Quantitative Assessment of <i>Fusarium graminearum</i> Infection in Resistant and Susceptible Barley Genotypes Beyond Visual Symptoms

Fusarium Head Blight (FHB), primarily caused by <i>Fusarium graminearum</i>, damages cereal crops by inducing discoloration of spikes and producing deoxynivalenol (DON), which contaminates grains. A TaqMan multiplex real-time PCR previously demonstrated a strong correlation between DON and <i>Fusarium</i> biomass measured by <i>Fusarium Tri5</i> over barley <i>Actin</i>, though a weaker association with severity. To investigate this observed disconnection, we analyzed data from 340 spikes of susceptible (Explorer) and resistant (AAC Synergy) barley at rating, sourced from a single field plot inoculated with <i>F. graminearum</i> and categorized according to severity rating categories (SRC). <i>F. graminearum</i> load was measured externally via the <i>Tri5</i> gene, internally through <i>Fusarium</i> biomass and colony-forming units (CFU), in addition to DON levels, to identify reliable infection indicators. Significant Genotype × SRC interactions were observed across all measurements, including <i>Tri5</i> (<i>P</i>-value = 0.0003), biomass (<i>P</i>-value = 0.0031), CFU (<i>P</i>-value = 0.0012), and DON (<i>P</i>-value = 0.0022). The genotype influenced <i>Tri5</i> and CFU but did not significantly affect biomass or DON levels. SRC had a notable impact on all four metrics, with higher visual severity correlating with increased <i>Tri5</i> levels, <i>Fusarium</i> DNA ratios, and CFU counts, thereby indicating greater fungal presence and biomass. Although severity correlated strongly with these measurements, it did not account for differences in genotype response. The combination of real-time PCR and CFU assays offers potential for improving future screening for FHB resistance. These findings underscore the importance of employing diverse measurement approaches to accurately evaluate <i>F. graminearum</i> infection in barley spikes.