TRANSMISSION OF SPIROPLASMA CITRI IN CARROT SEEDS AND DEVELOPMENT OF A REALTIME PCR FOR ITS DETECTION
A. Alfaro-Fernández, I. Ibañez, E. Bertolini, D. Hernández-Llopis, M. Cambra, M.I. Font
Spiroplasma citri is a phloem-limited bacterium that causes citrus stubborn disease and vegetative disorders in Apiaceae, such as carrot and celery, and in other hosts. Detection methods include indexing in indicator plants, in vitro culturing in specific media, and serological and molecular amplification methods, e.g., conventional and real-time polymerase chain reaction (PCR). A protocol for the detection of S. citri in carrot seeds, roots and leaves by real-time PCR with a TaqMan probe was developed and assayed. The protocol was specific and sensitivity was 102 times higher than conventional PCR for S. citri detection. One and two samples out of 20, which consisted of five whole seeds per sample, were positive for S. citri by real-time PCR, which implies an infection rate of at least 1-2%. Seed transmission was assessed when analysing the roots of the seedlings that emerged from both cultivars grown in an insect-free containment growth chamber (P2 level) 240 days postsowing. Transmission rates of 30.1% and 37.6% were estimated. The symptoms observed in all the seedlings that tested positive for the pathogen included twisted petioles, filiform, mosaic, yellowing or red leaves, and protuberances, deformation or branching in roots. The epidemiological consequences of S. citri transmission in carrot seeds are discussed. The role of S. citri inoculum in carrot crops cannot be ruled out at increasing infection rates in other hosts. The control of carrot seed lots for this quarantine pathogen must be undertaken.