Alternative Hosts, Infested Soil, and Infected Roots Serve as Independent Sources of Ceratocystis fimbriata Inoculum and Drive Infection in Sweetpotato Slips

Ceratocystis fimbriata, the causal agent of black rot in sweetpotato, compromises the phytosanitary integrity of vegetative propagation and persists through poorly understood inoculum sources. Although crop rotation is widely used to reduce pathogen pressure, the ability of alternative hosts and reservoirs to sustain inoculum has not been clearly defined. This study investigated (i) the susceptibility of commonly rotated crop species to C. fimbriata using no-wounding, wounding, and seedling inoculation assays and (ii) the extent to which inoculum from infested soil or inoculated roots independently initiates infection in sweetpotato slips. Our study revealed significant crop × time interactions across all host assays ( P < 0.001). Under no-wounding conditions, squash and watermelon supported high infection incidence, serving as asymptomatic reservoirs. Conversely, wounding and seedling assays increased disease development and mortality in the most susceptible hosts (sweetpotato and morning glory), whereas most rotation crops showed low or no mortality. Survival analysis identified sweetpotato as the most vulnerable host (log-rank test, P < 0.001), whereas most alternative hosts exhibited a significantly reduced hazard relative to sweetpotato (hazard ratio < 1; P < 0.05). In the slip experiment, inoculum from both infested soil and infected roots independently caused infection. Pathogen detection via qPCR confirmed colonization up to 76 mm from the stem base, with the T3G9 marker exhibiting significantly higher sensitivity than T5G26 ( P = 0.0079). Together, these findings identify alternative hosts, infested soil, and infected roots as independent inoculum reservoirs that drive black rot epidemiology and threaten the sustainability of sweetpotato propagation systems.