Black Stem Rust

Washington State University Bulletin

SP0004 -- 1993

Diseases of Washington Crops.

Otis C. Maloy and Debra Ann Inglis

 

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Cause

Hosts

Symptoms and Signs

Disease Cycle

Control

1997 PNW Pesticide

Recommendations

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Other interesting Black Stem Rust sites

Black stem rust, or stem rust, normally has not been a major problem in Washington since the state began a barberry eradication program in 1944. However, that program ended in 1977, and since then black stem rust has become more apparent. Some damage has occurred on late maturing wheats.

Cause
Stem rust is caused by Puccinia graminis f. sp. tritici, a heteroecious, long cycle rust fungus with many pathogenic races. The fungus has five distinct stages: the pycnial and aecial stages occur on common barberry (the alternate host), and the uredial (orange or summer) and telial (black or winter) stages occur on wheat. Basidia arise from teliospores and basidiospores infect barberry. Urediospores are orange-red, spiny and oblong with four median germ pores. The brown-black, two- celled teliospores have a terminal cell that is pointed at the top.

Hosts
Telial hosts are wheat, barley and several grasses, but wheat is the only economic host in Washington. Aecial (alternate) hosts are the common European barberry, Berberis vulgaris, and two related species native to North America, B. fendleri and B. canadensis. The Japanese barberry, B. thunbergi, and others now widely planted as ornamentals are not alternate hosts and are resistant to the rust. Some Mahonia species are reported as hosts, but apparently our native Mahonias, Oregon grape, are not susceptible.

Symptoms and Signs
Uredial pustules are rust-colored, oval or spindle-shaped, up to 3 x 10 millimeters, and occur on wheat leaves, stems and leaf sheaths. The pustules become dark brown or black as the telial stage develops (Photo 11). The uredia, and later the telia, erupt through the epidermis of stems, leaf sheaths and blades, glumes and awns.

Disease Cycle
Teliospores over-winter on plant refuse and soil and germinate to produce four basidiospores (sporidia). These basidiospores infect newly developing barberry leaves. Very small pycnial pustules soon occur on the upper barberry leaf surface. Minute pycniospores then fertilize receptive hyphae in pycnia of the opposite mating type. Aecia (cluster cups) from the cross-fertilized pycnia develop on the underside of the barberry leaf.

Windborne aeciospores infect wheat plants. Once the uredial stage forms, urediospores re-infect wheat plants throughout the growing season-this is the repeating stage. Finally, as the plants mature, telia develop and teliospores form.

Warm weather and moisture from dew and other sources favor stem rust. Free moisture, as dew or rain, is required for spore germination and infection. Of the three cereal rusts in Washington, stem rust needs the warmest temperatures for development (Figure 5). The optimal temperature is 68F; fungal development is hampered below 59F or above 104 degrees F.

Control
Chemical. The fungicides effective against stripe and leaf rusts also are effective against stem rust. For current recommendations see the PNW Plant Disease Control Handbook.

Cultural. B. vulgaris, originally introduced from Europe, spread widely because birds eat the berries and pass the seeds. The plant played an important role in the early establishment of rust in the United States and in the development of new races. Cooperative federal and state barberry eradication programs, initiated in the early 1900s, were effective in reducing the frequency of epidemics in many areas where teliospores are important for over wintering. Eradication of this alternate host is still recommended as a method of controlling teliospore inoculum.

Resistance. In most areas where stem rust is a serious problem, such as the Midwest, barberry is not important in the disease cycle of P. graminis. Each year, waves of urediospores windblown from Mexico northward to Canada, provide inoculum for epidemics. In such areas, researchers have developed resistant cultivars, but occurrence of numerous pathogenic races of the fungus reduces their effective use. Genetic deployment is used to thwart the rapid buildup of pathogenic races. This tactic relies on planting scattered genotypes to avoid a continuum of the same source of resistance, thus preventing the proliferation of a single rust race. Pathologists also recommend planting early maturing cultivars that ripen before rust can become severe.

References
Roelfs, A.P. 1982. Effects of barberry eradication on stem rust in the United States. Plant Disease 66: 177-181.

Roelfs, A.P. 1989. Epidemiology of the cereal rusts in North America. Can. J. Plant Path. 11 :86-90.

Other Interesting Black Stem Rust sites:
University of Nebraska-Lincoln NebGuide covers the life cycle, symptoms, and control of stem rust of wheat.