Another one of the rules of the hadas presented in the Mishna (32a) is that if there are more berries than leaves, the hadas cannot be used. If the berries are removed, however, then the hadas is considered kosher for use.
R Hisda said: This statement was stated by our great Rabbi, Rav, and may the Omnipresent come to his assistance. The Sages taught this only if the berries were concentrated in one place. However, if they were distributed in two or three places throughout the branch, it is fit.
Rava said to Rav Hisda: If the berries are distributed in two or three places, the myrtle branch is speckled with different colors in different places. It lacks beauty and is certainly unfit.
Rava explains that Rav must have been making a different point: that if the berries were green they would not be a problem; it is only if they are black (or red, according to Rav Papa) that the hadas cannot be used.
At no point does the Gemara explain why the berries create a problem for the hadas. The implication of the Gemara (as interpreted by Rashi) is that the problem is one of hadar – that the four species must be particularly beautiful, and the contrasting color of the berries is considered a blemish, marring the hadar of the branches.
The Jerusalem Talmud suggests two possible problems with the berries. One suggestion is that the berries – with their distinct color – appear to be foreign to the branch; another possibility is that the commandment in the Torah is to perform the mitzva with the branch (anaf) – not with the fruit (peri). Once the berries have ripened – as is indicted by the change in color – they are considered fruit, which cannot be used for performance of the mitzva.