'Since karma is an essential property of every sentient individual, there is no trace in the Buddha’s teaching of such a thing as “collective karma”. This fact has profound moral implications.' -- Richard Gombrich expert on the Pali Canon of Buddhism and contributor to the SGI Institute of Oriental Philosophy [ http://www.iop.or.jp/Documents/1424/Gombrich.pdf ]
Of course there is collective karma, as a teaching in the texts [especially the Lotus Sutra] and as observable reality. This brings us back to the teachings of Nichiren:
"Question: If I have doubts, it is because I am an ignorant person. But if I doubt the words spoken to me by all persons of wisdom, then I will end up with nothing to believe in, and will merely live my life in vain, will I not?
Answer: In his dying instructions, the Buddha said, “Rely on the Law and not upon persons.” This means that if what a person says is not in agreement with what is expounded in the sutras, one should not believe it, no matter how fine the person may be...."