Pear psyllids [Cacopsylla pyri (L.), C. pyricola (Förster), and C. pyrisuga (Förster)] are major arthropod pests of pear (Pyrus communis L.) throughout North America and Europe. Both adults and nymphs feed primarily in the vascular tissue of the leaves, petioles, and succulent shoots. Prolonged feeding in the phloem by nymphs affects the plant directly by inducing necrosis and premature defoliation. The nymphs excrete excess sugar as honeydew, which can cause russetting of young fruit. Sooty mold fungus grows on the honeydew, resulting in reduced photosynthesis and marked fruit. The insect is also the vector of the pear decline phytoplasma (Hibino et al., 1971). In North America, Cacopsylla pyricola has rapidly developed resistance to commonly used pesticides, including organophosphates and pyrethroids, and biological controls are not always sufficiently effective. Resistant cultivars would reduce grower costs and enhance the sustainability of the pear industry.
Resistance within the genus Pyrus to pear psylla has been demonstrated in the East Asian pear species P. betulifolia Bunge, P. calleryana Decne., P. fauriei Schneid., P. pyrifolia (Burm.) Nakai, P. ussuriensis Maxim., and P. ×bretschneideri Rehd.; in P. ussuriensis × P. communis hybrids (Berrada et al., 1995; Harris, 1973; Harris and Lamb, 1973; Quamme, 1984; Westigard et al., 1970); and in a few genotypes of the European “snow pear”; P. nivalis Jacq. (Quamme, 1984; Westigard et al., 1970). Cultivars within the European pear species, P. communis L., with moderate to high levels of resistance include the Italian cultivar Spina Carpi (Briolini et al., 1988; Quarta and Ruggioni, 1985), ‘Doyenne du Poitiers’ (Robert et al., 1999), and 15 landrace cultivars from Europe (Bell, 1992, 2003; Bell and Stuart, 1990; Bell and van der Zwet, 1998). These studies used a variety of objective orchard and laboratory studies, including choice and no-choice tests of oviposition and antibiosis. Ovipositional antixenosis (i.e., inhibition of egg laying), feeding antixenosis (i.e., inhibition of feeding), and antibiosis have all been identified as modes of resistance. Nymphal feeding antixenosis has been associated with inhibition of oviposition, increased mortality, and delayed development of nymphs (Bell, 1991; Butt et al., 1988, 1989; Puterka, 1997; Puterka et al., 1993) in both P. communis and P. ussuriensis-derived germplasm and, thus, was used as the measure of host resistance in this study.
Based on the distribution of counts of pear psylla nymphs in seedling trees derived from crosses of susceptible P. communis cultivars with P. ussuriensis 65, a resistant open-pollinated selection, Harris and Lamb (1973) concluded that resistance was heritable and dominant. The only other published study of the inheritance of resistance to this pest confirmed the multigenic nature of genetic control and identified one quantitative trait locus (QTL) that explained 15% of the phenotypic variability in a cross of ‘Angelys’ and NY 10355 and a few other minor QTLs (Bouvier et al., 2012). The variable analyzed was a subjective estimate of the number of first to third instar nymphs and presence of honeydew. The purpose of the present study was to assess the degree of inheritance of resistance to pear psylla nymphal feeding antixenosis as a key indicator of resistance and to determine the relative breeding values of a set of six resistant P. communis cultivars of European origin.
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