Black-eyed Susan is a hardy perennial flower that spreads and weathers the elements fairly well. Despite their resilience, these tall daisy-like plants are susceptible to some diseases and fungi.
Verticillium wilt, a fungal disease that affects plants, enters the plant from the root structure and may cause leaf loss, yellowing and slow growth. This fungus can live on black-eyed Susan and similar plants for years. Many herbaceous perennials — including black-eyed Susan — also wilt and lose stems to a white mold known as Sclerotinia. Powdery mildew may also affect black-eyed Susan.
Reduce the humidity that fungus thrives on by replanting black-eyed Susans further apart. Dig fungus-infested plants up by the root to prevent them from spreading the problem to other plants.
In addition to molds and fungal diseases, black-eyed Susan plants are susceptible to bacterial diseases that cause spotting on leaves.