Evaluation of Germplasm Resistance in Several Soybean Accessions Against Soybean Fusarium Root Rot in Harbin, Heilongjiang Province, China

Soybean root rot, caused by diverse soil-borne pathogens, is a major constraint on production worldwide, with yield losses ranging from 10 to 60% under epidemic conditions. Symptomatic plants were collected from three locations in Harbin, Heilongjiang Province, China, and 23 fungal isolates were recovered using standard tissue isolation procedures. Integrated morphological characterization and rDNA-ITS sequencing identified these isolates as three <i>Fusarium</i> species: <i>F. oxysporum</i> (18 isolates, 78%), <i>F. equiseti</i> (3 isolates, 13%), and <i>F. brachygibbosum</i> (2 isolates, 9%). Pathogenicity assays following Koch's postulates confirmed <i>F. oxysporum</i> as the predominant and most aggressive pathogen in this region. To identify resistance resources, 200 soybean germplasm accessions adapted to Northeast China were screened using an etiolated seedling hypocotyl inoculation method with <i>Fusarium oxysporum</i> isolate A3 (DSI = 68.5) as the test pathogen. Disease severity indices exhibited a continuous distribution (mean = 52.84, range = 0-100), suggesting quantitative inheritance. Accessions were classified as highly resistant (13, 6.5%), resistant (40, 20%), moderately susceptible (67, 33.5%), susceptible (43, 21.5%), or highly susceptible (37, 18.5%). To explore potential molecular mechanisms underlying resistance, RT-qPCR analysis was performed on two extreme genotypes-a highly resistant line (H9477F5, DSI = 15.3) and a highly susceptible line (HN91, DSI = 88.7) at 1, 3, and 5 days post-inoculation. The resistant line maintained consistently higher expression of positive regulators <i>GmFER</i> and <i>GmSOD1</i>, with <i>GmFER</i> reaching 15.89-fold induction at day 3. Conversely, expression of negative regulators <i>GmJAZ1</i> and <i>GmTAP1</i> remained lower in the resistant line, with susceptible plants showing 5.62-fold higher <i>GmJAZ1</i> expression at day 3. These findings provide characterized pathogen isolates, resistant germplasm resources (53 accessions with HR or R classifications), and preliminary molecular insights that may inform breeding strategies for improving root rot resistance in Northeast China.