Powdery white spots that enlarge and spread over the leaves and stems of plants. The powdery stuff is composed of asexual spores that can spread the fungus to other plants. Requires high humidity and moderate temperatues so most likely to be seen in fall or in a cool wet spell in summer.
A nasty plant pathogen that can be a serious agricultural problem, but also occurs in the wild. Often, though it causes only cosmetic damage since it attacks late in the growing season. In the Park, expect it in rainy periods in late summer or early fall.
A large number of species in the family Erysiphaceae form powdery mildews. Generally they are fairly specific in the species they attack, so one can narrow the identification by knowing the infected plant. The species in the pictures is likely either Erysiphe cichoracearum or Sphaerotheca fusa, since it is attacking eaves of green-headed coneflowers (Rudbeckia laciniata).