You'll notice from the photo that some of the leaves nearest our quince are a bit brown. I haven't identified any specific pathology causing the browning and drying, and I've noticed the same pattern of browning on the apple tree over the years; apple and quince are closely related. There may be a specific cause; it may even be a "treatable" cause. But as both trees continue to grow, thrive, and produce fruit, it's probably not worth worrying about.
It may have something to do with the fact that the high temperature in this little slice of the Desert Garden has been over 110° for at least the last 10 days. Once it's been 10 days, it's best to stop counting.
Think about that for a moment: our quince is in full sun and, while adequately watered, is subject to brutal temperature and light levels. If we were to pluck the little fruit off the tree and set it on the ground, it would be shrivelling and discolored by the sun within hours. Yet while attached to the tree by it's tiny stem, it does just fine and continues to grow and mature.
We tend to get worried about the little things that happen to our plants, while the truly amazing thing is that any of them manage even to survive.