Powdery mildew haustoria are easier to be observed by light microscopy in trichome cells compared to epidermal cells of infected leaves. The objective of this study was to explore the relationship between the hyphal length and the number of haustoria in type I trichome cells of tomato (Solanum lycopersicum Mill.) leaves. The trichome cells of tomato cv. Moneymaker were inoculated with conidia of tomato powdery mildew (Pseudoidium neolycopersici L. Kiss), isolate KTP-04. On these cells, the P. neolycopersici isolate produced a maximum of four vigorously elongated hyphae per conidium. At 12 days after inoculation, KTP-04 formed two to five haustoria per conidium. Field-emission scanning electron microscopy showed that the haustorium consisted of a haustorial body and several lobes embedded in an extrahaustorial matrix. The number of haustoria per hypha and hyphal length on trichomes were positively correlated. Also, the tips of one to four hypha per conidium (excluding germ tubes and primary appressoria) were injured using a minute glass needle installed on micromanipulator under a high-fidelity digital microscope, and their total hyphal lengths were compared. Wounded hyphae possessed the same number of haustoria in trichome cells as non-wounded hyphae, and total hyphal lengths were similar between treatment groups. In this study, a new model was developed to study the infection mechanisms of powdery mildews that will be useful in future gene expression studies.