Originally posted by Paulieb
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Not much scientific study has been done on older varieties. Most new varieties seem to quickly become prone to canker without spraying, even if they are bred to be resistant to scab. It often doesn't take long for the shallow disease resistance of modern varieties to be over-run.
Many very old varieties have outstanding disease resistance (that's why they've lasted so long - and that's why so many "old-timers" are triploid) but because they've never been widely grown - or at least not much in modern times - the modern fruit "experts" simply don't know how resistant or susceptible the trees are.
Disease resistance also can vary with the soil and climate. Some varieties definitely prefer certain soils or climates - and not necessarily all prefer the best soils!
Some of the older varieties seem to prefer difficult soils, or grow just as well in horrible soil as they will in rich soil, and crop better in the nasty soil; probably because when they were raised many centuries ago the tree simply had to grow and crop well on whatever land and conditions its owner had to make do with.
Many old-timers also show surprisingly good resistance to pest/insect problems too, because three hundred years ago there were no pheromone traps or insecticide to deal with codling moth, and no way to deal with woolly aphid other than varieties with tolerance or low attractiveness to the pest
If it came to a competition for all-round disease resistance, pest resistance and tolerance of a wide range of conditions, I'd back some of the really old triploids (and a few of the modern triploids).
Contrary to popular belief, triploids tend to have a greater level of self-fertility than most diploids, and will often set an acceptable crop with minimal pollination. Triploids regularly have few pips or no pips at all in the fruit. Funnily, some old books list varieties now known to be triploid as "self fertile". Some varieties now known to be triploid were once recommended as "good pollinators".
The trusty old Scotch Bridget is triploid yet "it will produce regular heavy crops even after a harsh winter, late frosts and a poor summer".
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