"Oomycetes, also called water molds, have similarities to fungi and were once classified with them. However, they differ from fungi in several important ways. Unlike the chitinous cell walls of fungi, oomycetes have walls composed of cellulose. Additionally, they are generally diploid, whereas the dominant life forms of fungi are typically haploid. Phytophthora, the plant pathogen found in the soil that caused the Irish potato famine, is classified within this group."