Powdery Mildew is a fairly common seasonal disease affecting chrysanthemums. The disease manifests itself as a white powdery  covering on stems and leaves, occasionally on flowers.

Infection is typically first seen on the undersides of lower leaves but if left untreated it will spread to upper leaves. It is more commonly seen in a glasshouse environment but will occur outdoors in late summer. 

A few facts about powdery mildew:
Powdery mildews are a group of closely related species of fungi that are very similar in appearance, producing a 'powdery' effect on the surfaces of plants. 

Each species of powdery mildew has a limited host range, and powdery mildew on one species of plant will generally not spread to other unrelated plants.

All powdery mildews prefer dry conditions on closely spaced plants with poor air circulation and low light.

Low levels of fungal infection may show no symptoms but develop when conditions are favourable.

Powdery mildew spores are spread far and wide by wind currents.

Impact/effects on chrysanthemums
Leaves and stems become unsightly and leaves fail to photosynthesise to full potential resulting in a reduction in plant vigour and overall health. Plants may become distorted and premature leaf death may ultimately occur.

Fungicides available to the home gardener for powdery mildew control are based largely on two chemical ingredients - myclobutanil and penconazole. Sulphur applied as a dust is another form of control. 

There are numerous products available containing these chemical ingredients.