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

Abstract

Two diagnostic assays involving the polymerase chain reaction (PCR) were evaluated to detect and quantify phytoplasma in aster leafhoppers, Macrosteles quadrilineatus. One assay was used to detect the pathogen in individual insects and monitor the number of insects carrying the pathogen. The other assay was a quantitative PCR, which was used to estimate the total amount of phytoplasma DNA in a leafhopper population. The presence of the pathogen in aster leafhoppers was compared with the incidence of aster yellows disease in lettuce plots at Bradford, Cambridge and Grand Bend, Ontario, from 1992 to 1995. Except for 1995, peaks in phytoplasma in the aster leafhopper population preceded or coincided with the appearance of aster yellows symptoms in the crop. The presence of phytoplasma-infected leafhoppers was a better predictor of aster yellows than the total number of aster leafhoppers. Results from 1995 were unusual because no aster yellows symptoms were observed in the crop even though phytoplasma-infected leafhoppers were present, and this may have been due to unusually high temperatures which have been reported to prevent transmission of aster yellows and eradicate the phytoplasma from infected plants. The results of this research show the strengths and limitations ofa monitoring program for aster yellows using PCR-based assays.