Assessment of infection efficiency and symptom severity of rhizogenic Agrobacterium strains

Hairy Root Disease (HRD) is a significant threat to hydroponic crops such as tomato, cucumber, melon, eggplant, and bell pepper. Rhizogenic agrobacteria are the primary causative agents of HRD and infection of host plants results in excessive root growth, which leads to substantial yield losses. This study investigated infection efficiency and symptom severity across 24 rhizogenic Agrobacterium strains and explored potential correlations with specific genomospecies, opine types or host of isolation. This was done using a laboratory-scale assay, i.e. an in vitro carrot disk bioassay, and a soil-based tomato bioassay. In the carrot disk assay, substantial variation was observed in infection efficiency and symptom severity, in particular excessive root development, across strains. While infection efficiency showed no clear correlation with genomospecies, opine types or host of origin, the carrot disk assay showed that hairy root biomass as a measure for symptom severity varied depending on genomospecies, opine type, and plant host from where the isolates are originally isolated. Based on the outcomes of this carrot disk assay, six strains were selected for additional assessment under greenhouse conditions in the soil-based tomato bioassay. Similar to the carrot disk assay, large variations were observed between strains regarding infection efficiency and symptom severity. The use of these complementary bioassays not only allowed identification of strain-specific differences in virulence, but also provided valuable insight into the diversity of rhizogenic Agrobacterium and its interaction with different host plants.