By Gregory Lang.
Edited, adapted and commented by Carlos J. Tapia T. Technical director of Avium and content director of SmartCherry 2019.
ABSTRACT. Cherries (Prunus avium L.) can be one of the most profitable fruit species grown in temperate climates. While cherry trees naturally grow at relatively high altitudes (≥ 10 m asl), new cherry rootstocks with size control similar to those used in high density are now a reality. Gisela (GI.) and Weiroot (W.) series from Germany, the Grand Manier (GM.) series from Belgium, the P-HL series from the Czech Republic, “Tabel Edabriz” from France, and others of international origin are in various stages of scientific and field testing in various parts of the world, with some of these series already being used for commercial production. These stocks confer several advantageous traits in addition to vigor control, including early fruiting and high productivity. While these traits are sometimes beneficial, serious problems, such as low fruit size, have also been documented. Since many of these rootstocks are interspecific hybrids of Prunus L., could there be significant limitations to fruit quality and orchard longevity? What is known about its tolerance to various soil types and/or climatic stresses? What is known about its susceptibility to pathogens and pests? Furthermore, globally the area planted to cherries is already expanding to record levels and there is a traditional agricultural tendency towards overproduction until the farmer benefits are minimised, but what might be the future impact of early and productive rootstocks on cherry profitability and sustainable production? This summary addresses these issues, providing some answers and some areas for future scientific research and industrial discussion.
Cherries are among the temperate fruit tree species most appreciated by consumers, a fact reflected in the fact that they have one of the highest economic yields per hectare on a seasonal basis. They have an intense flavour and, typical of temperate species, are very fruity. Prunus, can be stored for only a couple of weeks, thus intensifying the apparent appeal (and value) to consumers due to its short-lived availability during the summer of each hemisphere. However, cherries are not easy to produce, being subject to numerous serious diseases and pests (Blodgett, 1976) and susceptible to numerous vagaries of the weather (severe winter cold, spring frosts, rain during ripening, summer heat).
Furthermore, profitable orchard management can be challenged by inefficiencies associated with large trees, a long establishment period before first fruiting, and relatively small, delicate fruit that must be hand-harvested for fresh markets. Likewise, the potential production efficiencies conferred by early rootstocks have long eluded cherry growers (Toyama et al., 1964; Webster, 1996).
Beginning in the late 1980s, several series of promising low-vigor cherry rootstocks (Table 1), developed largely from European breeding and selection programs, were widely tested in North America (Perry et al., 1996) and Europe (Kemp and Wertheim, 1996). Some of these have shown great potential for promoting early fruiting and high productivity, as well as providing a range of tree vigor levels to best match cherries to different training systems and soil characteristics.
However, many questions remain before commercial adoption of such rootstocks in high-density cherry orchards becomes widespread, including, among other things, whether large fruit size can be achieved despite increased crop loads and a widespread concern about how to facilitate overproduction and not depress orchard economics and how cherries would be easier to manage. With the selection of this first wave of rootstock improvements occurring only recently, more advanced studies on their adaptability, susceptibility and management dynamics are still relatively early.
Tree vigor.
Genetic control of vigor is the driving force in the development and selection of new cherry rootstocks. Prunus avium It is a forest tree in its native environment, therefore it is a challenge to maintain it in an orchard. The labor of pruning and managing the vigor of the shoots and harvesting small fruits has a higher production cost making it even more inefficient due to the time spent climbing and moving ladders.
Smaller trees have the potential to at least double labor efficiency, as well as facilitate other potential orchard efficiencies. For example, chemical protective spray volumes can be reduced and mulch improved as tree size decreases, benefiting both the orchard and the surrounding environment.
Orchards with cover systems for small trees can be developed at significantly lower costs to minimize potential damage from rain, birds, or hail. With a proper understanding of the relationships between vegetative and reproductive growth, small trees are better suited to facilitate light distribution throughout the canopy and optimal balance of crop loads to leaf area. Conversely, smaller trees also present some new challenges for growers. With a less permanent structure and less inherent vigor, balance, leaf area, and storage reserves with fruiting capacity become more critical to achieving high-quality fruit. As the proportion of fruit that can be picked from the ground increases, so does the vulnerability of the crop to spring frost damage.
