Robert M. Harveson, Extension Plant Pathologist University of Nebraska, Panhandle REC, Scottsbluff
Sugar beet seedling rust is caused by the fungal pathogen Puccinia subnitens (syn. P. aristidae). The pathogen produces 5 distinct spore stages, and the life cycle is completed on different hosts (unlike dry bean rust that infects only dry beans). Only two stages (pycnial and aecial) occur on sugar beet, while the other stages (uredial and telial) infect various species of saltgrass (Distichlis spp.) and needlegrass (Aristida spp.). Inland saltgrass is a warm season grass native to arid areas of the western United States, and is also commonly found in brackish, marshy areas or highly saline soils. The disease on sugar beets is associated with early season and infection is generally limited to the cotyledons, hence the name “seedling rust”.
Life Cycle and Symptoms
In early spring, teliospores germinate from telial pustules after overwintering on inland saltgrass to produce basidiospores, which then are blown by wind where they can infect sugar beets or a number of other alternate hosts, including common lambsquarters. Infections by the basidiospores give rise sequentially to the pycnial and aecial spore stages (Figure 1). Pycnial lesions are circular and light yellow, measuring 2-5 mm in diameter (Figure 2A). Flask-shaped pycnia are generally found on the upper leaf surface and after fertilization occurs, the aecia are formed on the lower leaf surface directly below the pycnia (Figure 3). The aecia consist of clusters of yellowish-orange, rounded structures (Figures 2B) containing aeciospores. Later the newly formed aeciospores then re-infect the inland saltgrass, creating new uredia and telia, completing the life cycle (Figure 1)
Early History of the Disease on Alternate Hosts
In the late 1890s, Puccinia subnitens was commonly recognized on various Distichlis species throughout North America, particularly in the western United States. A close association between P. subnitens infections on saltgrass and similar aecial rust infections on the sugar beet cousin, common lambsquarters (Chenopodium album) was also widely observed across the Great Plains and Western U.S. from North Dakota and Montana to New Mexico and Arizona to California.
In the summer of 1904, the Reverend John Mallory Bates postulated that rust infections he observed in Nebraska on species of Cleome (Beeweed), Sophia (tansymustard), Lepidium (pepperweed), Erysimum (wallflower), and Salsola (Russian thistle) were related to P. subnitens and closed resembled infection on lambsquarters. J. M. Bates was an Episcopalian clergyman widely known and respected as an authority on plant rust diseases and ornithology.
Bates communicated his observations to the noted rust pathologist, J. C. Arthur and sent him samples from Distichlis, and infected seedlings of Cleome serrulata and C. album, and numerous other plant species covered profusely with the yellowish-orange-colored aecia, all of which he thought belonged the same rust species. Arthur pronounced the disease to be identical to P. subnitens and confirmed Bates’ deductions later that summer by inoculating these same host plants with spores and reproducing the disease.
Ellsworth Bethel, with the Colorado State Museum in Denver, also found the same fungus distributed throughout Colorado from many of the same plant species as that of Bates in Nebraska. By 1934, the combined works of Bates, Bethel, Arthur and others, further confirmed and expanded the host range of P. subnitens to include a remarkable 84 species in 19 families and 52 genera. It is quite unusual to find a rust pathogen like this with the ability to infect such a wide range of plant species. Rust pathogens are generally limited in their pathogenic host ranges to a few related species.
Sugar Beet Seedling Disease
The first published report of this disease on sugar beets was from Colorado in 1914, where the pathogen was found during the 1912-1913 seasons from multiple fields in the Arkansas Valley near Rocky Ford, CO. They also noted lambsquarters that were heavily infected with aecial lesions within the same locations. By early July the disease intensity had diminished and was no longer an issue. They further added that the telial form was found abundantly on saltgrass stands near fields during the sugar beet epidemics.
New Report from Nebraska
In mid-May 2009, young sugar beet plants were first observed exhibiting signs indicative of sugar beet seedling rust in a field near Bayard, Nebraska after an extended period of unusually cool and wet weather had persisted throughout the western Nebraska Panhandle in April and early May. Disease incidence in this beet field approached 25% although lesions were restricted primarily to the cotyledons. It was also additionally noted that this particular infested sugar beet field was surrounded by stands of inland saltgrass also infected with the telial stage of a rust pathogen.
Survey of Sugar Beet Fields 2009-2010
Based on this initial identification of the pathogen in Morrill County, Nebraska a survey of sugar beet production fields in western Nebraska was conducted between late-May and mid-June to further document the incidence and number of fields infested with seedling rust caused by P. subnitens. After similar environmental conditions were experienced in the late spring of 2010, another similar survey was conducted in May and June 2010 from more than thirty locations throughout western Nebraska.
Over this two year period, 88 locations from 8 western Nebraska counties (Scotts Bluff, Morrill, Box Butte, Banner, Kimball, Sioux, Cheyenne, and Sheridan) were scouted and 47 were identified with plants infected with pycnia and/or aecia of P. subnitens. Forty-one of these locations were sugar beet fields, with 36% (15/41) of the fields additionally containing infected lambsquarters. Although the pathogen was readily found throughout both years, the incidence within and among sites was lower in 2010 with only 30% of the monitored locations yielding infected plants compared to 65% in 2009.
Red Rust on Spinach
P. subnitens has additionally been observed to cause disease on other plants related to sugar beets including table beets, Swiss chard, and spinach. On spinach, the disease is called “red rust”, and symptoms are similar to those on sugar beets, including small yellow spots with red centers that occur on upper leaf surfaces. Aecial clusters can then be found scattered profusely on both upper and lower leaf surfaces, often arranged in rings.
This disease was first identified on spinach from Colorado from multiple fields over two sequential seasons (1943-1944). Since that time the disease has appeared sporadically and severe losses in Colorado spinach production have been attributed to the same pathogen in 1952 and 1959 with additional reports of outbreaks in 1996 and 1999.
A survey conducted in the summer of 1959 identified the telial stage of the pathogen infecting saltgrass from 14 distinct counties, widely scattered across Colorado.
In the 1950’s the disease was shown to progress rapidly on plants over several days while being shipped to market, rendering them unsalable upon arrival at their terminal destination. Spinach plants obviously became infected in the field, but decomposed rapidly after infections were undetected by packers prior to shipping. Even with our advanced shipping technology today, this disease is difficult for packers to detect, and still poses a problem because disease can develop so rapidly during transit.
Sugar beet seedling rust, caused by Puccinia subnitens, is a disease that has rarely been observed in sugar beet production. In fact to my knowledge the reports from Nebraska in 2010-2011 were the first published records of the natural occurrence in the field of this disease on sugar beets since the original communication in 1914.
Although the disease has been shown to be very destructive to spinach growers, seedling rust is not considered economically damaging on sugar beets. However, the investigation into this epidemic has still been a valuable experience by documenting a new report for a rarely occurring disease, and also providing a foundation for recognizing future outbreaks. By correctly diagnosing the disease rapidly as we did in 2009, fungicide applications were avoided, which would otherwise have been made due to the unknown nature of the problem.
Figure 1. Life cycle of the sugar beet seedling rust pathogen (Puccinia subnitens).
Figure 2. Circular, light-yellow pycnial lesions on the upper surface of a sugar beet cotyledon (A); yellowish-orange aecial pustules arranged in rings on the lower cotyledon surface (B).
Figure 3. Thin section of infected sugar beet leaf showing flask-shaped pycnia on the top leaf surface, and aecial cups and developing aeciospores on the bottom leaf surface.