Crabapple is a common name for Malus domestica with bitter fruit under 5 cm in diameter of various wild species of Malus (Guo et al., 2009). These small-fruited species are mainly distributed in the northern temperate zone spanning Eurasia and North America. China is the largest genetic resource of crabapples, with a cultivation history of more than 2000 years (Li, 2001; Qian, 2005). Crabapples, together with peonies, plum blossoms, and orchids, are known as the “four outstanding flowers in spring” in China (Wang, 2010; Wang et al., 2015). They have been cultivated as ornamentals in China since the Han Dynasty (202 BC) and are esteemed by professional and amateur gardeners. Crabapples from China were introduced to Europe and the Americas in the 18th century. Breeding had produced ≈1000 crabapple cultivars, of which approximately half are from Canada and the United States (Dirr, 2010).
Crabapples have become an important ornamental tree in gardens and landscapes because of their diverse flower, fruit, and leaf colorations. Of the nearly 1200 crabapple taxa recorded by Fiala (1994), less than 5% of them are the semi-double or double flowers, such as the outstanding ‘Van Eseltine’ (pink doubles), ‘Kelsey’ (red doubles), ‘Diamond’ (red–purple doubles), and ‘Brandywine’ (pink doubles). During the past 20 years, only two double-flower crabapple cultivars, ‘Spring Bride’ (Spongberg, 1996) and ‘Jarmin’ (Jarmin, 2003), were released to the market in the United States. In China, there are only two double cultivars, ‘Fenghong Nichang’ (Fan et al., 2019) and ‘Fen Balei’ (Zhou et al., 2019), that have been released recently.
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