Oidia are formed by
Oidia also termed arthrospores originate via fragmentation of vegetative hyphae into short cylindrical cells separated by disarticulation of septa under nutritional or environmental cues. Process called arthroconidiation occurs under high nutrient or humidity causing hyphal segments to constrict, round up and separate into propagules. Each oidium behaves as propagule germinating into new mycelium quickly colonizing substrate. Endogenous formation, meiosis and budding not involved. Observed in Erysiphe and Coprinus. Thin-walled, hyaline, short-lived, aiding rapid multiplication but not stress resistance, distinct from thick-walled chlamydospores specialized for survival.
Ref: Campbell Biology 12th ed., Chapter 31: Oidia Fragmentation; NCERT Class 11 Fungi - Fragmentation Oidia; NCBI Arthrospore Formation