Echinacea sp. (Coneflower) may get Pseudomanas or Xanthomonas leaf spot. Both are bacterial diseases. Purplish leaf spots start on lower leaves first and progress upwards through the growing season.
Powdery mildew develops a whitish to light grayish thin growth (fungal mycelium). Leaves may turn brown and die under the fungus growth.
Botrytis causes brown spots in flowers (flower buds may be killed), leaves and stems. Entire plants may die.
Aster yellows causes a witches broom in the flower head (looks like a broccoli flower head), greening of petals sometimes, stunting and possible death of the plant.
There is an eriophyid mite that causes the flower to develop lumps or bumps or leaf like structures growing out the side.
Coneflower may get Pseudomanas or Xanthomonas leaf spot during wet weather. The bacterial diseases are weak pathogens so need some kind of opening to get in. These openings can be natural (stomata in the leaves), insect feeding damage, or some other injury. Avoid working among the plants when they are wet.
Powdery mildew will also occur when weather conditions are warm to hot and dry during the day and cool and Humid (not wet) at night. These conditions must occur three consecutive days and nights minimally for the symptoms to be seen.
Botrytis can survive the winter on both live and dead plant tissue. It can infect plants when temperatures are between 32 and 84 degrees fahrenheit. Spores are released on a rising as well as a lowering of humidity. Spores infect when plant tissue is wet.
Aster yellows is carried by insects that over winter in tall grassy/weedy areas.
There is very little known about the life cycles of this eriophyid mite.
Sanitation is the main control for the bacterial leaf spots.
Powdery mildew and botrytis can be managed by using your local University Extension organic or inorganic recommended fungicide. Use according to directions on fungicide label.
Aster yellows infected plants should be destroyed by burning (where legal) or burying six or more inches deep in the soil. Since aster yellow is spread by insects that over winters in tall grassy/weedy areas. Keep these areas mowed. Insecticides do not kill the insects fast enough to prevent the insect from transmitting the aster yellows to the plant thus insecticides are NOT recommended.
Remove eriophyid infested flowers and destroy them by burning (where legal) or burying six or more inches deep in the soil. (Note that to some gardeners, the aster yellows and the eriophyid mite can cause similar looking distortions if not checked closely.)