Asparagus rust is characterized by different color spots or pustules on spears and fern branches. Orange pustules are present on new spears in spring and brick red pustules develop on stalks, branches, and leaves in summer. Black spores are present in fall and winter. Fern yellowing and browning, defoliation, and dieback may occur.
Life cycle
Rust is favored by temperatures between 55 and 90°F. Several hours of dew or rain are necessary for spores to germinate and infect the host. Rust is most common on fern growth after the harvest season is over. Infections begin in spring from spores that overwintered on crop debris. These infections produce the orange stage of the disease. Light green patches that mature into yellow or pale orange pustules in concentric ring patterns develop on new spears. Spores produced by these spring stages are airborne to new fern growth. Infection occurs and brick red pustules develop on stalks, branches, and leaves of the fern. These red pustules produce airborne, rust-colored spores in a powdery mass, which can reinfect the fern and increase disease incidence. As ferns mature and die, the black-spore stage may develop. These black spores are the overwintering stage of the fungus.
Solutions
Rust is most prevalent in more humid areas. Provide adequate irrigation during the spring and summer fern period so that the plants are neither over- or underwatered. Sulfur and other fungicides applied when rust first occurs are helpful in control. Cut down diseased ferns at the crown and destroy. Resistant varieties may be available.
