At the left you see some very sick-looking Flowering Dogwood leaves. The newly developed spring leaves are pale, puckery, and appear to be covered with a white dust or powder. The close-up at the lower right shows that there's not much to this white stuff -- it really is like powder. That powder is the fungus causing the disease known as Dogwood Powdery Mildew, and the genus name for it is either Microsphaera or Phyllactinia.
The white matter on the leaf consists of masses of tangled hyphae that obtain their food by sending rootlike haustoria into the leaf's living cells. At this early stage in the mildew's lifecycle it is reproducing with special kinds of asexual spores called conidia. This species can reproduce when the tips of certain vegetative hyphae simply constrict in certain places (no sex involved) forming egg-shaped spores that then can blow away and under proper conditions sprout new hyphae. Later in the year sexual reproductive structures will form in the powdery area (tiny items barely large enough to see with the naked eye) and they will produce regular sex-based spores.
At the right is a close-up of another mildew, this one Erysiphe cichoracearum, causing a late-summer mildew on the leaf of a yellow crookneck squash plant. The picture shows white mycelium and conidiophores with long chains of white conidia on the leaf's sunny, upper surface. You can see some very nice photomicrographs (photos taken through a microscope) showing this species' conidiophores and conidia here. As with all powdery mildews, this species does not kill its host, but rather just stresses it and reduces growth, robbing it of nutrients and slowing down photosynthesis.