Pear psyllids (Cacopsylla spp., Hemiptera: Psyllidae) are major pests of European pear (Pyrus communis L.) in orchards in most temperate regions where the crop is grown (Berrada et al., 1995; Westigard et al., 1970). In North America, the only species present is Cacopsylla pyricola (Förster), whereas in Europe, Cacopsylla pyri (L.) and, in the more northern regions, Cacopsylla pyrisuga (Förster) are also major pest species. 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 russeting 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). Pear psylla have rapidly developed resistance to commonly used pesticides such as pyrethroids (Pree et al., 1990), azinphosmethyl, endosulfan, Perthane, and fenvalerate (Follet et al., 1985), and biological controls are not always sufficiently effective (Shaltiel and Coll, 2010).
Host resistance and pear psylla behavior have recently been reviewed by Bell and Puterka (2003) and Nin et al. (2012). The behavioral and physiological responses of arthropod pests to resistant host genotypes are termed modes of host plant resistance, as distinct from the underlying mechanisms of resistance. Major modes of host plant resistance are as follows: 1) antixenosis, which can be observed in terms of difficulty in orientation and reduced settling (i.e., host finding), adult and nymphal feeding, and oviposition; 2) antibiosis, which delays development, increases mortality, reduces fecundity, and reduces egg hatch; and 3) tolerance, or the ability to withstand infestation without damage. The combination of these effects determines the overall level of pest colonization and damage to the host plant.
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. ussuriensis Maxim., and P. ×bretschneideri Rehd., in P. ussuriensis × P. communis hybrids (Harris, 1973; Harris and Lamb, 1973; Pasqualini et al., 2006; Quamme, 1984), in P. pyrifolia (Burm.) Nakai × P. communis hybrids (Braniste et al., 1994; Robert et al., 2004, 2005), 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 old Italian cultivars Spina Carpi (Quarta and Puggioni, 1985; Briolini et al., 1988), Doyenne du Poitiers, and D’Aoǔt Lamer (Robert et al., 2004, 2005), and 15 landrace cultivars from Eastern Europe (Bell, 1992, 2003; Bell and Stuart, 1990). Two wild pears of uncertain parentage were found in Israel to have reduced susceptibility to C. bidens (Šulc) (Shaltiel-Harpaz et al., 2014). A high level of nymphal feeding resistance has also been identified in ×Sorbopyrus auricularis (Knoop) C. K. Schneid. (Bell, 1992; Bell and Stuart, 1990), an intergeneric hybrid of possible use as a rootstock for pear or as a parent for the development of a novel fruit type. These studies identified antibiosis and ovipositional and feeding antixenosis as modes of resistance.
Nymphal feeding antixenosis has been associated with increased mortality and delayed development of nymphs (Bell, 1991; Butt et al., 1989; Puterka, 1997) in a limited number of both P. communis- and P. ussuriensis-derived cultivars and breeders selections. The purpose of this study was to further characterize host resistance in additional germplasm of interest for breeding.
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