If it was me, I would go with a non-apple tree to replace the one that is dying but keep the healthy one until it gets diseased. Squirrels got all of my apples before they got close to ripening this year but in the three years of growing the apple trees, they have not gotten any diseases that I have noticed. If the apple tree in my front yard, dies I will likely replace it with a peach. It is not thriving as it is in horrendous soil and is in the shadow of a big hackberry tree, so nothing is likely to really produce fruit in that location, but that will likely not stop me from trying. Serviceberry is another option which seem to be overlooked by a lot of garden references. They have smaller fruit but may be less prone to the diseases.

Eric, I agree with Mars......growing apples here is just a really difficult exercise in frustration. I decided long ago to devote our fruit tree space to something easier---so I grow peaches, plums and figs. Ironically, of them all, the most reliable producers are the fig trees. Even if they freeze back to the ground in a cold winter (which is common here), they regrow quickly and produce a crop every single year, and two crops some years. The peaches and plums don't produce every year as they tend to bloom too early and get frozen back after blooming and once small fruit have set. In the years that they do produce, the harvest is wonderful to have. I have a neighbor who grows apricots and cherries, but you have to fight the birds for every single cherry and it isn't easy.

Even if you can find a pollinator for your apple (you might go to the website of Stark Brothers and see what pollinators they recommend for Yellow Delicious), you will have to maintain a strict spraying schedule to try to keep those apple trees alive and thriving here in OK. There's a lot more to worry about than just Cedar Apple Rust, including pests that will ruin your fruit if you don't spray at just the right time and fire blight as well. I have two friends near me who grow apples and, honestly, I don't think what they have to do in order to get apples is worth the trouble. They both grow Granny Smith, essentially for pie apples.

Pears can be grown here too, but will have a lot of the same issues as apples, but particularly fire blight which tends to strike the trees about the time they finally reach bearing size and are capable of producing a crop.

Dawn

You need another apple or a crabapple to cross pollinate your remaining apple - pears or any other type of fruit tree will NOT work! And despite what you might read or how they are publicized, ALL apples need a second variety for cross pollination. None are really fully self fertile and although you may get a few fruit from just a single tree, it is nothing like a normal crop that you would realize with proper fertilization.

Red Delicious, Liberty, McIntosh and Redfree are all very resistant to cedar apple rust. And all with the exception of Liberty will work as pollination partners for the Golden Delicious. And there are a good number of other disease resistant apples that you could plant that will not require a lot of spraying attention. Many people are able to grow fruit trees without excessive spraying - you just need to select for diseases resistant varieties and the ones that will perform best in your climate. The Fruit Tree forum can provide a lot more detailed information.