Collect leaves that show signs of powdery mildew. Recently infected leaves are best for observing hyphae and conidia. Conidia can be more easily observed by folding the leaf and looking for the conidia in profile with a dissecting microscope.  Look for infected leaves on azalea, grape, filbert, sweet pea, lilac, zinnia, Kentucky bluegrass growing in shady areas, wheat, viburnum, rose, phlox,  cucurbits, and many other common weeds and garden plants.

Collect leaves with mature (black) cleistothecia on plants with powdery
mildew in late summer and autumn.  The cleistothecia should be visible with your eyes, although a hand lens may help. Azalea, grape, filbert, sweet pea, lilac, wheat, cherry, oak, viburnum, horse chestnut, and many other garden plants and shrubs are good sources of cleistothecia. Leaves with abundant cleistothecia may be pressed and dried for classroom use for many years.

Leaves can be stored in large envelopes or cardboard boxes for use throughout the year.  Students may find it easier to remove cleistothecia
from fresh leaves and can be encouraged to bring their own samples to  fall classes. Dried leaves tend to crumble. It might be easier to remove cleistothecia by gently scraping the surface with a moistened, single-edge razor blade. If this is done over a white sheet of paper, it is easier to see if cleistothecia have been successfully transferred to the water drop on the slide.  Dried leaves also can be softened by steaming for about 10 minutes on a screen over boiling water.