With high density orchards having open “streets” between rows of trees rather than a closed canopy over the tractor “street,” there is less light interception per acreage and possibly resulting in lower yields.
The results of the 10-year NC-140 project in the USA (Perry et al., 1996) and other trials around the world have revealed that a wide range of trees influenced by different rootstocks, vigor is possible, from very dwarf to very vigorous (Table 2).
In these initial trials, the dwarf rootstocks, 'Inmil' (tested as 'GM.9') from Belgium and 'Gisela 1' ('GI.1', tested as 'Giessen 172/9 [Gi.172/9]') from Germany, have not been satisfactory, for reasons that will become apparent later in this review.
However, few rootstocks were classified in vigor ranges from “dwarfing” to “semi-dwarfing”, most notably “GI.5” (tested as “Gi.148/2”) and “GI.12” (tested as “Gi.195/2”).
In France, ‘MaxMa 14/Brokforest’ (virus-free clone of ‘MxM.14’) has emerged as an important ‘semi-dwarf’ rootstock; widespread comparative trials of ‘MaxMa 14’ have not yet occurred in North America or Europe, some early North American trials of ‘MxM.14’ have been reported (Perry, 1987).
While some of these new rootstocks are equal to or superior to Mazzard in vigor, and further evaluation has revealed some to be significantly earlier (e.g., “GI.6,” tested as ‘Gi.148/1’), more productive (e.g., “GI.6,” “MxM.2”), or more adaptable to specific conditions (e.g., “Colt” at replant sites, [Webster, 1996]) they might be of specialized interest in more traditional orchard systems.
Having a range of vigor levels available to growers will likely be important to match higher density orchard targets with different soils, varietal growth types and/or habits. For example, high density orchards of vigorous growing varieties on fertile soils would be good candidates for a dwarf graft such as ‘GI.5’, while moderate density orchards with similar conditions may be better planted to a “semi-dwarf” rootstock such as ‘GI.12’.
On poorer soils, high-density orchards could be planted with a somewhat more vigorous rootstock such as ‘GI.12’ and moderate-density orchards with a less vigorous one such as ‘GI.6’. It should be noted that in the NC140 trial reported by Perry et al. (1996) ‘GI.6’ produced full-sized ‘Bing’ trees on fertile, irrigated soils in Washington, Oregon and British Columbia, but produced very dwarf ‘Hedelfingen’ trees on poorer soils in Michigan and New York.
Therefore, in addition to soil type and management factors, varietal differences can also have a significant impact on orchard rootstock decisions.
Virus sensitivity detection
While rootstocks are screened for susceptibilities or tolerances to several important diseases (and will be discussed later), it has recently been recognized that
One of the first rootstock tests, tests should be for reaction to hilarvirus, like the prune dwarf virus (PDV) and prunus necrotic ring spot virus (PNRSV).
These viruses are prevalent in most cherry growing regions of the world, can be transmitted through infected pollen, and in fact are often found in cherry orchards that do not cause negative symptoms in trees grown on Mazzard rootstocks (P. avium) either P. mahaleb L. (better known as Mahaleb).
However, some genotypes of P. cerasus L. (sour cherry), P. canescens (grey-leaf cherry) and P. fruticosa, are known to exhibit varying levels of sensitivity to these viruses, and thus make up some of the novel rootstocks that have been selected or hybridized from these species. Lang et al. (1997, 1998) have shown that the virus can pass from the point of infection (young flowering shoots) to the graft union within 10 weeks, whereupon a hypersensitive rootstock may begin to exude gum, followed by yellowing and premature leaf abscission. During the second growing season following infection, hypersensitive trees collapse and die. Sensitive trees, which may only reveal a bronze leaf color during the initial season of infection, subsequently put out small pale green leaves and minimal growth, eventually leading to tree collapse and death after several growing seasons. The rootstocks that have been selected for PDV and PNRSV sensitivity so far are listed in Table 3. It is likely that this virus sensitivity may explain several of the cases of tree loss in European trials (Wertheim et al., 1998) that were attributed to delayed graft incompatibility.
Since there are no protective measures for these viruses once they infect a tree, in the American NC-140 project, scientists have concluded that virus tolerance should be a primary screening criteria for new cherry rootstocks.
