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Soybeans
Volume 61 Number 19 Date 12/08/2016


SOYBEAN APHID - The annual survey found a statewide average count of eight soybean aphids per plant. This average compares to 35 aphids per plant last year and is only marginally higher than the record-low count of seven aphids per plant documented in 2012. One hundred and seventy soybean fields in the R2-R5 growth stages were sampled during a three-week period from July 25-August 15. Aphid densities were below 151 per plant in all fields, and the majority of sites had counts of fewer than 25 aphids per plant. No field sampled had an average exceeding the 250 aphid-per-plant treatment threshold. Results of the survey suggest that aphid populations remained low or moderate in most soybean fields this season and widespread treatment for aphid control was not required.

WHITE MOLD - Above-average rain, favorable June temperatures, and early canopy closure contributed to a greater incidence of white mold than expected in 2016. Disease pressure was highest in the northern half of the state where ratings in mid-August ranged from 0-30%, with most fields showing 10% of plants with symptoms. Late-planted soybeans with extended bloom periods (i.e., longer infection periods) were disproportionately affected.

JAPANESE BEETLE - Defoliation was observed in about 74% of the soybean fields examined in late July and August, indicating that Japanese beetle injury was more widespread than ever. Defoliation estimates were mostly below the 20-30% treatment threshold, but chemical intervention was justified in some instances. Once primarily a fruit and landscape pest, the Japanese beetle has become an increasingly serious threat to Wisconsin's agronomic crops that more soybean and corn growers now have to manage for the first time.

OBLIQUEBANDED LEAFROLLER - This generalist leafroller was common in Wisconsin soybean fields for the second year in a row. Larvae began emerging by early June and were prevalent in fields throughout July. Most of the larval population pupated by early August. Despite their abundance, the OBLR damage to soybeans observed in 2015 and 2016 was minor and not of economic importance.

-- Krista Hamilton, DATCP Entomologist


PHYTOPHTHORA ROOT ROT - During the 2016 survey, 32% (17 of 53) of soybean fields tested positive for root rot disease caused by Phytophthora sojae, compared with 38% (19 of 50) in 2015. Positive samples were collected in Barron, Columbia, Crawford, Dodge, Dunn, Green, Jefferson, Marathon, Outagamie, Racine, Rock, and Walworth counties. Also detected was P. sansomeana in soybean roots from a Dunn County field. Since 2012, five new Phytophthora species have been found in Wisconsin soybeans as a result of this survey: P. inundata, P. iranica, P. sp. "personii", P. pini, and P. sansomeana.

-- Anette Phibbs, DATCP Plant Pathologist