Avocado (Persea americana Miller) is a significant and nutritious fruit crop grown in both the tropic and subtropical regions in many parts of the world. World production of avocados in 2008 was estimated at ≈3.2 million tons with the world leader being Mexico followed by Indonesia, Columbia, Brazil, the United States, Chile, Peru, China, and South Africa (Imbert, 2008). Within the United States, California dominates avocado production (≈90%) and in the 2010–2011 season, the crop was valued at $460,209,682 (California Avocado Commission annual report, <http://www.avocado.org>). Three botanical races of P. americana have been domesticated from their putative centers of origin: the Mexican race (P. americana var. drymifolia), the Guatemalan race (P. americana var. guatemalensis), and the West Indian race (P. americana var. americana) (Bergh and Ellstrand, 1986; Popenoe, 1941). Each race possesses distinct agronomic characteristics such as the shape, taste, and color of fruit; timing and length of fruit set; cold-hardiness; disease resistance; and salinity tolerance (Bergh and Lahav, 1996) However, cross-fertility among and within the botanical races has led to extensive genetic variability within P. americana and many extant cultivars are racial hybrids, thus possessing variable characteristics (Ashworth and Clegg, 2003; Davis et al., 1998; Douhan et al., 2011). This extensive genetic variability is important because it potentially provides ample germplasm for breeders when developing rootstocks or scions with desirable characteristics (Bender and Whiley, 2001).
Phytophthora cinnamomi Rands. is the most serious disease of avocado worldwide (Zentmyer, 1980) and has actually eliminated commercial production in many areas in Latin America and is the major limiting factor of production in Australia, South Africa, and California (Ploetz et al., 2002). In California, 60% to 75% of California groves are affected by the disease with estimated yearly losses of over $40 million (Coffey, 1992). The use of tolerant rootstocks to control Phytophthora root rot (PRR) of avocado has long been proposed as the ultimate method for managing the disease (Zentmyer, 1957, 1963) and research on developing PRR-tolerant rootstocks has been a major focus of avocado research at the University of California Riverside (UCR) since the 1950s. The UCR program has developed or been actively involved in the development of most of the clonal rootstocks that are now used in the global avocado industry.
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