Monitoring phytoplasma in populations of aster leafhoppers from lettuce fields using the polymerase chain reaction

Two diagnostic assays involving the polymerase chain reaction (PCR) were ev aluated to detect and quantify phytoplasma in aster leafhoppers, Macrostele s quadrilineatus. One assay was used to detect the pathogen in individual i nsects and monitor the number of insects carrying the pathogen. The other a ssay was a quantitative PCR, which was used to estimate the total amount of phytoplasma DNA in a leafhopper population. The presence of the pathogen i n aster leafhoppers was compared with the incidence of aster yellows diseas e in lettuce plots at Bradford, Cambridge and Grand Bend, Ontario, from 199 2 to 1995. Except for 1995, peaks in phytoplasma in the aster leafhopper po pulation preceded or coincided with the appearance of aster yellows symptom s in the crop. The presence of phytoplasma-infected leafhoppers was a bette r predictor of aster yellows than the total number of aster leafhoppers. Re sults from 1995 were unusual because no aster yellows symptoms were observe d in the crop even though phytoplasma-infected leafhoppers were present, an d this may have been due to unusually high temperatures which have been rep orted to prevent transmission of aster yellows and eradicate the phytoplasm a from infected plants. The results of this research show the strengths and limitations of a monitoring program for aster yellows using PCR-based assa ys. (C) 1999 Elsevier Science Ltd. Al rights reserved.