Effects on precocity, productivity and quality of the fruit
Cherry trees on Mazzard or Mahaleb rootstocks often do not flower significantly until the 6th or 7th leaf. Some of the new hybrid rootstocks begin flowering on the 3rd leaf (2nd year in the orchard), with potential economic cropping on the 4th to 5th leaf.
Such earliness is a tremendous economic advantage, helping to recover orchard establishment costs much sooner and thus advance the financial break-even point in the life of the orchard by several years. Gisela rootstocks were the precocious, with the earliest flowering ranges from 3rd to 5th leaf (Perry et al., 1996), followed by Mahaleb and MxM, Mazzard and ‘Colt’ rootstocks. While the most vigorous rootstocks were generally the least precocious. ‘GI.6’ exhibited strong vigor in the US Pacific Northwest being earlier and dwarfer than ‘GI.5’ and ‘GI.7’ (tested as ‘Gi.148/8’). For large-sized ‘Bing’ trees in ‘GI.6’, cumulative yields were more than double those of trees in Mazzard, and even trees in semi-dwarf ‘GI.12’ and dwarf ‘GI.5’ had yields of around 25% up to 30% higher, per tree. Planting such rootstocks at higher densities to take better advantage of their reduced size will increase early yields per hectare even further. In addition to early flower bud formation, the early Gisela rootstocks also promote a higher number of flowering centres (darts). This results in the potential for continued high productivity, compared to Mazzard, even after the impact of early flowering.
The productivity column in Table 5 reveals the highest possible yield during years 7 to 10 of the orchard on the three Gisela rootstocks currently recommended for commercial trials. Yields were similar, on a base tree, among the dwarf trees in “GI.5”
and the more vigorous trees like Mazzard, even in commercial orchards the dwarfs would be planted at up to twice the density of the vigorous trees. This bodes well for maintaining good production levels even as some reduction in total light interception per hectare due to the more open alleys in high density orchards.
Regardless of the variation in vigor of the Gisela rootstock, it maintains yields approximately 25% and 50% higher than Mazzard, respectively. These productivity traits can be used to particular advantage in promoting varieties with lower setting potential. Regarding the influence of the rootstock on fruit quality, a critically important issue in global cherry production is how to increase market competition: in the NC-140 project trials, the largest-sized fruit were generally obtained from the most vigorous trees.
Balancing rootstock loads with respect to their leaf ratio (leaf/fruit ratio) is a critical factor in fruit size variation, so potential genetic and physiological effects of rootstocks on fruit size can only be truly examined by tightly regulated comparative experiments.
Fruit from trees on Gisela rootstocks were generally smaller than fruit at Mazzard in high crop load years, but of similar size (except ‘GI.1’) to fruit at Mazzard when crop load was moderated by spring frosts. Therefore, the hypothesis that good fruit size can be achieved, even on these highly productive rootstocks, through intensive orchard management remains valid and in need of testing. A preliminary study (Lang and Ophardt, 2000) that thinned flower buds just before flowering to alter crop loads on unpruned trees of a very productive variety, ‘Rainier’, on a very productive dwarf rootstock, ‘GI.7’, resulted in very significant differences in fruit size, but at the same time respectable yields (Table 6). Compared to the control crop load, which was similar to the low management in the NC-140 trials, altering the crop by leaving 1 or 2 flower buds per scion reduced total yield by up to 25%, but increased fruit size by up to 43%. Fruit were of higher quality and better flavor as well, with significantly higher soluble solids and up to 87% of the crop was “Cat 1” for all markets, compared to only half of the crop of the thinned control. On a per hectare basis, minimally managed trees would have yielded about 3,500 kilos of marketable fruit, while trees managed by crop load would have produced about 5,000 kilos of marketable fruit, a significant achievement at fifth leaf.
Thus, the challenging aspect for those new rootstocks that are both precocious and highly productive is that over-cropping is a strong possibility from the fifth leaf, earlier than a typical tree in Mazzard. However, in the case of those rootstocks from this first wave that have been recommended for the breeding trial ('GI.5', 'GI.6' and 'GI.12'), this does not appear to be a genetic limitation of the rootstock, but rather a challenge to develop new ways of managing cherry orchards now that precocity and over-vigour are less of a problem. Combining varieties, training models and management systems is likely to become more important, as is the appropriate number of rootstocks and their diversity of unique traits offering growers a greater set of orchard tools to choose from.
