Multi-year evaluation and release of improved soybean lines for resistance to Phomopsis seed decay

Soybean [Glycine max (L.) Merr.] is one of the most economically important oilseed crops grown in the world. Soybean diseases not only affect yield but can also affect seed quality resulting in significant economic losses. Phomopsis seed decay (PSD) causes poor seed quality and is one of the most damaging soybean seed diseases. Diaporthe longicolla (syn. Phomopsis longicolla) and other pathogens in the Diaporthe/Phomopsis complex are the cause of PSD. Developing and releasing PSD-resistant germplasm lines is an important step toward the development of resistant cultivars that can protect soybean from PSD seed damage. The objective of this study was to identify improved soybean lines with PSD resistance for the mid-southern U.S. after inoculation with D. longicolla in a 5-year delayed-harvest field trial. In this study, seed assays were conducted to evaluate a total of 266 soybean entries in field experiments with inoculation of D. longicolla from 2019 to 2023. Results from five years of testing showed that maturity group (MG) late III 65-414-132-1 (DS65-1) had significantly less infection (38.6%) than susceptible late III LG03-4561-14 (86.2%). Early MG IV lines 14119-211-10 and 10031-243-12 (released as DS31-243) had statistically lower levels of PSD (29.9% and 46.6%, respectively) than susceptible early IV 'LD00-3309' (63.9%). These improved breeding lines were derived from PI 587982A. Moreover, early MG V 10049-142-31 (released as DS49-142) had less infection (7.9%) than early V 'AG5335' (25.4%), and late MG IV 12060-260-2 (released as DS1260-2) had less infection (19.6%) than susceptible late IV 'P46T59R' (55.3%). DS1260-2 was derived from PI 587982A, and DS49-142 was derived from PI 603756. Resistant lines were transferred to breeders for potential use in developing soybean cultivars with resistance to PSD. This research provides useful information for the development of durable resistance to PSD, which will aid in the management of soybean seed decay.