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2010 Plant Management Network.
Accepted for publication 17 December 2009. Published 15 March 2010.


First Report of Sugar Beet Seedling Rust Caused by Puccinia subnitens in Nebraska


Robert M. Harveson, Panhandle Research and Extension Center, University of Nebraska-Lincoln, Scottsbluff, NE 69361


Corresponding author: Robert M. Harveson.  rharveso@unlnotes.unl.edu


Harveson, R. M. 2010. First report of sugar beet seedling rust caused by Puccinia subnitens in Nebraska. Online. Plant Health Progress doi:10.1094/PHP-2010-0315-03-BR.


Sugar beet seedling rust, caused by the macrocyclic, heteroecious pathogen Puccinia subnitens (syn. P. aristidae), is rare in sugar beet production. Only the pycnial and aecial stages occur on sugar beet, while the other stages infect the alternate host, inland saltgrass (Distichlis spicata) (1). Seedling rust was reported to cause moderate damage to sugar beets in the Rocky Ford area of Colorado in 1912-1913 (4), and since the 1940s periodic severe losses in Colorado spinach production have been attributed to the same pathogen (3). Stands of saltgrass infected with the telial stage of the pathogen were found in close proximity to spinach and sugar beet fields infected with the aecial stage (3,4). Inland saltgrass is a warm-season grass native to arid areas of the western United States. It is commonly found in brackish, marshy areas or highly saline soils (2). The plant is very drought tolerant and thrives in the strongly alkaline soils of western Nebraska.

In mid-May 2009, signs of sugar beet seedling rust were found on young sugar beet plants in a field near Bayard, in Morrill Co., NE. Unusually cool and wet weather during April and early May had prevailed throughout the western Nebraska Panhandle. Temperatures during these two months were 13C cooler with 12 cm higher rainfall than the 30-year average for this area. Rust incidence in the field approached 25% although lesions were restricted primarily to the cotyledons.

Puccinia subnitens was identified by the presence of circular, light-yellow pycnial lesions (Fig. 1A), 2 to 5 mm in diameter, containing flask-shaped pycnia (Fig. 2) and yellowish-orange aecia aggregated in rings (Fig. 1B). Aeciospores were globoid with finely verrucose walls that measured 17-22 m × 15-20 m (1). The field was monitored 6 times over the entire season and no uredial pustules were observed on plants, effectively ruling out beet rust, a typical macrocyclic, autoecious rust, caused by Uromyces betae, as the cause. The affected sugar beet field was surrounded by stands of inland saltgrass infected with the telial stage of a rust pathogen presumed, but not confirmed, to be P. subnitens.


 

Fig. 1. (A) Circular, light-yellow pycnial lesions on the upper surface of a sugar beet cotyledon; (B) yellowish-orange aecial pustules arranged in rings on the lower cotyledon surface.

 

Fig. 2. Thin section of a pycnial lesion on a sugar beet leaf with flask-shaped pycnia embedded in the epidermal surface. Close-up of one pycnium exhibiting receptive hyphae and spermatia (inset).


Survey of Sugar Beet Fields

After initial identification of the rust near Bayard, ten other fields within 24 km were scouted in mid-May, but no rust was observed. Because of the continued cool weather and above average precipitation in May, a more comprehensive rust survey of 53 sugar beet fields in eight counties (Scotts Bluff, Morrill, Box Butte, Banner, Kimball, Sioux, Cheyenne, and Sheridan) was conducted between late May and mid-June. Sugar beets in thirty three of the fields were infected with P. subnitens. Most of the diseased fields (89%) were located in Scotts Bluff and Morrill counties in the North Platte River valley although four diseased fields were located outside the river valley (two each in Kimball and Box Butte counties). The reason why seedling rust was so heavily concentrated in the valley is not known.

More than half (30) of the fields were resurveyed in June and of those 12 contained sugar beets with pycnia on newly emerged leaves. Five of these fields were among those initially surveyed near Baynard in mid-May and found to be rust free. This indicates that leaf infection (up to the 6th true leaf) by basidiospores occurred after May (Fig. 3).


 

Fig. 3. Sugar beet plant in the 5-6 leaf growth stage exhibiting a pycnial lesion of P. subnitens. Close-up of pycnial lesion (inset).

 

Pycnia and aecia of P. subnitens were found on common lambsquarters (Chenopodium album), another reported host (3,4) in 30% of the diseased sugar beet fields (Fig. 4). The rust was also found on lambsquarters at four additional sites in Scotts Bluff County: two ditchbanks, an uncultivated field, and a home garden.


 

Fig. 4. A true leaf of common lambsquarters with pycnia of P. subnitens (arrows). Common lambsquarters cotyledon exhibiting aecia of P. subnitens (inset).

 

Conclusions

Seedling rust on sugar beets is not considered economically damaging (1) and infection is usually restricted to cotyledons and occasionally the first true leaves (1,4), hence the name seedling rust. However, diagnosing the disease rapidly and correctly, as was done during the 2009 growing season in Nebraska, was important because it convinced growers that a fungicide application was not needed.

This is the first report of seedling rust on sugar beets and common lambsquarters in Nebraska. It is also the first report of the pycnial stage of P. subnitens from natural infections and documentation of multiple infections of sugar beet leaves beyond the seedling stage. The unusually cool and wet weather during May and June 2009 likely contributed to the high incidence of fields with rust.


Literature Cited

1. Hanson, L. E. 2009. Beet rust and seedling rust. Pages 12-13 in: Compendium of Beet Diseases and Pests, 2nd Edn. R. M. Harveson, L. E. Hanson, and G. L. Hein, eds. American Phytopathological Society, St. Paul, MN.

2. Newman, S. D., Gates, M., and Materne, M. 2006. Plant Guide: Saltgrass. USDA-NRCS, Ecological Sciences Division, Washington, DC.

3. Oshima, N., Henderson, W. J., and Dickens, L. E. 1960. The occurrence of spinach rust in Colorado. Plant Dis. Reptr. 44:828-829.

4. Pool, V. W., and McKay, M. B. 1914. Puccinia subnitens on the sugar beet. Phytopathology 4:204-206.