Soil and climate adaptations
In general, vigorous rootstocks such as Mazzard and Mahaleb are rootless and tolerate drought conditions better than clonally propagated rootstocks which tend to have shallower roots. This may be the reason that ‘Colt’ is even considered ‘semi-dwarf’ under non-irrigated conditions (dry soils), which can occur in European orchards, whereas on irrigated and fertile soils it can be at least as vigorous as Mazzard. ‘Colt’ also appears to be less vigorous on clay soils (R. Perry, personal communication). However, some of the MxM series, which are clonally propagated, develop extensive root systems (such as MxM.2 and MxM.60 [Longstroth and Perry, 1996]) and have been found to be drought tolerant (Wertheim, 1998). Rootstocks derived from P. cerasus They tend to be shallow-rooted and drought-sensitive, such as 'Tabel Edabriz' (Webster, 1996. Experience at Washington State University with unnoticed irrigation problems in a test block suggests that 'GI.1', 'GI.5', and 'GI.7' are quite sensitive to drought stress. The Russian rootstocks 'L-2', 'LC-52', 'VC13', and 'VSL-2' were selected under non-irrigated conditions and are presumed to be drought tolerant, although they have been found to perform poorly on rocky soils (G. Eremin, personal communication).
As for some of the other species that have been used to create new cherry rootstocks through selection or hybridization, P. canescens and P. cerasus tend to have shallow roots and are sensitive to anaerobic conditions, although some rooting has been found (Perry, personal communication). Although the P. cerasus Based on the Weiroot series, they are recommended for well-drained soils not subject to flooding (Wertheim, 1998), P. cerasus in
It has generally been reported to be quite tolerant of heavy soils (Perry, 1987). P. fruticosa It has shallow roots and is somewhat tolerant of anoxia. Regarding specific rootstocks, ‘Colt’ and ‘Damil’ are somewhat tolerant of anoxia, as is ‘GI.4’ (tested as ‘Gi.473/10’), ‘GI.6’ and ‘Gi.169/15’ (ASHS, 1997; Franken-Bembenek, 1996; Webster, 1996). ‘Gi.196/4’ does not tolerate anoxia well.
While there has so far been very little research on rootstock interactions with different soil chemicals. The Belgian rootstocks, 'Inmil' and 'Damil', as well as 'Colt' and 'Tabel Edabriz', are sensitive to high pH calcareous soils, while rootstocks based on P. mahaleb are well adapted to such soils (ASHS, 1997; Callesen, 1998; Webster, 1996). Callesen (1998) summarised several reports that ‘Colt’ and ‘Damil’ take up little nitrogen and potassium, one of which (Ystaas and Froynes, 1998) also showed that trees on ‘GI.1’ had low N and trees on ‘Colt’ also had lower levels of P and higher levels of Ca and Mg in the leaves. Some of these reports are contradictory, requiring further study before they can give useful conclusions. Of particular interest, in the matter of soil relations, is the tolerance of ‘Colt’ for the so-called “replant disease” based on allelopathies common in woody orchards (Webster, 1996), which typically causes decline and possibly death of young trees on Mazzard or Mahaleb that have been planted on old cherry soils.
Cold resistance tests carried out by Strauch and Gruppe (1985) revealed good resistance of P. avium in selections of mountainous regions, P. cerasus x P. subhirtella Miq., P. mahaleb ['St. Lucie 64' ('SL.64')], 'GI.6', 'GI.8' (tested as 'Gi.148/9'), 'GI.12', and 'Gi.196/4'. Cummins et al. (1986) found good early winter hardiness with 'GI.6' and 'GI.10', but mixed results with 'GI.11' (tested as 'Gi.195/1') and 'GI.12'. Strauch and Gruppe (1985) rated 'GI.5' as similar in hardiness to the Mazzard clone 'F.12/1', although Cummins et al. (1986) found 'GI.5' to be very cold hardy to early winter frosts. Lang et al. (1997) reported that Bing flower blights on ‘GI.5’ were equally resistant to those on Mazzard, but that acclimatization occurred more rapidly on ‘GI.5’ during late winter. A feature also evident in the data of Strauch and Gruppe (1985). The least resistant is ‘Colt’, to early winter and midwinter frosts (Strauch and Gruppe, 1985; Cummins et al. 1986; Perry et al., 1996). This would suggest caution in planting ‘P-50’ (Table 4), which appears to be derived from ‘GI.5’. P. pseudocerasus Lindl, in climates with severe winter chill potential until such time as this can be assessed accordingly. In addition to losing several trees at ‘Colt’ to a severe freeze in early winter (Perry et al., 1996).
