Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/3/1998
Publication Date: N/A
Interpretive Summary: Phytoplasmas are responsible for a growing list of diseases affecting economically important crop plants around the world. Some phytoplasmas are presently restricted in their geographical distributions, and therefore are subjects of quarantine measures to prevent their spread to new territories. The disease of carrot observed in the Canary Islands of Spain has been causing serious economic losses, but the pathogen responsible for the disease was unknown prior to this study. Using the polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) analysis of amplified DNA, we found that the diseased carrot plants were infected by strains of stolbur phytoplasma-a pathogen which has not been found in North America but which is known to cause serious diseases in grapevines, tomato, potato, pepper, and other plants in Europe. We also found that a psylla, Bactericera trigonica, is capable of transmitting the pathogen to healthy carrots, which subsequently develop disease. These findings are significant in pointing both to the pathogen responsible for the carrot disease and to an insect vector capable of spreading the disease. The results of our study shed new light on the disease in carrot and on the geographic and host distribution of stolbur phytoplasma and provide data that will interest farmers, diagnostics laboratories, and plant quarantine agencies.
Technical Abstract: Plants of carrot exhibiting symptoms suggestive of phytoplasmal infection were observed in the Canary Islands. The use of phytoplasma-specific, nested PCRs and restriction fragment length polymorphism (RFLP) analysis of PCR products confirmed infection by phytoplasma. RFLP patterns of amplified DNA were indistinguishable from those of rDNA from stolbur phytoplasma strain STOL, the type strain of subgroup 16SrXII-A. Greenhouse-grown, healthy plants of carrot, that were fed upon by field collected individuals of the psylla B. trigonica, developed symptoms similar to those observed in field-grown plants. RFLP patterns of amplified rDNA from the experimentally inoculated plants indicated that stolbur phytoplasma strains had been transmitted to carrot by B. trigonica. Using the nested PCR protocol and RFLP analysis of PCR products, DNA of stolbur phytoplasma strains was also detected in the B. trigonica, confirming that this psylla species had acquired the phytoplasma in the field. No symptoms developed in comparable carrot plants fed upon by aphids (Cavariella aegopodii) that had been collected in carrot fields. Indicator plant species developed no symptoms when mechanical transmission of a pathogen from field-collected diseased carrot was attempted. These results gave no evidence for the presence of viruses including carrot red leaf virus, carrot thin leaf virus, and carrot mottle virus. The findings in this study point to stolbur phytoplasma as the cause of the disease in carrot and to a psylla as a vector of the disease agent.