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Soybeans
Volume 54 Number 18 Date 08/28/2009 SOYBEAN APHID - Surveys conducted from July 15-August 11 showed 94% of 247 soybean fields examined did not develop economic infestations of 250 or more aphids per plant during the R2-R4 stages of growth, while 6% of fields did develop significant populations. Specific counties in which economic densities were noted include Columbia, Dunn, Eau Claire, Marquette, Pepin, Pierce, Taylor, St. Croix and Wood, principally in the south-central, west-central and central districts. The vast majority of surveyed fields contained low to moderate numbers of aphids throughout July and early August. Populations in the southwest and southeastern counties were exceptionally low. Average densities per plant by agricultural reporting district were as follows: northwest 49, north-central 89, northeast 22, west-central 102, central 93, east-central 16, southwest 6, south-central 71, and southeast 3. The 2009 state average density of 51 aphids per plant compares to 72 per plant in 2008, 164 per plant in 2007, 69 per plant in 2006, 118 per plant in 2005, 11 per plant in 2004, and 758 per plant in 2003. In contrast to the survey findings, soybean aphid populations are now very high in many untreated fields. Reports from County Extension agents and consultants, as well as recent field observations, indicate that populations have escalated sharply as far north as Polk County in the last 2-3 weeks. Soybean fields in the R4-R5.5 stages that still qualify for treatment must be evaluated immediately to determine if colonies have reached or surpassed the economic threshold of 250 or more aphids per plant. NORTHERN CORN ROOTWORM - Soybean fields in Green County are reported to be showing light defoliation by this insect. Generally the infestations are minor, but very large numbers of beetles are present. --Krista Hamilton, DATCP Entomologist WHITE MOLD - Signs and symptoms of this fungal disease, also called Sclerotinia stem rot, are appearing widely across soybean fields from Kenosha to Sauk counties. Foliar symptoms include chlorotic, wilted leaves that eventually die and turn brown, but remain attached to the stem past maturity. The disease is easily diagnosed by the presence of white, fluffy mold growth on the lower stems of plants. As the infection progresses, the fungus produces black, hardened survival structures called sclerotia that persist in plant residue and soil (see image below). Sclerotinia is expected to be a greater problem this year than most, as a cool July and frequent periods of leaf wetness both extended the soybean flowering period (the critical time for infection) and provided appropriate conditions for pathogen survival. Ironically, sclerotinia stem rot is a greater problem for soybeans managed for high yield, since factors that create a dense canopy favor the disease. Sclerotinia has a notably wide host range, with over 450 species of dicots having been shown to be susceptible. In some crops, such as sunflower, Sclerotinia can be a production-limiting factor under proper conditions. While varietal differences in response have been noted, useful resistance has thus far eluded plant breeders. Management options include fungicide treatments, a biocontrol agent, and cultural practices such as longer rotations and canopy management. Further description and control recommendations may be obtained at the UW Soybean Plant Health website: http://www.plantpath.wisc.edu/soyhealth/cause.htm. --Adrian Barta and Kara Geertsma, DATCP Plant Pathologists 
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