What seems to be a mildew on Black Eyed Susan, Spreading to other beds but...
This is probably a leaf spot disease, possibly septoria. These plants can become very unthrifty and unsightly. It can spread to other black-eyed susans and infect them until they look burned out and die back. It is challenging to control with fungicides, but cultural managment can help.
Regular scouting and plant inspection, especially on lower and inner leaves, will alert growers to foliar problems. Samples should be taken and symptoms noted for follow up inspections and evaluation of the disease progress. Have the disease identified so that management decisions are based on the appropriate life cycle. Management
Most foliar diseases can be lessened with proper watering and humidity regulation. Water as early in the day as practical, to allow foliage to dry before nightfall. Alleviate poor air circulation, crowded conditions, or standing water to help lessen humidity levels. Inspect new foliage and older foliage regularly for signs of infection to catch infections early. Sometimes simply removing the infected spotted leaves or plants will solve the problem. Selection of resistant varieties will also help to eliminate the application of costly controls.
In some cases, under heavy disease loads, it may be better to grow something else.