Fruit can be both countable and uncountable. When talking about all fruit in general or unspecified fruit it is uncountable. When talking about different kinds of fruit you can say fruits. (oranges, mangoes and other fruits). So both would be possible. 'Ripe fruit falls from the trees' or 'Ripe fruits fall from the trees.' The same can be said of cheese. We can say 'They sell different cheese' or they sell cheddar, stilton and other cheeses.'

I agree with JacquiD. Ripe fruits fall from the trees - this sentence would never be said because we would state which fruit we are talking about eg. "ripe apples fall from the tree" or talk about it collectively as a whole "ripe fruit falls from the tree" when it doesn't matter which type of fruit we mean.

ripe fruit = uncountable SINGULAR noun. **All uncountable nouns are singular!** ripe fruits is WRONG

"Ripe fruit falls..." is definitely more natural. "Fruits" is less and less common nowadays; we would tend to form sentences like 'fruit is good for you' or even 'I eat lots of different fruit'. The only real times 'fruits' is used is in phrases like 'the fruits of your labour' and so on - if in doubt use the uncountable noun.