From Shulchanaruchharav Web site:
[and milk]. If fruit juice is mixed with water then it is even more of a problem then plain water regarding its ability to ferment, being that plain water takes at least 18 minutes to ferment, while when mixed with fruit juice it can ferment immediately. Thus care must especially be taken that when eating Matzah with fruits etc, that there is no water around, including perspiration, on the fruit. Practically one may eat Matzah with avocado, tomato, oil. But one must beware that cheese and butter may not be placed on the Matzah as they contain water.