High Humidity and Rainfall During Late Grain Filling Promote Deoxynivalenol Accumulation rather than Fusarium Head Blight Severity in Wheat

Fusarium head blight (FHB), caused mainly by Fusarium graminearum and Fusarium asiaticum in China, severely threatens wheat production and food safety via deoxynivalenol (DON) contamination. We investigated FHB at eight wheat-producing sites across six provinces in China during the 2023 growing season, quantifying Fusarium-infected spike rate (FISR), Fusarium-infected spikelet rate (FISLR), FHB-symptom spike rate (FSSR), and grain DON content at early and late grain filling stages, and analyzing their correlations with meteorological factors. Results showed widespread Fusarium latent infection at early grain filling (FISR up to 90% in Shaanxi), with post-flowering infection increasing FISR by ≥17% at four sites even under routine fungicide application. FSSR was consistently lower than FISR, indicating that asymptomatic infection is common and poses a hidden risk. Grain DON content varied drastically (15-2433 μg/kg), with samples from Shaanxi exceeding twice the national safety threshold. Spearman correlation and linear regression analyses revealed that humidity at late grain filling was the most critical driver of DON accumulation (R² = 0.89, p < 0.001) rather than FSSR (R² = 0.32, p = 0.182). A second fungicide application (7 days after the initial treatment) at Haian, Jiangsu significantly reduced FHB symptoms and DON accumulation, with observed DON content much lower than the predicted >1000 μg/kg. Our findings confirm that rainfall and high humidity at late grain filling, rather than FHB severity, dominate DON accumulation in wheat. They also highlight the prevalence of latent infection at early grain filling and widespread post-flowering Fusarium infection during grain filling, which would also lead to hidden but severe FHB-related toxin risks. A second fungicide application effectively reduces DON contamination, providing a scientific basis for optimized FHB management strategies.