigation and fertilization. Damage occurs on leaves on all sides of the tree.
Controls: Leach out excess salts by applying large amounts of water to the soil periodically.

iron deficiency (only on new leaves = ends of the branches) yellow leaf with green veins, very common

zinc deficiency yellow leaf with green veins Zinc deficient leaves are undersized, while iron deficient leaves are normal-sized. Deficiencies may be corrected by foliar or soil applications of zinc salts or zinc chelates.

manganese deficiency yellow leaf with green veins manganese sulfate or a manganese chelate to treat. Soil applications of manganese sulfate in our alkaline soils are less effective than foliar applications.

90% of the time it is iron deficiency. Take an iron solution with a few drops of Ivory liquid detergent and, making sure the iron solution is slightly acidic with a little bit of vinegar, dip the leaves in the solution for a few minutes. You should see a color change in the leaves in about 24 hours or less. If it is something other than iron, it won't do anything. Then go ahead and try your manganese application.

If this color change occurred during the winter it is possible it could be cold damage. This appears more like a bronzing of the leaves rather than yellowing.

Completely Pale Green or Yellowish Leaves:
Causes: Nitrogen deficiency, over-watering. Occurs mostly on the older growth. Leaves also fall prematurely.
Controls: Apply nitrogen containing fertilizer - follow recommended watering and fertilization practices.

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Iron deficiency often appears in winter due to low soil temperatures, and root inactivity. High soil pH and poorly drained soil may also cause iron deficiency. Iron deficiency symptoms occur on the younger leaves, whereas nitrogen deficiency symptoms occur on the older foliage.

First, check the watering schedule on your fruit trees and make sure the moisture is reaching a depth of 2 - 3 feet. You can check with a probe after waiting a few hours after your irrigation system runs. Push the probe into the soil. It will be easy to probe where the water has moistened the soil but harder when you reach the zone where the water stopped. If you can only easily probe down to one foot, you need to water longer. Or you can use a moisture sensor available at home improvement stores or nurseries.

Your goal is to water deeply, but infrequently, allowing the soil to dry just a bit before applying more water. Roots need oxygen as well as water in order to absorb nutrients and if soils are soggy, plants are less efficient at this process. This time of year fruit trees should be watered about once every 1 - 2 weeks.

You can apply iron or nitrogen as a fertilizer, being careful not to apply too much as fertilizers can burn roots. Use a 'chelated' or water soluble iron product that contains iron that can be absorbed by the roots.
Nitrogen is available in packaged fertilizers such as ammonium sulfate, ammonium phosphate, ammonium nitrate, etc. Follow package directions for the size and age of your trees, broadcast over the root zone and water in well.

Thanks for the responses. It looks like it was lack of water since we have not had any rain lately and the tree didn't get watered as much as the other ones. I am only there on the weekends so it doesn't get watered during the week. It should have plenty of fertilizer. It is planted near a old cattle pen where the water runoff washes the old manure down to the trees. If anything they get too much fertilzer. It seems to be doing a lot lot better since it was soaked the last couple weekends.