Resistance to phomopsis seed decay identified in maturity group V soybean plant introductions

Phomopsis seed decay (PSD) of soybean, Glycine max (L.) Merr., caused primarily by Phomopsis longicolla Hobbs, is the major cause of poor seed quality in most soybean-growing countries. To identify soybean lines with resistance to PSD, seeds of 208 representative maturity group V soybean plant introductions obtained from the USDA Soybean Germplasm Collection in 2006 were plated on cultural medium and assayed for the percentage of Phomopsis seed infection. From these data, 122 PIs without seed infection were selected and field screened under natural infection in Stoneville, MS, in 2007. On the basis of assays of naturally infected seeds from 2006 and 2007, 14 PIs were selected for further evaluation with P. longicolla-inoculated and noninoculated treatments in 2008 and 2009. Plant introduction 424324B was identified as the most PSD-resistant line. It had no Phomopsis infection in the seed plating assays from 2006, 2007, and 2008. In 2009, frequent rainfall during seed maturation led to high levels of seed infection by P. longicolla (up to 80%) and other fungal pathogens on most of the soybean lines tested in Stoneville, MS. However, only 1 and 2% of the seeds from PI 424324B were infected by P. longicolla in the noninoculated and inoculated treatments, respectively. Plant introduction 458130 was also resistant to PSD, with no seed infection from the naturally infected trials in 2006 and 2007 and less than 3% Phomopsis seed infection in the 2008 and 2009 inoculated trails. These resistant accessions can be used to develop cultivars resistant to PSD.