China is the largest apple producer in the world, accounting for 44% of world production in 2009 (FAO, 2011). Apple production in China has increased 7-fold in the last two decades to 31 million tonnes in 2009 (FAO, 2011; Gao et al., 2011). In recent years, many apple cultivars and rootstocks have been introduced to China from foreign countries (Gao et al., 2011). In addition, apple breeding has accelerated in China and several cultivars and rootstocks have been released in the last few decades (Gao et al., 2011). Currently, ≈90% of apple orchards are planted with wild non-dwarfing standard seedlings of species such as M. sieversii, M. baccata, and M. prunifolia used as rootstocks and cultivars as scions (Gao et al., 2011; Wan et al., 2011). A minority of orchards use “semi-dwarfing” trees: wild non-dwarfing seedlings as rootstocks with dwarfing interstems (e.g., ‘M.26’) and commercial cultivars as scions (Gao et al., 2011; Rong et al., 2011; Wan et al., 2011). The industry is transitioning to a greater use of dwarfing rootstock clones as both rootstocks and interstems (Rong et al., 2011). Researchers throughout China are developing cultivars with high quality and rootstocks with high performance, both with disease resistance (Gao et al., 2011; Wan et al., 2011). The importance of disease resistance of scions is obvious but resistance of rootstocks is also of interest. Disease-resistant rootstocks would be beneficial to nurserymen who grow rootstock liners and need to have disease-free materials. There is also some evidence that when grafted, rootstocks may be able to confer resistance to scions (Cristinzio et al., 2001; Jensen et al., 2003, 2012).
The development of such genotypes is expected to improve both fruit quality and production efficiency (Gao et al., 2011; Wan et al., 2011). However, breeding new cultivars and rootstocks is complex as a result of their highly heterozygous genetic background and long generation times (Gao et al., 2011; Janick et al., 1996; Wan et al., 2011). Therefore, careful selection of breeding parents is essential.
Marssonina coronaria (Ellis and J.J. Davis) [also known as Diplocarpon mali (Y. Harada & Sawamura)], a causative agent of Marssonina blotch, and Alternaria alternata apple pathotype [previously known as A. mali (Roberts, 1924)], a causative agent of alternaria leaf blotch, are fungi causing the two most important diseases that devastate apple production in China (Lee et al., 2011; Shou et al., 2009; Zhao et al., 2011). In China, the annual loss of apple production caused by M. coronaria is 12.4% to 28.7% and to A. alternata apple pathotype is 15.9% to 30.1% (Shou et al., 2009; Zhao et al., 2011). These two diseases primarily occur in Asian countries and seriously affect their apple industries (Lee et al., 2011; Sharma and Sharma, 2006). Symptoms of M. coronaria appear as round or oval small brown leaf spots that later become black, enlarged, and may coalesce. Eventually, leaves become chlorotic around the sites of infection and abscise. Symptoms of A. alternata apple pathotype appear as small, gray leaf spots with chlorotic margins that later extend into gray patches. Finally, the area surrounding the lesions withers and leaf abscission occurs (Harada et al., 1974; Lee et al., 2011; Sharma and Sharma, 2006; Shou et al., 2009; Tamietti and Matta, 2003; Zhao et al., 2011). In epidemic years, these two diseases can cause serious defoliation and reduce tree growth, flowering, fruit quality, and yield (Harada et al., 1974; Lee et al., 2011; Sharma and Sharma, 2006; Zhao et al., 2011).
To date, there have been few reports concerning evaluation and identification of resistance to these two fungal pathogens in apple cultivars and rootstocks commonly used in China. This study is the first to evaluate field tolerance to these two pathogens in 74 apple cultivars and rootstocks to provide basic information for the apple industry and apple disease resistance breeding programs.
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