Oomycetes are no longer considered true fungi because they have
Oomycetes excluded from true fungi and placed in Oomycota within Stramenopila, Chromista supergroup based on converging ultrastructural, biochemical and molecular divergences. Cell walls contain cellulose and β-1,3, β-1,6 glucans with negligible chitin contrasting fungal chitin-glucan architecture. Vegetative mycelium diploid throughout, mitochondria bearing tubular cristae not flattened, lysine biosynthesis via diaminopimelate pathway not aminoadipate, and biflagellate zoospores with anterior tinsel and posterior whiplash flagella driving motility. Life cycle diplontic producing oogonia and antheridia forming thick-walled oospores through oogamous sexual reproduction aligning closer to brown algae.
Ref: Campbell Biology 12th ed. Chapter 28 Protistan Relatives Oomycota; NCBI Taxonomy Oomycetes APSnet Table 1