Development of southern stem canker disease on soybean seedlings in the greenhouse using a modified toothpick inoculation assay

- Modified toothpick assay resulted in high incidence of stem canker in the greenhouse.
- Soybean with known resistance genes did not develop disease.
- Isolates of Diaporthe aspalathi were observed to differ in virulence.

Southern soybean stem canker, caused by Diaporthe aspalathi, has caused major soybean losses for growers in the Southeast U.S. The most effective disease management tool for growers is the use of stem canker resistant soybean varieties. A fast, reliable greenhouse assay for stem canker would facilitate identification of resistance in soybean germplasm. An existing toothpick assay was modified to include culturing D. aspalathi on oxgall agar on toothpicks autoclaved in oxgall liquid medium. Three week-old seedlings were inoculated with toothpicks inserted in the stem between the cotyledon and the first trifoliate leaf. Inoculation sites were sealed with petroleum jelly, and seedlings were incubated in humidity chambers for 72 h. Stem canker disease was highly consistent on susceptible lines four weeks post-inoculation and was not observed on soybean lines with known Rdm genes. High levels of disease (≥98.3%) were observed with cultivars Braxton, Davis, and Centennial previously reported to have resistance in field studies. Isolates of D. aspalathi were observed to differ in virulence. This modified greenhouse assay will assist in the efforts for breeding stem canker resistance and better understanding the differences in disease phenotypes for some cultivars.