Oidium is a genus of Deuteromycetes, where traditionally most anamorphs of the order Erysiphales are included. Most of them are plant pathogens causing different forms of powdery mildew, for example:

• Oidium alphitoides (= O. quercinum) = Microsphaera alphitoides (on oaks)
• Oidium anacardii (= Acrosporium anacardii) (on cashew)
• Oidium arachidis (on peanut)
• Oidium asteris-punicei (= Erysiphe cichoracearum) (on mangos)
• Oidium balsamii = Erysiphe verbasci (on mulleins)
• Oidium begoniae (=O.cyclaminis, O.lini, O.violae) = Erysiphe polyphaga (on Valerianella, Calluna and Erica)
• Oidium candicans (= Monilia candicans)
• Oidium caricae (on papayas)
• Oidium caricae-papayae (on papayas)
• Oidium caricicola (on papayas)
• Oidium chrysanthemi = Erysiphe cichoracearum (on numerous plant species, especially Asteraceae)
• Oidium carpini
• Oidium chartarum (= Torula chartarum)
• Oidium cyparissiae (= Acrosporium cyparissiae) (on cypress)
• Oidium erysiphoides = Sphaerotheca fuliginea (on melons, watermelons and cucumbers)
• Oidium euonymi-japonicae (on spindle-trees)
• Oidium farinosum = Podosphaera leucotricha (on apples)
• Oidium fructigenum = Monilia fructigena (on Rosaceae)
• Oidium helichrysi (on strawflowers)
• Oidium heveae (on rubber tree)
• Oidium hortensiae = Microsphaera polonica (on bear's breeches and hydrangea)
• Oidium indicum (on papayas and other tropical fruits)
• Oidium lauracearum (on laurel)
• Oidium lini ( = Euoidium lini)(on flax)
• Oidium lycopersicum (on tomatoes)
• Oidium mangiferae
• Oidium manihotis
• Oidium monilioides (=O.tritici)= Blumeria graminis (on cereals)
• Oidium neolycopersici (on tomatoes)
• Oidium oxalidis = Microsphaera russellii (on sorrels)
• Oidium papayae (on papayas)
• Oidium pullorum
• Oidium sesami
• Oidium tingitaninum
• Oidium tuckeri = Uncinula necator (on grapes)

Oidium albicans has been a parallel name of the yeast Candida albicans (not related to the powdery mildew).

Some other species of medical importance previously classified as Oidium include:

• Oidium cutaneum = Trichosporon cutaneum (cause of some human fungal diseases)
• Oidium lactis (=Oidium pulmoneum) = Geotrichum candidum (in soil, water and air, also belongs to normal human flora)