Rusts are foliar diseases caused by several species of Puccinia and Uromyces fungi. P. sorghi and P. polysora cause common and southern rust, respectively, on field and sweet corn. Southern rust is much less common in Illinois but has the potential to be more damaging to sweet corn than is common rust. P. asparagi causes rust on asparagus. Uromyces phaseoli causes rust on snap bean.
Regardless of host, these diseases are fairly easy to diagnose in the field. Rust can be identified by the minute, circular to elongate, golden or reddish brown to cinnamon brown pustules that form on the upper and/or lower leaf surfaces. These pustules break through the leaf surface and release rust-colored, powdery spores. On asparagus, the first symptom, occurring in April or May, is the presence of inconspicuous, light green, oval spots on the first shoots or spears. If the spears are not harvested, these spots develop into yellow, cup-shaped lesions containing fruiting bodies in a concentric ring.
Spores are released from these fruiting bodies, infecting new tissues and producing the pustules and powdery spores described above. Rust life cycles are complex. Some species require infection and alternation between two completely different hosts to fully complete the life cycle. However, once rust is present in a field, the fungus can produce large numbers of repeating spores that reinfect the same or additional plants during a single growing season. Repeating spores of the sweet corn rust come from previously infected corn plants in the South (where the rust survives year-round) and are windblown progressively northward. Rust progresses most rapidly in susceptible hybrids or varieties when the temperature is near 80 F with high humidity and frequent dews.
Whenever possible, plant hybrids or varieties that are resistant to rust. Early season sweet corn hybrids often escape infection due to low levels of inoculum (spores) or environmental conditions that are not favorable to disease development. In susceptible hybrids or varieties, or when conditions are extremely favorable for development of an epidemic, foliar fungicide applications may be feasible, starting when pustules first appear on the leaves.
One method to help manage asparagus rust involves breaking the life cycle by removing the fruiting bodies and spores. Because the fungus can remain on the stubs of spears, you can eliminate them and break the rust life cycle by cutting spears below the soil line. Beds that are three years old or older should be cut until the danger of infection has passed, usually around the first part of July. Old, unused beds or wild asparagus within 300 yards of the planting should be destroyed. After harvest, suggested fungicides can be applied to the fern stage on a seven- to ten-day schedule to protect against infection.