SUMMARYEpidemiological studies, especially of obligate pathogens, often require healthy test plants and the incubation of infections in isolation. Ventilation of glasshouses with spore‐free air is helpful but costly, and provides only a few discrete environments. The apparatus described is cheap and simple and enables many separate covered plant pots to be supplied with filtered air. It was designed to help measure the incubation time and frequency of viable conidia ofErysiphe graminis, but may also be useful for studying other pathogens and, with some modification, for plant breeding or quarantin