The purple coneflower (Echinacea purpurea) is a perennial that grows widely across the United States. It averages between 2 and 4 feet tall at maturity and blooms in July and August. Purple coneflowers are susceptible to a number of problems.
A viral-like pathogen causes a disease in purple coneflowers called aster yellows. Aster leafhoppers spread the disease from one plant to another. The Botrytis cinerea fungus causes gray mold, while the Pseudomonas and Xanthomonas bacterium cause bacterial leaf spot. Fungi such as the Oidium, Erysiphe and Microsphaera species cause powdery mildew infections, according to The Connecticut Agricultural Experiment Station.
Purple coneflowers with aster yellows have yellowed or deformed leaves and slow growth, according to the Missouri Botanical Garden. Botrytis blight or gray mold attacks the petals. Brown lesions form in the inner petals; plants may also develop thin masses of fungal webbing. Xanthomonas causes brown leaf spots with yellow borders, while pseudomonas is characterized by brownish-red spots and leaf distortion. Powdery mildew shows as patches of white fungal growth over the leaves, stems and blossoms that grow until the entire plant is covered.
Aster yellows is an incurable disease; remove infected flowers promptly and control weed and insect populations. Fungicides are not effective against botrytis blight; deadhead infected coneflowers and burn or bury diseased plant tissue. Control bacterial leaf spot by removing diseased plant tissue; avoid overhead irrigation and space plants so that they have adequate air circulation. Treat powdery mildew by spraying with an appropriate fungicide at the first sign of disease.