This looks like severe magnesium deficiency to me. The chlorosis begins at or near the leaf edge and progresses inward between the primary and secondary veins. Some green color remains along the main veins and the chlorotic area may become almost creamy-white. The remaining green tissue surrounding the main veins is best described as a "Christmas tree pattern." The chlorotic margins can become necrotic and turn brown as the deficiency becomes more advanced.

You can use an all-purpose fertilizer that has magnesium as a micronutrient. Broadcast over the rootzone and water in well. Use the recommended amount from the product label - no more.

Be sure to check your irrigation schedule. You want to water deeply, but not too often. Its possible that you are leaching away nutrients from the soil by watering too often. A good schedule would be to irrigate once every 10 days this time of year, increasing to once a week in summer. The trick is to apply a generous amount of water allowing the soil to dry a bit in-between.

A tree the size of yours (I'm guessing it has a 6 foot wide canopy) would probably need about 25 gallons each time you water. Don't water right next to the trunk. Instead place your emitters at the edge of the branches - that's where the feeder roots are.

Its also a good idea to remove the stake that is right next to the trunk. The tree doesn't need it and it can rub on the trunk and branches creating wounds.

Good luck.

Ever smell a bradford pear in bloom?

You're better off unless you like the smell of an old fishing dock. Hehe...

On the East Coast they're a mistake many of us wish wouldn't have made it into so many roadway plantings.

Thanks for the question, runawayoctober, and the response, aztreelvr! This looks like the same thing that's happening to one of my gardenias, though not quite as severe yet. I've got some palm food with magnesium that I'll apply tonight and start adding the gardenias to my fertilization schedule. A couple additional questions:
1. What should I look for to let me know that the magnesium deficiency is resolved (i.e. will leaves that are showing yellow, but not browned return to green or will it impact only new growth)?
2. How long should it take to start showing results (i.e. if no impact in 30 days, fertilize again, or wait until product recommended interval)?

Magnesium is mobile and unless they're far gone to the point of damage the leaves can return to green on old growth. The old growth is currently "feeding" the new growth.

Thanks for the quick response, nc.

Thanks for your responses guys!

Could it be a double-tap of Fire Blight AND Magnesium deficiency? I was cruising around Google today and learned that pears are susceptible to Bacterial Leaf Scorch and Fire Blight. The dying twig ends look like the images that I found of FB.

Should I fertilize first and wait for a change or treat for FB? I worry that if it is FB that it'll spread or get worse. Or both at the same time?

NC, I thought the decaying fish smell trees were ginkgo trees? It bloomed in Feb and we didn't smell anything, good or bad. Maybe that's another sign it sick.

I could also be wrong that it's a ornamental pear, but, again through the wonders of Google, I'm pretty sure I ID'd it right.

They smell way worse in mass plantings. I'm exaggerating it's awfulness a bit.

runawayoctober this is not fire blight. Give it a slow release fertilizer like Osmocote for Citrus & Avocado's. To treat it for the Mag deficiency take a gallon container and fill it with water and a half cup of Epsom Salts. Shake it well and pour it around the tree before you water. If the new growth is not green two weeks after give it another dose of the Epsom Salts and that should be enough to do it.

If giving it magnesium doesn't help, it may be prometone damage, possibly from that "touch of herbicide" near the dripline. If so, treat the soil with humates to detox.