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SOYBEAN

Early Incidence of Soybean Seedling Pathogens in Iowa

^a Centro para Investigaciones en Granos y Semillas (CIGRAS), Univ. de Costa Rica, San José, Costa Rica
^b Dep. of Agronomy, Iowa State Univ., 2104 Agronomy Hall, Ames, IA 50011

^* Corresponding author (adriana.murillowilliams{at}ucr.ac.cr).

Soybean [Glycine max (L.) Merr.] seed yield is often limited by root-infecting pathogens. This study was conducted to determine the prevalence of three root-infecting seedling pathogens in a fumigated vs. a nonfumigated soil, to determine if incidence is greater in cultivars that lack resistance to soybean cyst nematode (Heterodera glycines Ichinohe, SCN), and how incidence of these pathogens relates to yield. From 2004 through 2006, a 2 ha section of a field was fumigated each year with a mixture of 1,3 dichloropropene + chloropicrin on a Webster clay loam at Nevada (fine-loamy, mixed, superactive, mesic Typic Endoaquolls), and on a Salix silty clay loam at Whiting (fine-silty, mixed, superactive, mesic Typic Hapludolls), IA. Average incidence in the nonfumigated sites (determined by enzyme linked immunosorbent assay [ELISA] tests) was 86, 47, and 9% for Pythium spp., Rhizoctonia solani, Phytophthora sojae, respectively. Fumigation increased yield by 10%, reduced Pythium spp. incidence 27%, and no reductions in incidence of R. solani and P. sojae were detected. Incidence of Pythium spp. and P. sojae was less for cultivars with resistance to SCN. Negative correlations were found of yield with root health ratings (–0.43), SCN initial populations (–0.37), the presence of Pythium spp. (–0.28), and P. sojae (–0.15). Pythium spp. were more prevalent than R. solani and P. sojae in soybean seedlings in Iowa, and it was more related to yield than the other two pathogens. More research is needed to investigate the possibility of SCN-susceptible cultivars being exposed to greater Pythium spp. and P. sojae damage which may influence yield.

Abbreviations: ELISA, enzyme linked immunosorbent assay • RHR, root health ratings • SCN, soybean cyst nematode

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Received for publication October 17, 2007.