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Downy mildew on hops is caused by the pathogen "Pseudoperonospora humuli", an oomycete protist. "P. humuli" is an obligate biotrophic pathogen, meaning that it can only live and grow in living host tissue. "P. humuli," like most downy mildews, is highly host-specific and thus will only infect hop ("Humulus lupulus") and also Japanese hop ("Humulus japonicas").