Diversity and Management of <i>Pythium</i> sensu lato Associated with Sweet Corn in Virginia

Abstract

Sweet corn (Zea mays L. var. saccharata), particularly shrunken-2 hybrid cultivars, is highly susceptible to seed decay and damping-off caused by Pythium sensu lato (s.l.), a disease complex that includes many Globisporangium species. Production risk is greatest when early planting, incentivized by lucrative market prospects, coincides with cool, wet soil conditions. Despite the importance of these diseases, the diversity within regional Pythium s.l. populations remains poorly characterized in most sweet corn production regions, limiting pathogen-specific management recommendations. This thesis characterized the diversity of Globisporangium species associated with Virginia sweet corn production and evaluated the efficacy of commercially available fungicide and nutrient-based seed treatments against the predominant species. Soil baiting from 44 sweet corn fields during 2024 and 2025 recovered 226 isolates representing 10 Globisporangium species, with G. sylvaticum as the predominant species statewide (39.3% in 2024; 44.7% in 2025), followed by G. irregulare (30.4% in 2024; 11.4% in 2025), G. spinosum (14.3% in 2024; 21.9% in 2025), G. cryptoirregulare (8.0% in 2024; 1.7% in 2025), and G. ultimum (7.1% in 2024; 8.8% in 2025). Community composition varied significantly among physiographic divisions, with G. spinosum associated with the Coastal Plain and G. cryptoirregulare with the Blue Ridge division. All five dominant species were confirmed pathogenic on sweet corn seedlings. Species differed in temperature-dependent aggressiveness. G. ultimum was substantially more aggressive at 25°C (94.2% disease severity index) compared to 12.8°C (71.4%), whereas G. cryptoirregulare demonstrated the opposite pattern and was more aggressive at cooler temperatures. G. sylvaticum maintained substantial aggressiveness across both temperature regimes. Picarbutrazox and mefenoxam provided the greatest protection during pre-emergence damping-off, with mefenoxam significantly increasing seedling emergence in four of five species tested. In post-emergence experiments, fungicide- and nutrient-based seed treatment efficacy was substantially reduced across all tested products, indicating reduced performance after seedling emergence. These findings demonstrate that Virginia sweet corn production is challenged by a diverse and geographically structured Globisporangium community with species-specific temperature-dependent aggressiveness and differential fungicide responses. Collectively, these findings provide a foundation for developing regionally informed, pathogen-specific seed treatment recommendations.