"It has small, red fruit in September and October."
Translation:Il a de petits fruits rouges en septembre et en octobre.
I'll try to translate this for those who just need the information:
"My professor explained it to me yesterday: For the better half of the last century, des is possible. The French rule of grammar had previously stated that you can not put des if a plural adjective precedes the noun. Both versions are correct, but de is more formal."
I think one of the points of this question is that "fruit" is not used as an uncountable mass noun in French (or at least not typically in this sort context – I don't know if it applies to all sorts of expressions involving fruit). In French, then, you can have a fruit or some fruits, but generally you can't have something that translates directly to "(some) fruit", in a mass or collective sense, as you can in English.
Another rule is that "des" changes to "de" when it precedes an adjective: