Development of young pear trees with different rootstocks in relation to psylla infestation, pear decline, and leaf curl

Abstract

Young Bartlett, Winter Nelis, and Hardy pear Pyrus communis L. trees with six different rootstocks were studied during 1961 through 1965. Non-topgrafted trees of each type of rootstock were included. Three treatments were used: (1) saran cages placed around entire trees (caged-control); (2) spray applications (exposed-control); and (3) psylla infestation with Psylla pyricola Foerster confined to branches of trees by means of organdy sleeve cages (psylla-infested). This study confirmed the decline tolerance of trees with Bartlett, Winter Nelis, or Pyrus calleryana Decne. seedling, or own-rooted Old Home P. communis rootstocks. Decline susceptibility was confirmed with trees with either P. serotina Rehd. or P. ussuriensis Maxim. rootstocks. Psylla-tight cages protected decline-susceptible trees—corroborating previous reports that the pear psylla is the pear decline vector. Differences in growth among trees with decline-tolerant stocks were small. Non-topgrafted rootstock trees attained greater size than their topgrafted counterparts; their vigor was correlated with the vigor and decline tolerance each type could impart as a root-stock for P. communis varieties. The size of root systems was correlated with trunk circumference and shoot growth. Tests also indicated that trees without graft unions may succumb to decline. Contrary to previous reports, these experiments showed that girdling from phloem sieve-tube necrosis is not always a prerequisite to the development of red foliage. Also, red foliage may indicate damage from psylla feeding alone or psylla feeding plus pear decline virus for non-topgrafred trees and decline-tolerant scion-stock combinations, as well as decline-susceptible ones. Leaf curl symptoms occurred in a large percentage of the exposed-control and psylla-infesred topgrafred trees, and in non-topgrafted Bartlett and Winter Nelis seedlings, and own-rooted Old Home. Differences in variety, source of scionwood, species or type of rootstock had little effect on incidence of curl. A few of the caged-control trees remained free of curl for more than a year after the cages were removed. This indicated that cages prevented or delayed its transmission by psylla. Psylla strains from New York and other sources, presumably free of pear decline virus, apparently induced curl in young pear trees, suggesting that the two diseases are separate entities. Evidently, complete exclusion of psylla would be required to control curl. The study revealed no outstanding differences among trees of one variety with the different decline-tolerant rootstocks. A mixture of the decline-tolerant rootstocks is recommended over any single rootstock for commercial plantings of the three varieties studied, however, because no one rootstock can be expected to be superior in all orchard situations.