The epidemiology of pear decline in the UK

Polymerase chain reaction (PCR) was used to detect phytoplasmas in pear psyllids, the insect vector of pear decline, using primers to sequences in the 16S rRNA gene. Psyllid samples from eight commercial pear orchards and from one experimental plot of pear seedlings were rested over a period of one year. The highest proportion of insects indexing positive occurred in late summer when the concentration of phytoplasmas in pear trees was also high. However, there was no correlation between the numbers of trees affected by pear decline in the commercial orchards and the numbers of pear psyllids or the proportion of pear psyllids yielding a positive result by PCR. Positive results were obtained from overwintered adults sampled in spring, a time of year when phytoplasmas could not be detected in the aerial parts of pear trees. It is proposed that the overwintering generation of adult psyllids retain phytoplasmas during the winter months at a time when the phytoplasmas an unable to survive in dormant pear trees and that new infections are initiated by the feeding of overwintered adults the following spring.