Hairy Root Disease in Hydroponic Crops: Taxonomy, Pathogenesis, Microbiome Dynamics and Management

ABSTRACT

Hairy root disease (HRD) occurs in hydroponic production of tomato, cucumber, eggplant, bell pepper and other protected horticultural crops. Rhizogenic bacteria carrying root‐inducing (Ri) plasmids cause HRD by delivering transfer DNA (T‐DNA) into plant cells at wounded root sites. Stable expression of integrated T‐DNA genes reprogrammes root development and produces persistent adventitious root proliferation. Because Ri plasmids occur in different bacterial backgrounds, species identity cannot by itself define rhizogenicity or reliably predict disease expression. Recirculating nutrient solution promotes within‐system pathogen spread, while contaminated irrigation infrastructure may contribute to persistence. Diagnosis combines symptom recognition, PCR‐based detection of Ri‐plasmid‐associated markers, and recovery of representative isolates for taxonomic characterisation and crop‐relevant pathogenicity testing. Management relies mainly on exclusion, sanitation and early detection, while bacterial antagonists and bacteriophages have reduced disease or bacterial populations in experimental studies. Microbiome analyses identify disease‐associated community shifts but do not establish causation. The same bacterial T‐DNA transfer machinery is used under controlled conditions in hairy‐root biotechnology. Diagnosis and management therefore require bacterial taxonomic identity, Ri‐plasmid evidence and pathogenicity to be considered separately.