All commercially grown sweet cherries use a rootstock in combination with a grafted or budded scion variety. Rootstocks can directly influence productivity, precocity, tree size, tree architecture, fruit size, and fruit quality. The choice of certain rootstocks will also influence many horticultural decisions such as pruning, training, tree support, and labor management. Traditionally, both sweet and tart cherry growers have relied on vigorous rootstocks which develop large (15-20 feet) trees that take several years to reach full production potential. Recently, dwarfing or semi-dwarfing rootstocks (i.e., Gisela® 5 and 6, Edabriz, and Weiroot) have become available. These rootstocks allow growers to produce smaller trees at a higher density. In addition, most size-controlling rootstocks are precocious, meaning they begin flowering earlier in the life cycle of the tree.
Our research group has or is currently involved in several different research projects dealing with sweet cherry rootstocks. Of particular interest is new rootstock evaluation, characterization of rootstock influence on flower bud density, Armillaria root rot resistance, virus sensitivity of rootstocks, rootstock incompatibility with commercial scion varieties, and gene expression changes associated with rootstock induced dwarfing.
All original research data (including graphics, photos, and text) presented on these web pages are preliminary and are provided for the benefit of growers and producers. Duplication and/or use of this information is not permitted without authorization by Dr. Greg Lang at Michigan State University. Thank you for your cooperation.
Sweet and tart cherry rootstock evaluation at Michigan State University is currently done at the Northwest Michigan Horticultural Research Station in Traverse City, Michigan. Michigan State University is a cooperator in the NC 140 Regional Rootstock Research Project. The goal of this project is to evaluate and identify new rootstocks for cherry production. The current sweet cherry rootstock planting was made in 1998 with 19 different rootstocks using the scion variety Hedelfingen. A sour cherry rootstock trial, also planted in 1998, includes 12 different rootstocks using Montmorency as the scion variety. Rootstocks are evaluated for their influence on survival, tree growth, flower density, yield, fruit size, soluble solids, and suckering.
For more information about the NC 140 rootstock trials, please visit the NC 140 website.
Cherry rootstocks have been selected for a number of reasons, including precocity, productivity, vigor control, disease tolerance, and adaptability to different soils or climates. With some of these rootstocks, there is a potential problem of excessive cropping levels when grafted with very productive scion cultivars. Excessive cropping can result in poor fruit quality and stunted vegetative growth. One of the ways to study this, and perhaps eventually develop potential strategies to manage it, is to more precisely characterize how rootstock genotype influences floral architecture and placement on the scion; that is, how different rootstocks affect the patterns of floral node development along shoot growth, including quantification of spurs, buds, and flowers.
Figure 1. Karen Maguylo measuring rootstock influence on floral and vegetative growth in sweet cherry
Using the sweet and tart cherry NC 140 rootstock planting at Traverse City, Michigan, rootstock influence on floral and vegetative growth were measured on two-year-old shoots (Figure 1). Three relatively uniform branches on five trees for each rootstock were selected and divided into three equal parts based on length (Figure 2). For each of these equal parts, # of nodes, # of spurs, # of blind nodes, # of vegetative bud-only nodes, # of lateral shoots, # of buds per spur, # of flowers per bud, and # of fruit set were measured. Currently, only the sweet cherry data have been analyzed.
Figure 1. Typical two-year-old sweet cherry branch delineated by red bars
Analyses thus far show that rootstock genotype affects sweet cherry floral architecture and placement, including relative amount of spur and lateral branch development. Graphical representation of the percentage of bud type (blind node, vegetative, lateral branch or spur) for Hedelfingen on 18 different rootstocks can be seen in Figure 3. Basal, middle, and apical sections correspond to the representative branch in Figure 2. Tables 1, 2 and 3 show the rootstock induced differences in floral architecture (# buds per spur, # flowers per bud, total # flowers per branch section) and illustrate the differences between low (dwarfing), intermediate (semi-dwarfing), and high (standard size) rootstocks.
Figure 2. Percentage of nodes within each section (basal, middle, apical) of two-year-old sweet cherry shoots with: no buds (white), only vegetative buds (green), lateral branches (brown), or spurs (red). Number of nodes in each section is indicated in parentheses for comparison.
Table 1. The number of flowers per bud, buds per spur, and mean number of flowers for six low vigor (dwarfing) sweet cherry rootstocks.
