Development of a Rhizobox-Based Inoculation Method for <i>Pythium graminicola</i> to Observe Nonfatal Stunting on Corn Plant Maturity

Pythium root rot (PRR), caused by species of Pythium sensu lato (s.l.), is typically characterized as a seedling disease, but field observations suggest that early infections causing nonfatal stunting have season-long impacts on corn (Zea mays L.) growth and yield. In Delaware, Pythium graminicola is the predominant species associated with PRR in corn. Controlled studies evaluating PRR impact through corn physiological maturity (R6) are limited, with no inoculation method specifically aimed at inducing nonfatal symptoms identified. We evaluated five rates (1.0, 2.5, 10.0, and 20.0 g) of P. graminicola-colonized sand-cornmeal substrate in small pot (200 g potting media) greenhouse experiments for impacts on corn shoot length (cm) and root biomass (g) at V2–V3. Higher rates of inoculum substrate (10.0 and 20.0 g) reduced shoot length and root biomass without inducing plant death. These rates were scaled to rhizoboxes (450, 900, or 1800 g per 6 kg potting media) to evaluate impacts on shoot length (cm) 28 days after planting (V3–V4); and shoot length (cm), average stalk diameter stalk width (mm), root biomass (g), root rating (1–10 scale) and kernel count (n) at R6. Lower corn shoot lengths were observed in V3–V4 plants inoculated with 1800 g of colonized sand-cornmeal, but differences were not detectable at R6, suggesting that plant competition exacerbates nonfatal PRR field infection effects. These findings provide a foundation for future studies on nonfatal PRR infections that will inform germplasm screening or other efforts targeting improved resilience of corn to PRR to minimize yield loss.