"Fruit usually tastes sweet" is perfectly correct.
Thanks, Bhaisahab. I was started to think I was going mad as I was about to post exactly that then I saw the other replies!!!
There is nothing wrong with "Fruit usually tastes sweet" - used as a noun to incorporate all different types of fruit, fruit takes the singular. Conversely, in the same context, vegetables would take the plural form:
Vegetables usually taste savoury.
That's why, when you see a sign at a greengrocer's shop, it will say "Fruit and Vegetables". Obviously, they don't only sell one type of fruit yet we rarely say "Fruits and vegetables".
That is not to say that "fruits" doesn't exist. Of course it does.
My favourite fruit is banana.
My favourite fruits are bananas, strawberries and apples.