Rodrigues, S. D.; Kim, N.; Moons, J.; Rediers, H.; Pauwels, L.; De Coninck, B.

Hairy root disease (HRD), caused by rhizogenic Agrobacterium, is an economically important disease affecting hydroponic tomato (Solanum lycopersicum L.) production worldwide. HRD-affected plants show extensive root proliferation, resulting in decreased energy expenditure towards fruit production. Host plant susceptibility to rhizogenic Agrobacterium is typically evaluated through artificial wounding-based infection bioassays. However, under natural infection settings, rhizogenic Agrobacterium can induce disease symptoms without deliberate, artificial wounding. We developed a soil-based, non-wounding bioassay that closely mimics natural rhizosphere interactions and permits quantitative and qualitative assessment of HRD symptoms. The assay measured root dry weight, documented agravitropic root development typical of HRD and confirmed in planta T-DNA gene expression using reverse transcriptase quantitative polymerase chain reaction (RT-qPCR). We used this bioassay to evaluate disease symptoms towards rhizogenic Agrobacterium in tomato cv. Moneymaker and the rootstocks Optifort, Maxifort, and Arnold. Optifort and Maxifort exhibited significantly higher root biomass than Arnold and Moneymaker, indicating more pronounced symptom development. The bioassay also differentiated virulence levels amongst various rhizogenic Agrobacterium strains isolated from HRD-affected plants. Together, these results show that our soil-based bioassay provides a robust and ecologically relevant platform for screening tomato genotypes and comparing virulence levels of rhizogenic Agrobacterium strains supporting resistance breeding and disease management efforts.