The oomycetes are not true fungi but are fungus-like organisms. They include some of the most destructive plant pathogens including the genus Phytophthora, which includes the causal agents of potato late blight and sudden oak death. Particular species of oomycetes are responsible for root rot.
Despite not being closely related to the fungi, the oomycetes have developed very similar infection strategies. Oomycetes are capable of using effector proteins to turn off a plant's defenses in its infection process. Plant pathologists commonly group them with fungal pathogens.

Significant oomycete plant pathogens

• Pythium spp.
• Phytophthora spp., including the causal agent of the Great Irish Famine (1845–1849)

Some slime molds in Phytomyxea cause important diseases, includind club root in cabbage and its relatives, and powdery scab in potatoes. These are caused by species of Plasmodiophora and Spongospora, respectively.