Symptoms
Asparagus spears are usually harvested before extensive rust symptoms appear. Symptoms are first noticeable on the growing shoots in early summer as light green, oval lesions, followed by tan blister spots and black, protruding blisters later in the season. The lesions are symptoms of Puccinia asparagi during early spring, mid-summer and later summer to fall, respectively. Severe rust infections stunt or kill young asparagus shoots, causing foliage to fall prematurely, and reduce the ability of the plant to store food reserves in the crown.

Fig. 1: Oval to elliptical-shaped lesions develop in the first stage of this disease.

Fig. 2: Tan blisters open to expose rust-colored spore masses in mid-to late summer.

Life Cycle
There are three distinct stages of rust disease. In the first stage, occurring from April to July, lesions develop. These oval lesions are raised, light green in color, and 10-20 mm in length [fig. 1]. The lesions are sometimes inconspicuous and decrease in number from the base of the shoots upward. The lesions turn cream-orange in color and become sunken in the center as they mature. During summer months, the second stage of rust disease begins as reddish-brown, blister-like pustules develop on the asparagus shoots [fig. 2]. When the pustules mature, they release large numbers of rust-colored spores that cause new infections throughout the summer. Reddish, rust-colored, powdery spores are seen when rubbed against light-colored clothing. Later in the season the third stage replaces rust-colored spores with black, over-wintering spores. In some lesions, both reddish-brown and black spores appear together. Plants affected by rust are more susceptible to Fusarium crown and root rot.

Spores overwinter on host plant residue, germinate in early spring, and produce new infections on growing asparagus spears. The light green, oval lesions are surrounded by a concentric ring pattern. In young plantings, before stalks are harvested, lesions develop yellow spore-bearing structures in concentric rings.

Wind and splashing rain can spread spores to branches and fern needles, where they germinate in the presence of water drops.

Signs
The orange spores are the key sign for this disease. Run your hand over an asparagus spear and examine your palm for orange-colored spores. Laboratory techniques may also be used for diagnosis of asparagus rust.

Management
Plant rust-resistant varieties of asparagus, such as Viking KB3, Jersey Giant, and Martha Washington, all of which are reported to grow well in Minnesota. Remove volunteer asparagus within 300 meters of commercial plants, and locate new plants away from established fields. Plant well-spaced rows oriented in the direction of prevailing winds to maximize air movement and facilitate drying after rain. At the end of the season remove and destroy any infected stems.

References:

• Gould, S. L. Disease-Resistant Vegetable Varieties. University of Minnesota Extension Service folder 02412, 1994.
• Howard R. J., J. A. Garland, and W. L. Seaman, eds. Diseases and Pests of Vegetable Crops in Canada. The Canadian Phytopathological Society & Entomological Society of Canada, 1994.
• Sherf, A. F., and A. A. MacNab. Vegetable Diseases and their Control, 2nd Ed. New York: John Wiley & Sons, 1986.
• Johnson, D. A., D. F. Mayer, R. Parker, and G. L. Mink. Asparagus Integrated Pest Management. Cooperative Extension, College of Agriculture & Home Economics, Washington State University, EB1383.