Other disease sensitivities.
Less research has been done on the disease susceptibility of these rootstocks, with the exception of bacterial canker (Pseudomonas syringae pv. Syringae van Hall) (Krzesinska and Azarenko, 1992), Phytophthora and Armillaria (Cummins et al., 1986; Proffer et al., 1988), and susceptibility to hilarvirus described above (Lang et al., 1997, 1998). While the severity and longevity of bacterial canker infections on wood vary with climate and can sometimes be managed, selection of rootstocks that are less susceptible are a high priority in certain areas of greater sensitivity. Cherry rootstocks that have been previously reported (ASHS, 1997; Webster, 1996; Wertheim, 1998) to be somewhat tolerant or less susceptible to bacterial canker include vigorous rootstocks ‘F.12/1’, ‘Colt’, and the MxM series. Krzesinska and Azarenko (1992) found 'GI.10' (tested as 'Gi.173/9') and 'Gi.169/15' to be more sensitive to bacterial canker than 'F.12/1'; 'GI.5' and 'GI.6' were similar in susceptibility to 'F.12/1'.
Prevent root rot caused by infection with Phytophthora spp. is important in many cherry growing areas of the world (Mink and Jones, 1996).
Some resistance to root rots Phytophthora has been reported for the MxM series, “Damil”, “GI.10” and “Gi.169/15” (Cummins et al., 1986). The “Mahaleb” rootstock series is known to be sensitive to several Phytophthora sp., and evidence has suggested that tracking is also sensitive: 'GI.1', 'GI.6', 'GI.11', 'GI.12', 'Gi.196/4'.
Conclusions
The first wave (Table 1) of new cherry rootstocks for testing in North America has yielded at least 3 highly productive, early-bearing genotypes (‘GI.5’, ‘GI.6’, and ‘GI.12’) with varying levels of vigor worthy of cultivar trials in more intensive orchard management strategies. The extraordinary productivity of some very vigorous rootstocks, such as ‘MxM.2’, may also be worthy of trial by growers interested in more traditional orchard systems at the earliness of the next wave (Table 2). Virus sensitivity of the second and third waves (Table 7) will also become known over the next few years.
Cherry production is bound to undergo significant change in the new millennium, with a diversity of new rootstock traits altering the foundations of orchard management that changed little during the 20th century. Induction of early maturing cultivars is an extremely strong economic incentive that will become common, either through rootstocks or new cultural manipulation of standard rootstocks, to remain competitive as orchards capitalize on sustainably increasing costs. Likewise, smaller tree stature and higher density orchards will become common as experience with highly productive crop load management and vigor-controlling rootstocks increase. Rather than focusing management decisions on minimal early pruning to speed crop growth and then late pruning to manage excessive vigor, as is current practice, high-quality cherry orchards will likely be pruned and fertilized more aggressively throughout their life to generate new leaf area and balance their potential, resulting in a more labor-efficient orchard that may also be better protected from some of the many risks inherent in cherry production.
Dwarf and early rootstocks have revolutionised apple production worldwide, with production levels increasing in the late 1990s with sustainable profits. Some traditional cherry growers have expressed concern that rootstocks and management that confer greater efficiency and ease of production in orchards may now lead to a profitable but long-term sustainable industry along similar paths.
Certainly, any orchard innovation that promotes more sustainable production in an environment typically subject to numerous serious climatic and pathological challenges will make the production of that product more attractive to new producers and/or expansion of existing producers.
In the logical hierarchy of sustainable production challenges, working in efficient high-density orchards (based on a combination of genetic improvement of rootstocks and more intensive management) will clearly be a factor in maintaining the viability of the economic potential of cherry production. Grower-packer-marketer communication and coordination to anticipate market demand and possible saturation will clearly be related, but separate factors.
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