Development of a seedling inoculation technique for rapid evaluation of soybean for resistance to Phomopsis longicolla under controlled conditions.

Background: Phomopsis seed decay (PSD) of soybean ( Glycine max L. Merr.) is caused primarily by the seed-borne fungal pathogen Phomopsis longicolla T. W. Hobbs. The PSD disease reduces seed quality and yield worldwide. Development of effective techniques to evaluate soybean for resistance to PSD can facilitate identification of new sources of host resistance to manage the disease. This study was undertaken to develop and utilize a rapid cut-seedling inoculation technique to evaluate soybean genotypes for resistance to P. longicolla under controlled conditions.

Results: There were no significant differences in stem lesion length determined as the area under disease progress curve at 24 °C and 30 °C. The 21 and 14-day-old seedlings were more susceptible than the older seedlings. Inoculation with 7 or 14-day-old pathogens caused higher values of AUDPC than older pathogen cultures. Isolates MS17-1 was the most aggressive isolate from the test of 25 isolates from seven states in the U.S. Eighteen previously reported field PSD-resistant accessions had significantly lower AUDPC than the susceptible checks and other entries ( P  ≤ 0.05).

Conclusion: This study provided rapid evaluation of soybeans for reaction to P. longicolla and identification of PSD-resistant genotypes. Although PSD is a soybean seed disease, results from the cut-seedling inoculation assays without waiting a whole growing season were comparable to those obtained from field tests. Additionally, concerns about the environmental effects and uneven distribution of the pathogen in the field were ameliorated. The cut-seedling inoculation technique can also be used to speed up evaluation of PSD populations for the discovery of PSD-resistance gene(s), and high throughput phenotyping of seed diseases at seedling stage for genetics and genomic studies.