Resistance to Phomopsis seed decay and purple seed stain in soybean, and virulence differences among Phomopsis spp. causing seed

Phomopsis seed decay (PSD) caused by Phomopsis longicolla and Diaporthe phaseolourm var. sojae and purple seed stain (PSS) caused by Cercospora kikuchii on soybean reduces quality of seed and grain. Many strategies have been suggested to control PSD and PSS, however, resistance is the most desirable. Several genotypes have exhibited different levels of resistance to these diseases. Reports on PSD resistance have been inconsistent, this might be attributed to variables including pathogen differences in field studies where selection is done. Resistance in PI 417479, parent of MO/PSD-0259, and PI 80837 has been studied in detail. Studies have shown that PSD-resistance in PI 417479 was due to one or two complimentary dominant gene(s) on linkage groups (LG) F and H. Additionally, studies have shown that PSS-resistance in PI 80837 is under strong genetic control. The objectives of this study were to: (a) characterize the resistances to PSD in MO/PSD-0259 and PI 80837 and to PSS in PI 80837. (b) determine expression of resistance to D. phaseolorum var. sojae and P. longicolla. (c) investigate differences in virulence of P. longicolla isolates. (d) determine if smaller sampling sizes will improve identification of resistant and susceptible genotypes. Inheritance and molecular studies were done using F2 and F2.3 lines from crosses of PI 80837 and MO/PSD-0259 and the susceptible genotypes AP 350 and PI 91113. Additionally, populations and lines from a cross of PI 80837 x MO/PSD-0259 were studied. Results showed that different single dominant genes on LG B2 and LG F confer PSD resistances in PI 80837 and MO/PSD-0259, respectively. Furthermore, five genotypes reported to have resistance to D. phaseolorum var. sojae and/or P. longicolla, along with one susceptible genotype were screened with both pathogens. Results indicated that resistance is expressed differently to these pathogens. P. longicolla isolates collected from PSD-resistant genotypes and a susceptible genotype were tested for virulence differences. Results showed that two isolates from PI 80837 and two from MO/PSD-0259 were virulent on their respective genotypes, while isolates from MO/PSD-0259 were also virulent on PI 417479, the resistant parent of MO/PSD-0259. Additionally, experiments were done comparing recovery of P. longicolla from 10-mm and 5-mm leaf disk cut from first trifoliate leaves to seed infection. Results showed that 5-mm disks correlated better with seed infection and were more reliable in identifying resistant or susceptible genotypes. This research has provided information, materials, and an adequate screening procedure to aid breeders in the development of resistance genotypes to PSD and PSS.