Cherry Scion & Rooststock Selection
It is important to select a rootstock that matches the local soil and pest conditions in an orchard. Rootstocks are bred to tolerate a wide range of soil conditions and pests. Although the scion (grafted onto a rootstock) produces fruit, the rootstock is an important determinant of cropping and fruit quality. Each of the common sweet cherry rootstocks used in California has advantages and disadvantages that should be carefully weighed in the context of the specific location of an orchard.
Colt (P. avium x P. pseudocerasus) Colt is tolerant of periodic high water tables (Ingels and Arceo 2012), and is less susceptible to Phytophthora cherry stem pitting, and gophers than most other cherry rootstocks. However, in some instances it confers excessive vegetative vigor and lower cropping than Mahaleb and other semi-dwarfing rootstocks. Reduced cropping is most problematic in early years of orchard development.
Mazzard (Prunus avium) is compatible with many scion cultivars and produces good fruit quality with moderate inputs. Mazzard is also tolerant of a wide range of soil types and colder winter temperatures (Inels and Arceo 2012). However, Mazzard produces tall, vigorous trees that are difficult to access during pruning and harvest. Because of its vigor, trees on Mazzard take slightly longer to reach their full productivity than trees on Mahaleb and other semi-dwarfing rootstocks.
Mahaleb (P. mahaleb) produces a tree 75 to 80% the size of Mazzard and Colt rooted trees (Brown et al., 1989). As a result, cherry trees on on Mahaleb tend to bear more heavily than those on Mazzard and Colt, especially during the early orchard development years. Mahaleb is tolerant of drought and high levels of soil calcium carbonate (lime). However, Mahaleb is very susceptible to gopher damage (Long and Kaiser 2010) and is more susceptible to Phytophthora than Mazzard or Colt (UC IPM 2013).
Maxma 14 (P. mahaleb x P.avium) is precocious and semi-dwarfing, with high yields. Maxma 14 results in trees that are only ¾ the size of a tree grafted to Mahaleb, and bear fruit 4 to 5 years earlier (Ingels and Arceo 2012; Muna et al., 2000). Maxma 14 is also tolerant of a range of soil types and environmental conditions. However, Maxma 14 does not perform well in high density plantings and requires proper management to ensure productivity (Long and Kaiser 2010).
Semi-dwarfing rootstocks, including Mahaleb and Maxma 14, and dwarfing rootstocks, such as some Gisela and Krymsk clones, are of interest to growers because they reduce tree height, make fruit more accessible during harvest and enable higher density plantings. Semi-dwarfing rootstocks bear fruit earlier than traditional rootstocks, beginning in year three. Although advancements have been made in breeding semi-dwarfing rootstocks, traditionally sized rootstocks are still the most common in California commercial orchards.
Bing is the most common sweet cherry cultivar grown throughout the U.S. Bing cherries have a desirable taste, large size, and remain firm postharvest (particularly when treated during fruit development with gibberellic acid), with low vulnerability to bruising (Brown et al., 1989; Crisosto et al., 2003). However, Bing is susceptible to both crinkle-leaf and deep suture disorders (Southwick and Uyemoto 1999).
Brooks is another red cultivar grown in California that was introduced in the 1980s. Brooks matures earlier than Bing and has less spur and double fruit production (Van Deynze 2000).
Rainier is a popular light colored (blushed) cultivar that is productive and hardy, producing large and durable fruit. However, Rainier fruit are highly susceptible to bruising (on and off the tree), scuffing (on the tree) and pre-harvest rain cracking (Brown et al., 1989).
Montmorency is the most common sour cherry cultivar. Montmorency cherries are high yielding, but flower buds are susceptible to late spring frosts (Brown et al., 1989).
There are public cherry breeding programs located throughout the United States. The University of California at Davis (Van Deynze 2000) had a breeding program during the 1950’s, and Washington State University (Lang et al., 1998) currently has a very active sweet cherry breeding program; Michigan State University has a prominent sour cherry breeding program (Iezzoni and Karle 1998). Common goals in sweet cherry breeding programs include higher quality fruit, fruit resistant to cracking and disease (Iezzoni 2008), fruit with longer ripening periods, and self-compatibility (Long et al., 1998). In California sweet cherry breeding has also been aimed at developing earlier season varieties that require less winter chilling. Sour cherry breeding programs have focused on developing new cultivars with the same productivity as Montmorency and improved fruit quality, suitable for mechanical harvesting (Iezzoni 2008).
Self-compatible cherry cultivars were introduced in 1954 in the United States (self-compatible pollination, Iezzoni 2008). All sour cherries in commercial production are self-compatible. However, the main sweet cherry commercial cultivars, including Bing, are self-incompatible and need to be planted near one or more compatible pollinizer cultivars with supplemental bees to ensure pollination and fruit set (all cherry pollen is carried by bees as the pollen grains are too large for wind pollination). Pollenizers must be genetically compatible - and have a bloom period that overlaps well - with the main variety. It is recommended to plant two to three cultivars (usually one main variety and two pollenizers) in the same orchard to ensure good pollination (Brown et al., 1989). Local climate and temperature determine the effective pollination period (the time in which the stigma is receptive to pollination, pollen tubes can grow in the style, and the ovule remains viable). Higher temperatures yield shorter periods and lower temperatures yield longer periods (Polito 2003).