Rhodococcus fascians

Taxonomy and pathogen type: Rhodococcus fascians is a species within the kingdom Bacteria, genus Rhodococcus. It is an actinobacterium known as a significant phytopathogen. Unlike many other bacterial pathogens, its virulence is primarily attributed to the presence of a linear plasmid that encodes for the production of cytokinins, which manipulate the plant's hormonal signaling.

Diseases and host range: The pathogen causes distinctive morphological abnormalities, most notably fasciation (flattening of stems), hyperproliferation, and the formation of witches' brooms. Its host range is exceptionally broad, affecting ornamental species such as lilies, chrysanthemums, and geraniums, as well as agricultural crops like strawberries and various leafy vegetables.

Biology and life cycle: The bacterium thrives on the surface of plant tissues and gains entry through wounds or natural openings. Once inside, it colonizes the apoplast and secretes cytokinins. These signaling molecules induce the host plant to undergo abnormal cell division in the meristems, resulting in the characteristic stunting and dense clusters of short, distorted shoots.

Conditions for development and spread: Rhodococcus fascians proliferation is favored by moist environments and high relative humidity. The pathogen is easily disseminated in greenhouse conditions via contaminated irrigation water, soil splashes, and vegetative propagation material. Mechanical transmission through pruning equipment is a major vector in commercial production settings.

Damage, protection, and control: The impact is primarily economic, as symptomatic plants are deemed aesthetically unsuitable or unmarketable. Management relies on stringent sanitation:

• sourcing pathogen-free propagating material
• strictly sanitizing cutting tools with disinfectants
• removing and destroying infected plants promptly
• maintaining good greenhouse ventilation to reduce humidity levels