MOLECULAR DIAGNOSTICS TO ASSESS THE PROGRESSION OF PHOMA TRACHEIPHILA IN CITRUS AURANTIUM SEEDLINGS AND ANALYSIS OF GENETIC DIVERSITY OF ISOLATES RECOVERED FROM DIFFERENT CITRUS SPECIES IN TUNISIA
L. Kalai, M. Mnari-Hattab, MR. Hajlaoui
This study was focused on the genetic diversity of a collection of Phoma tracheiphila isolates recovered from different orchards and host species in Tunisia. Knowledge of the sequences permitted molecular assessment of the pathogen’s downward progression from the point of inoculation on citrus leaves. Analysis of 58 isolates of P. tracheiphila including four Italian isolates assessed by ITS-RFLP molecular markers revealed that Tunisian isolates of P. tracheiphila are homogenous and genetically similar to the Italian isolates. Development of this pathogen is likely clonal under Mediterranean conditions and this should be taken into account when developing management strategies based on resistant varieties. The result made possible the molecular monitoring of the pathogen’s migration in sour orange vessels using only one representative and virulent isolate. PCR showed that within 10 days post inoculation (dpi) fungal DNA was detected 10 cm from the inoculation point and at 30 dpi it was present in the stem vessels at a distance of 50 cm and more. P. tracheiphila basipetal translocation is very quick and occurs prior to symptom expression. Consequently, diagnosis of mal secco based on typical symptom like shedding of leaves, wilting and the salmon pink discoloration of wood is not appropriate to estimate disease development and must be replaced by a molecular technique that allows earlier detection of the infection in still symptomless material.