Rust

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 (pycnia and aecia) of the disease. Occasionally, this stage can be found in spring on emerging spears from new or established plantings. The orange stage is characterized by light green patches on new spears that mature into yellow or pale orange pustules in concentric ring patterns. 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 (urediospores) in a powdery mass, which can reinfect the fern and increase disease incidence. Fern yellowing and browning, defoliation, and dieback may occur. As ferns mature and senesce, or autumn weather begins, the black spore stage may develop. The same pustules that produced the red spores begin producing black spores (teliospores). The pustule will slowly convert in appearance to a powdery mass of jet-black spores. These black spores are the overwintering stage of the fungus.

The overall effect of rust on asparagus is reduced plant vigor the following year and reduced yields.

Rust diseases have several stages, some of which may occur on different hosts. In asparagus rust, however, all the life stages (orange spore in spring, red spore in summer, and black spore in fall and winter) occur on asparagus. Therefore, what may appear to be a different disease, could be a different stage of rust.

Rust is favored by temperatures between 55° and 90°F. Several hours of dew or rain (free water) are necessary for spores to germinate and infect the host.

Good field sanitation and irrigation practices are important components of managing rust. Treatments are necessary when monitoring indicates rust is present.

Provide adequate irrigation during the spring and summer fern period so that plants are neither over- nor under-watered. Orient rows with the prevailing wind, if possible, to allow free flow of air through the field. This will allow faster drying of the soil surface when irrigations or rainfall occur. At the end of the fern season, cut and destroy diseased ferns. One of the best solutions is to incorporate the cut fern with a power-driven rotary tiller two times, once in each direction. The fern may also be removed from the field. Cut young spears to keep infections from occurring, thus breaking the cycle of the fungus in spring. Destroy volunteer asparagus within 400 yards of commercial asparagus fields.

Cultural control and sulfur dust treatments are acceptable to use in an organically certified crop.

Monitor spears and ferns for the appearance of rust lesions. Begin treatments when rust first appears.