Among the cultivated hardy trees and shrubs of North America, five subgenera are defined for Prunus by Rehder (1940), among which are the species of Amygdalus. This subgenus includes peach (P. persica), a widely grown tree fruit throughout North America, and almond (P. dulcis), a tree nut cultivated almost exclusively in California orchards in North America. Both species are economically important for California and have significant acreage throughout its various growing regions. In 2007, almonds were a $2 billion U.S. crop grown on nearly 250,000 ha, whereas peaches (fresh market freestones and canning clings) occupied ≈25,000 ha in California (Anon., 2008).
Xylella fastidiosa (Xf) is a genetically diverse Gram-negative bacterium with numerous hosts (Freitag, 1951), including a number of Prunus species, typically causing irregular patterns of leaf necrosis or “scorching” in infected plants (Hopkins and Purcell, 2002). In California, Xf is responsible for both Pierce's disease (PD) of grapes and almond leaf scorch disease (ALSD) in almond. Xf biotype differences based on single nucleotide polymorphisms of 16S rDNA sequences have been determined (Chen et al., 2005) with “A-genotype” biotypes causing only ALSD and “G-genotype” biotypes capable of causing both PD and ALSD. Using root-graft transmission studies and serology, Wells et al. (1981) demonstrated biotype differences for Xf causing both phony peach disease and plum leaf scald in the southeastern United States. Healthy plums grafted with infected root segments from peach trees expressing phony peach symptoms developed leaf scald symptoms after 9 months of growth. However, similarly, healthy peach trees grafted with roots from plum trees expressing plum leaf scald remained symptomless even after 2 years of growth (Wells et al., 1983). Similar results have been noted for Xf biotypes responsible for citrus variegated chlorosis and coffee leaf scorch diseases (de Lima et al., 1998; Li et al., 2001). Several other transmission and molecular-based studies have demonstrated the complexity of Xf biotype–host interactions with regard to pathogenicity and fulfillment of Koch's postulates (Hernandez-Martinez et al., 2006; Mircetich et al., 1976; Montero-Astúa et al., 2007; Raju et al., 1982).
Both random and aggregated patterns of ALSD distribution were observed in sampled almond orchards (Groves et al., 2005), indicating differences in almond cultivar susceptibility to the Xf biotypes present and raising questions relative to the original source(s) of primary inoculum. Using geostatistics to determine spatial patterns of disease development, Roberto et al. (2002) concluded that commercial citrus nurseries were the primary sources of citrus variegated chlorosis-infected orange trees in citrus orchards; however, primary inoculum sources of ALSD in almond orchards are as yet undetermined. Natural vegetation in habitats bordering California commercial grape and stone fruit orchards has been shown to harbor Xf and has been implicated as primary inoculum sources for PD in adjacent vineyards (Redak et al., 2004). When these same habitats border commercial tree nurseries, Xf-infected vectors are capable of inoculating susceptible rootstocks and budded trees throughout the growing season.
Susceptibility of almond to Xf biotypes causing PD and ALSD is well documented. Although accessions of peach are susceptible to phony peach Xf biotypes, several studies indicate that they resist infection from Xf biotypes responsible for PD and ALSD (Mircetich et al., 1976; Wells et al., 1983). Seed-propagated peaches are used extensively in California as rootstocks for almonds and other stone fruit species with rootstock cultivars such as ‘Nemaguard’ being favored by growers because of resistance to root knot nematodes (Meloidogyne incognita and M. javanica). In cases in which more scion vigor is desirable, peach × almond (PEAL) hybrids are being used, predominantly in newer almond plantings. The susceptibility of PEAL hybrids to Xf isolates causing ALSD is currently unknown. In this study, we examined disease development and bacterial titer within parental peach and almond genotypes together with an interspecific hybrid created from them over the course of a growing season.
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