Indexing for plant pathogens

Several factors account for the importance of producing plants of certified health status from tissue culture. First, while most pathogens are eliminated in seed propagation, they are transmitted at high frequency in vegetative propagation. This also applies to micropropagation, where the starting material is infected and where pathogens are not eliminated — e.g., by meristem culture at the establishment of the aseptic culture (Stage I; see Chapter 11) (George, 1993). Second, vegetative propagation is relatively slow and involves mature stock plants, so there is the likelihood that pathogen symptoms will be expressed. Pathogen symptoms, however, may not be expressed in tissues in vitro, and there is also the risk that uncultivable pathogens, especially fastidious bacteria, and inter-and intracellular endophytes may go undetected unless the tissues are specifically indexed for the latter. Third, there is the risk that the infected starting material may be clonally propagated and released locally or exported, where the contaminating microorganism may become pathogenic in the crop, establish a reservoir of a pathogen that will initiate infection in other crops, or, in an extreme case, introduce a new pathogen into a quarantine zone.