Reducing crop losses due to fungal diseases requires improved understanding of the mechanisms governing plant immunity and fungal pathogenesis, which in turn requires accurate determination of disease phenotypes of plants upon infection with a particular fungal pathogen. However, accurate disease phenotyping with unculturable biotrophic fungal pathogens such as powdery mildew is not easy to achieve and can be a rate-limiting step of a research project. Here, we have developed a safe, efficient, and easy-to-operate disease phenotyping system using the Arabidopsis-powdery mildew interaction as an example. This system mainly consists of three components: (i) a wooden inoculation box fitted with a removable lid mounted with a stainless steel or nylon mesh of ~50 µm pores for inoculating a flat of plants with fungal spores, (ii) a transparent plastic chamber with a small front opening for minimizing spore escape while conducting inoculation inside, and (iii) a spore-dislodging and distribution method for even and effective inoculation. The protocols described here include the steps and parameters for making the inoculation box and the plastic chamber at a low cost, and a video demonstration of how to use the system to enable even inoculation with powdery mildew spores, thereby improving accuracy and reproducibility of disease phenotyping.