August 23, 2004
From: ProMED-mail<promed@promedmail.org>
Source:  American Phytopathological Society, Plant Disease Notes [edited] <http:// www.apsnet.org>

Widespread occurrence of the perfect stage of powdery mildew caused by Erysiphe polygoni on sugar beets in Nebraska
R. M. Harveson, University of Nebraska, Panhandle Research and Extension Center, Scottsbluff 69361. Plant Dis. 88:1049, 2004; published on-line as D-2004-0625-01N, 2004. Accepted for publication 2 Jun 2004.

Powdery mildew, caused by _Erysiphe polygoni_ DC (synonym _E. betae_ [Vanha] Weltzien), has been a sporadic and relatively minor problem for sugar beet (_Beta vulgaris_ L.) growers in western Nebraska. Yield losses in this region have been limited, in part, because of the use of effective fungicides, but also because infection occurs late enough in the season that treatment has often been unnecessary.

The perfect stage had been reported only once in the United States until 2001-2002, when it was identified from Idaho and Colorado (1). The teleomorph was also noted from several fields in Scotts Bluff County in Nebraska in October 2002. The 1st appearance of the disease in 2003 occurred during the 2nd week of August within 5 miles of the fields where the perfect stage was noted in 2002.

On the basis of these observations, a survey was conducted between mid-August and mid-October to map the appearance and distribution of the perfect stage of _E. polygoni_ within the Nebraska Panhandle growing region. During this time, between 45 and 50 fields were surveyed in 6 Nebraska counties. This represented the majority (70 percent) of the sugar beet acreage in Nebraska.

The 1st finding of the perfect stage occurred in early September from multiple fields in the vicinity of and including the field where the asexual stage was 1st reported in August 2003. Ascomata measured 85 to 110 micrometers with one to 4 (mostly 3) ascospores per ascus, resembling previous pathogen descriptions (2). Subsequently, every other field in the North Platte Valley, where the oidial stage had been found, also contained
the perfect stage by the 3rd week in September, including the Nebraska counties of Scotts Bluff (15 fields) and Morrill (7 fields).

Outside the North Platte Valley, powdery mildew was not detected until mid-September and mid-October for the Northern Panhandle (Box Butte County) and Southern Panhandle (Kimball, Banner, and Cheyenne counties) growing areas, respectively. By 1 October, the perfect stage was found in 9 of 10 fields exhibiting the disease in the North Panhandle, whereas the perfect stage was not found in the Southern Panhandle before harvest. Over 85
percent of surveyed fields infected with powdery mildew also harbored the perfect stage (31 of 36).

Not only is the new and continued presence of the perfect stage potentially problematic for managing fungicide resistance, and for developing new cultivars with pathogen resistance (1), but it may also provide a means for
over-wintering in this area. This could result in earlier, and more severe, infections that would additionally require uncustomary treatment for powdery mildew control.

The unusually early appearance of the disease, and the high incidence of the perfect stage in Nebraska fields during 2003, further highlights these concerns and warrants closely monitoring future crops for continued epidemics.

References:
(1) J. J. Gallian and L. E. Hanson. Plant Dis. 87:200, 2003.
(2) E. G. Ruppel. Powdery mildew. Pages 13-15 in: Compendium of Beet Diseases and Insects. E. D. Whitney and J. E. Duffus, eds. The American Phytopathological Society, St. Paul, MN, 1986.

[Sugar beet is a globally important crop producing 27 percent of world sucrose supplies. It is grown in Europe, North America, Chile, Uruguay, China, the Middle East, North Africa, and countries of the former Soviet Union. _Erysiphe polygoni_ [Eg] infection of sugar beet by Eg causes a serious fungal foliar disease resulting in sugar yield losses of up to 30 percent. Eg occurs world-wide in all regions where sugar beet is grown, and it also infects other edible beet crops, e.g. garden beets, Swiss chard, and fodder beet. Eg infection of sugar beet is especially damaging in arid climates, e.g. Mediterranean countries, the Middle East, and California. Sources of genetic resistance to Eg are available in cultivated, and wild, _Beta_ germplasm and molecular markers developed linked to Eg, the only single major R gene described so far, and also to quantitative trait loci (QTL). The host range of Eg is specific to _Beta_ species. Disease management is based on the application of chemical fungicides and deployment of a single monogenic form of mildew resistance in lines derived from _B. maritima_. Production of the perfect stage (teleomorph) of Pg and  its over-wintering in western Nebraska is cause for concern for sugar beet producers, especially in terms of its survival as inoculum for long periods in soil.

An authoritative reference:
http://www.blackwellpublishing.com/pathogenprofiles/mppsugar.pdf> - Mod.DH]