Greenhouse results suggest that disease severity in soybean (Glycine max L.), caused by Diaporthe longicolla as both a stem and seed pathogen, may not be correlated

Diaporthe longicolla causes Diaporthe (Phomopsis) seed decay, as well as pod and stem blight in soybean (Glycine max L.) in the United States. However, our understanding of the genetic basis of resistance to D. longicolla is limited. Therefore, the objective of this study was to determine the relationship between stem and seed infection caused by D. longicolla. Separate experiments were conducted to assess stem and seed infection, both using a completely randomized design. Thirty-nine accessions were screened against a single isolate of D. longicolla in each experiment. In the stem experiment, a mycelial plug was placed on a wound created on the stem at the second trifoliate stage, and the inoculated plants were incubated at 24±3°C with 90% humidity for seven days. In the seed experiment, seeds of each genotype were surface-sterilized, soaked in a mycelial suspension for four hours, plated on 2% water agar, and incubated at 23±2°C for seven days. Seven days post-inoculation, significant differences in disease severity were observed among the accessions in both the stem (P<0.0004) and seed (P<0.0001) experiments. Twenty-nine accessions showed were significantly less susceptible to D. longicolla as a stem pathogen compared to the susceptible check 'Hawkeye', while 24 accessions were significantly less susceptible compared to ‘LG03-4561-14’ in the seed experiment. However, there was no significant correlation between the disease severity caused by D. longicolla on the stem and seeds (P>0.05). These findings indicate that further research on the genetic resistance mechanisms associated with D. longicolla is important for developing resistant cultivars.