The marks on the leaves of this Buddleia seedling, self-seeded by the side of our path, are a downy mildew caused by a fungus-like microorganism, Peronospora hariotii. This is one a large number of Pseudofungi that are highly specific to certain host plants. This one seems quite benign though it can be a problem in plant nurseries as it infects younger plants more than mature bushes. Other Peronospora, such as those effecting tobacco and brassicae, are a serious agricultural pest.