PRR is a catastrophic disease of the three North American Castanea taxa: american chestnut [C. dentata (Marsh.) Borkh.], allegheny chinquapin [C. pumila (L.) Mill. var. pumila], and ozark chinquapin [C. pumila (L.) Mill. var. ozarkensis (Ashe) Tucker] (Crandall et al., 1945; Jeffers et al., 2009, 2012; Sena et al., 2018). The principal causal pathogen, the oomycete Phytophthora cinnamomi Rands, is suspected to have been introduced to one of the coastal ports of the southeastern United States via ornamental plants from eastern Asia in the early 19th century (Anagnostakis, 2001; Crandall et al., 1945). Analyses of historical records have implicated P. cinnamomi in the widespread dieback of C. dentata and C. pumila var. pumila in the southeastern Coastal Plain and Piedmont during the 19th century (Anagnostakis, 2001; Crandall et al., 1945). Phytophthora cinnamomi has been isolated from dying chestnuts and chinquapins across the southeastern United States, as far north as Pennsylvania and as far west as Arkansas (Crandall et al., 1945; Fitzsimmons, 2016; Jeffers et al., 2009). Symptoms of PRR include necrosis of roots, leaf chlorosis, wilting of foliage, and branch dieback (Crandall et al., 1945; Jeffers et al., 2009). Unless symptoms are controlled with potassium phosphites or similar chemicals (James, 2011a), PRR will cause the death of susceptible plants 3 weeks to several years after the initial infection (Crandall et al., 1945; Jeffers et al., 2009). Phytophthora cinnamomi is one of the most serious obstacles to growth of C. dentata throughout its former range because of the prevalence of the pathogen in a variety of soils across the southeast and the fact that PRR is ultimately fatal to North American Castanea species.
In addition to PRR, the North American Castanea species are severely affected by chestnut blight, caused by the ascomycete Cryphonectria parasitica (Murr.) Barr (Alexander et al., 2005; Anagnostakis, 2001; Paillet, 1993, 2002). Since 1989, The American Chestnut Foundation (TACF) and collaborators have used the backcross method to introgress blight resistance from chinese chestnut (C. mollissima Blume) into a C. dentata genetic background (Hebard, 2005; Westbrook, 2017). In a recent review of TACF’s backcross breeding program, Steiner et al. (2017) reported that progeny tests have validated experimentally expectations that a blight-resistant population of C. dentata-type trees can be constructed using introgression of extraspecific alleles for blight resistance. However, because P. cinnamomi was only recently recognized as an impediment to C. dentata restoration, TACF’s breeding program has not selected for PRR resistance. As a result, PRR has caused the failure of entire experimental orchards and forest progeny tests before advanced backcross trees could be assessed for chestnut blight resistance (Clark et al., 2014; Jeffers et al., 2009, 2012; Sisco, 2009).
In response to PRR-induced mortality of potentially blight-resistant hybrid seedlings, a screening program was initiated in 2004 to detect PRR-resistant seedlings in TACF’s backcross lines (James, 2011a, 2011b; Jeffers et al., 2009, 2012). Results of this screening program have indicated that PRR-resistant seedlings are present at low frequency in some, but not all, TACF advanced backcross lines (BC2Fx, BC3Fx, and BC4Fx generations) that have been selected for blight resistance—most notably in lines derived from the blight-resistant hybrid cultivars Clapper and Graves (Jeffers et al., 2009). More recent resistance screening efforts showed that PRR resistance was also present in descendants of TACF’s third major source of blight resistance, C. mollissima ‘Nanking’ (Zhebentyayeva et al., 2014).
Studies of the quantitative trait loci (QTL) associated with resistance to P. cinnamomi have aided breeding efforts by demonstrating that this trait is controlled by a limited number of genomic regions and may therefore be a practicable target for marker-assisted selection (Santos et al., 2017; Zhebentyayeva et al., 2014). Santos et al. (2017) identified QTLs for PRR resistance on linkage groups E and K of japanese chestnut (C. crenata Sieb. & Zucc.), and QTLs for PRR resistance were found on the same two linkage groups of C. mollissima (T.N. Zhebentyayeva, The Pennsylvania State University, personal communication). These findings suggest common resistance mechanisms to P. cinnamomi infection across different species within the genus Castanea (Santos et al., 2017). However, candidate genes within these QTLs have not been tested for their effects, and there exists the possibility that the East Asian Castanea species may have multiple alleles that encode PRR resistance. Until the genetic control of PRR resistance is better understood, additional sources of resistance are desirable for chestnut breeding programs in the eastern United States, because different C. mollissima and C. crenata cultivars may have different alleles encoding resistance. Efforts to identify multiple sources of PRR resistance are also important because they will facilitate the introgression of different nut characteristics and other agronomically important traits into populations being tested for the commercial chestnut market.
Because of the high susceptibility of C. dentata to both PRR and chestnut blight, the growth of C. dentata-type trees across large portions of the southeastern United States requires planting trees that have host resistance to both pathogens. The objective of this study was to evaluate PRR resistance in 10 hybrid families derived from crosses between susceptible C. dentata trees and potentially resistant C. mollissima cultivars in a containment system, followed by outplanting the least symptomatic plants into a P. cinnamomi-positive orchard and assessing their survival midway through the second growing season.
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