Germplasm Screening for Resistance to Purple Seed Stain caused by Cercospora kikuchii in Soybean.

Cercospora kikuchii, (Matsumoto & Tomoyasu) Gardner is an important soybean pathogen that causes latent seed infections, seed decay, purple seed discoloration, and overall quality deterioration. The objective of this research was to screen diverse soybean germplasm accessions from the USDA germplasm collection for resistance to C. kikuchii. For this objective, 123 different accessions of Glycine Max from 28 different countries were selected representing maturity groups (MG) III, IV, and V. The experiment was conducted at the University of Arkansas Vegetable Substation at Kibler in a strip plot design with three replications and maturity group separation. Plants were inoculated in the field at R5 physiological stage, using a local isolate of C. kikuchii with confirmed pathogenicity on soybean. Visual ratings for leaf symptoms, purple seed stain (PSS), and percentage of seed infection were measured. Results showed a wide range of responses among the three different MG, PI accessions, and between inoculated and non-inoculated treatments. Percentage of infection for the inoculated plots varied, 0 to 51% for MG III, 0 to 33% for MG IV, and 0 to 16.33% for MG V. There was a positive correlation for percentage of infection between inoculated and non-inoculated plots. Based on percentage of PSS, we identified 11 possible highly susceptible accessions and 22 possible resistant accessions for MG III, and IV, as determined based on check responses.  Maturity group V entries showed relatively low percentage of PSS infection on seeds, probably due to unfavorable climate conditions for the development of the disease during the extended period of the growing season. In conclusion, with the germplasm identified, breeders will be able to develop soybean cultivars with improved resistance to purple seed stain.