Powdery mildew is a fungal infection, but when it strikes in the fall, it does not cause much damage because the plants are preparing to shed leaves for winter dormancy.  I suggest you cut back your dahlias now in October, as the season is almost over. By removing the top growth, you will also remove all the mildew spores. Do not compost the top growth. Often, late summer mildew is encouraged by warm weather and cooler, dewy nights. Do not wet the foliage by watering, and do not let the soil dry out, as a drought-stressed plant is more susceptible to mildew.  Good air circulation is crucial, so give your dahlia plants more space. If you dig, divide, and store your dahlia tubers, make sure this year you allow them to dry out before packing them away for the winter. Keep growing, Marianne Binetti