Aphanomyces is a genus of water moulds. As of 2003 there were about 45 described species. Many of these water moulds are known as ecologically important pathogens of species of plants and animals, including fish, crustaceans, and agricultural crop plants. Aphanomyces water moulds are "one of the most important yield-limiting factors in production of legumes and sugarbeet."

Some of these water moulds are host-specific, such as A. iridis, which only infects irises. Others can infest several hosts, such as A. euteiches, which grows on several legumes. The disease that manifests in plants infected with these water moulds is sometimes known simply as Aphanomyces root rot or common root rot.

Species include:

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