Evaluation of Pyrus Germplasm for Resistance to Pear Psylla In the Orchard

An orchard collection of 208 diverse Pyrus genotypes was evaluated by both subjective ratings and objective counts of eggs and nymphs of pear psylla (Cacopsylla pyricola Föerster) to determine whether differences in susceptibility could be detected with a degree of reliability sufficient for preliminary selection of germplasm.  Subjective scores assigned in two consecutive years were highly correlated (r=0.83), although the infestation was significantly more severe in the second year of the study.  The correlation of subjective score with counts of eggs and nymphs was highly significant and moderately high (r=0.64).  Germplasm with P. pyrifolia or P. ussuriensis parentage were more resistant than those with P. communis parentage.  Significant differences among genotypes could be detected in egg counts in March and April, and in egg and 1st‑3rd instar counts in April.  Significant differences among counts on fruit spurs, vegetative spurs, and terminal buds or shoots were consistently found during all three months, with egg counts on fruit spurs higher during March and April.  Counts of eggs and nymphs were generally higher on terminal shoots in May.  Significant variation among trees and genotype by sample type interaction developed by May.  Although the sampling scheme used in this study could detect large differences in host resistance, greater accuracy would require sample sizes too large for practical use with large collections.