Categories of Citrus Rootstocks and Their Effects on Tree Size, Yield, Fruit Quality, Adaptation and Disease Resistance

Historically, there have been several categories of common citrus rootstocks. Vigorous lemon-types are diploid hybrids that usually contain some citron or other acid-fruit in their heritage; these include Rough Lemon, Volkameriana (Volkamer lemon), Palestine Sweet Lime, Macrophylla and Rangpur lime. Rootstocks in this category are generally vigorous (producing very large trees) and high yielding, but produce poor quality fruit and are susceptible to citrus blight. Traditional Sour orange-types are diploid hybrids of pummelo with mandarin; these include sour orange, Kinjoki, Smooth Flat Seville and GouTou. Rootstocks in this group are widely adapted, moderately vigorous, and produce fruit of high quality that holds well on the tree. Some selections in this group are susceptible to sour orange quick decline caused by Tristeza virus, especially sour orange. Mandarin rootstocks include Cleopatra, SunChuSha, Sunki, Shekwasha and Amblycarpa.  This group produces vigorous trees, with some selections being slow to bear fruit. Selections in this group are often used for fresh fruit mandarin scions.  Trifoliate hybrids, especially citranges (sweet orange x trifoliate hybrids including Carrizo, Troyer and C-35), citrumelos (grapefruit x trifoliate hybrids including Swingle), and citrandarins (mandarin x trifoliate hybrids including x639) are now the most widely used rootstocks worldwide. These rootstocks capture the disease resistance of trifoliate orange (Poncirus trifoliata) and complementary attributes of the citrus parent. Due to emerging disease problems and efforts to improve production efficiency, the citrus industry worldwide is developing new advanced citrus production systems (ACPS) that feature high density plantings and earlier returns. The development of new tree-size controlling precocious rootstocks is essential to maximize the success of this approach. We have been exploring somatic hybridization of complementary diploid rootstocks via protoplast fusion to generate allotetraploid rootstock candidates, and more recently we have been conducting rootstock breeding at the tetraploid level by crossing superior somatic hybrids. We have coined the term ‘tetrazyg’ to describe zygotic allotetraploid rootstock candidates that are progeny from crosses of somatic hybrids. Several allotetraploid hybrid rootstocks are showing promise to facilitate ACPS. New breeding efforts, both at the diploid and tetraploid levels, have focused on combining parents from the various groups above with focus on resistance/tolerance to both biotic and abiotic stresses that are threatening citriculture around the world. Progress and strategies will be discussed, along with attributes of promising new hybrids.