It is important that stock plants remain disease-free to prevent the propagator from replicating plants that spread disease. Propagators use ELISA tests, PCR (polymerase chain reactions) and indexing to detect for disease infection in stock plants in order to maintain clean seed stocks.
Virus-free stock plants are used for grafting, cuttings and tissue culture. Periodically, new stock plants are produced (usually through tissue culture) that are certified to be disease free.
PCR (polymerase chain reactions)
Virus are translocated across grafts. It is common to index stock plants by grafting or budding indicator plants on to stock plants. If the stock plant is infected with a virus (even if symptoms are not obvious), it will be transmitted to the sensitive scion. The scion comes from an indicator species that shows clear visible signs of virus.
In the example to the right, arrow 1 indicates slow growth associated with virus infection, while arrow 2 shows clean virus free plants.
Indicator plant is budded on stock plant.