Sure, it tastes lousy now, but try it when it's a little rotten!
Sometimes it's pretty obvious why certain fruits fail to catch on. Take the medlar, for example. A tree fruit which shares a family (Rosaceae) with such mainstays of American fruit consumption as apples and pears, and native to the eastern Mediterranean, it seems as though it would have been quickly adopted in Europe and shipped along to the New World with all the various other things, good and bad, to become part of our culture heritage here.
Medlars, however, are not exactly wildly popular in Europe, and most folks in the United States have never heard of them. That's because of their one big catch: they're basically inedible fresh. Off the tree even when dead ripe, they are hard, green, and extremely astringent. There are other fruits with this issue, and in most cases the problem is solved by simply making them into jellies or preserves, but even that (alone, anyway) does not work for the medlar. The solution to this is "bletting": ripe medlars are taken from the tree and spread on some rather absorptive material (like straw, sawdust, or bran) somewhere rather cool, and allowed to decay for several weeks. Ideally, they're harvested from the tree immediately following a hard frost, which jumpstarts the bletting process by breaking down cell walls and speeding decay. Once the process is complete, the flesh will have broken down to the point where it can be spooned out of the skin. The sticky mush substance tastes a bit like something somewhere in between dates and a weird, dry applesauce, with maybe a hint of cinnamon. I'm told its an acquired taste, but I've yet to acquire it (not that I've tried very hard to). I've been warned that there is a fine line to be observed here, and that letting medlars blet too long is a good recipe for an upset stomach.
As is the case with so many things these days, most folks don't really want to be bothered with that anymore. The medlar enjoyed a run of popularity during the Middle Ages and Renaissance, particularly in Tudor England, and Charlemagne specifically ordered them to be planted on his royal (imperial?) estates. Part of their charm in those days was that because they were harvested late in the season and then allowed to blet for many weeks, medlars were some of the very few sweet things available to eat in Europe during the winter. They enjoyed another run of popularity in the Victorian era as desserts, but with the advent of controlled atmosphere storage and worldwide food distribution, that particular reason to eat medlars is essentially gone unless you're a hardcore homesteader or live somewhere without a supermarket. They maintain a small niche today among gourmets, and in French cooking (which appears to have found a place for pretty much every edible substance on the face of the planet.)
Botanically, medlars are interesting in that unlike other related fruit, the seeds are born in open chambers on the calyx end of the fruit, a fact which prompted Chaucer and other writers of his era to refer to it as "the open-arse-fruit", and the French to nickname it "cul de chien", meaning "dog's asshole" (Other off-color medlar metaphors are no doubt the reason for the medlar reference in Romeo and Juliet...I believe there's another one in As You Like It, too). Such names probably don't do much for its popularity, and the fruit are, to my eye anyway, rather unattractive: a heavily russetted green, with a flared open calyx, rather similar to a deformed crabapple.
The tree, however, is more attractive than its fruit, with large, abundant, white flowers which would justify its use as an ornamental if not for the need to dispose of all the resulting medlars somehow (it is self-fertile, so even a single tree will set fruit). I've seen it described as a large shrub, but it always seemed more tree-like to me, especially since it can hit heights of more than thirty feet (though it rarely does, as it is rather slow growing). The leaves are dense, downy, and dark (how alliterative!). Quince or hawthorn are frequently used as rootstocks.
There a handful of medlar cultivars around, some of which are carried by nurseries. 'Macrocarpa' (meaning "big fruit") is among the most common, and is available in the U.S. from Raintree Nursery among other places. Honestly, this is the only cultivar I can recall seeing for sale in the U.S. (Edible Landscaping has a medlar for sale, too, but doesn't list the cultivar). Also popular (if a medlar can be said to be so) is the 'Medlar of Nottingham', commonly known as 'Nottingham', and 'Large Dutch' which are available from a variety of sources in the U.K. and Australia. Keepers Nursery in England has perhaps the largest selection of medlar cultivars I've seen. I'm not sure how international my audience is here, but while I'm on such a track I might as well mention that they're also available at Perry's Fruit & Nut Nursery and Daley's Fruit Tree Nursery in Australia and at Diack's Nurseries in New Zealand.
For quite a while, the European Medlar, Mespilus germanica, was considered to be alone in its genus. All that changed, however, when in 1990 a new species called Stern's Medlar was described in Arkansas. The new species, designated Mespilus canescens, is confined to a single population of about 25 individuals in the Konecny Grove Natural Area (there have been unconfirmed reports of individual trees outside this area). The new species has been suggested to be polyploid and may in fact be of hybrid origin, but the limited amount of material and the rather small field of medlar researchers have so far limited the amount of work done on the subject. Interestingly, although Stern's Medlar is known to have fruited in the past, none have done so for several years, despite prolific flowering.
Note that so-called "Japanese Medlars" are actually Loquats, Eriobotrya japonica, another Rosaceous tree fruit (I've been meaning to do a write-up on these for a while).
I'll end this entry with a question I've had for a long time: can anyone think of anything else one blets besides medlars? Some one once suggested persimmons to me, but I don't know that I've ever heard it used in that context. I find it strangely appealing that a fruit as obscure as medlars has verb dedicated specifically to it.