Table 2. The number of flowers per bud, buds per spur, and mean number of flowers for seven intermediate vigor (semi-dwarfing) sweet cherry rootstocks
Table 3. The number of flowers per bud, buds per spur, and mean number of flowers for five high vigor (standard size) sweet cherry rootstocks
The characterization of flowering architecture and fruit placement on two-year-old shoots will provide a better understanding of potential crop placement within the canopy, and its proximity to different leaf populations. Managing both crop placement and leaf area populations, via pruning responses or flower or fruit thinning, should lead to a better balance of energy and resource partitioning within the tree canopy. Other practical applications of this information may be to better predict appropriate rootstocks for specific scion varieties and training systems.
Armillaria, a soilborne fungus that causes a root rot disease of cherry and other stone fruits, has become a serious and widespread problem in sweet and tart cherry production areas of Michigan. In the late 1980s, Armillaria root rot was identified as a problem in 35 orchards on sandy soil across Oceana, Mason, Manistee, Benzie, Leelanau, and Grand Traverse counties. Three of the eightArmillaria species present in North America were identified in these orchards.
Armillaria occurs most commonly in sandy soils where stone fruits have been grown for several generations. There is no known method of managing this disease. Armillaria persists in the soil even after an orchard is removed, resulting in loss of prime stone fruit sites. For these reasons, Michigan cherry industry leaders have concluded that Armillaria is a serious problem that threatens the survival of the industry.
In cooperation with Dr. Ray Hammerschmidt at Michigan State University, an Armillaria research trial was established at a site near the Northwest Michigan Horticultural Research Station. At this site, 21 different rootstocks for sweet and tart cherries were established in 2001 (Table 1). These rootstocks will be inoculated with Armillaria and screened for any potential resistance to the disease. As the project continues, new rootstocks will be added for testing, and other wild Prunus species will be evaluted for Armillaria resistance and potential rootstock use.
|CT 500||P. mahaleb|
|CT 2753||P. mahaleb|
|P-HL A||P. avium x P. cerasus|
|P-HL B||P. avium x P. cerasus|
|Colt||P. avium x P. pseudocerasus|
|Gisela® 196/4||P. canescens x P. avium|
|Weiroot 10||P. cerasus|
|Weiroot 13||P. cerasus|
|Weiroot 53||P. cerasus|
|Weiroot 72||P. cerasus|
|Weiroot 158||P. cerasus|
|Gisela® 5||P. cerasus x P. canescens|
|Gisela® 6||P. cerasus x P. canescens|
|LC-52||P. cerasus x (P. cerasus x P. maacki)|
|Damil (GM 61/1)||P. x dawykensis|
|VSL-2||P. fruticosa x P. serrulata|
|Maxma 14||P. mahaleb x P. avium|
|L-2||P. serrulata var. lannesiana|
In many sweet cherry production regions of the world, strains of prune dwarf virus (PDV) and Prunus necrotic ringspot virus (PNRSV) are commonly found in established orchards. In sweet cherry (Prunus avium) scions and standard rootstocks such as Mazzard and Mahaleb, infection with PDV and/or PNRSV is generally symptomless (tree longevity or mortality is not affected). Many nurseries propagate sweet cherry trees from certified virus-free budwood, and all foreign budwood legally imported into the U.S. is tested and if necessary heat therapy is used to produce virus-free budwood.
Many new rootstocks with potential for sweet cherry production are Prunus species other than P. avium, or hybrids between other Prunus species. Several of these new rootstocks have been found to be sensitive or hypersensitive to PDV and/or PNRSV. When tolerant scion varieties (Figure 1) are infected with PDV and/or PNRSV and grown on sensitive or hypersensitive rootstocks, obvious and sometimes rapid effects are observed. Changes in foliar color or leaf size and significant reduction in vegetative growth occurs when a rootstock is sensitive to these viruses (Figure 2). Gradual decline in productivity and tree health occurs, leading to eventual tree death. Hypersensitive rootstocks exhibit gum exudation from the graft union, premature foliar yellowing and defoliation, shoot death, and rapid tree death in one to two years (Figure 3).
Figure 1. Example of a rootstock tolerant to PDV and/or PNRSV.
Figure 3. Example of a rootstock hypersensitive to PDV and/or PNRSV.
All new rootstocks being evaluated for potential commercial use are screened for sensitivity to PDV and/or PNRSV. This work is done in collaboration with Bill Howell and the National Regional Support Project 5 at the Washington State University Irrigated Agriculture Research and Extension Center in Prosser, Washington.
Occasionally, a particular rootstock and scion combination will be incompatible. This the reason for this graft incompatibility is not well understood, but is not due to virus sensitivity. Chelan™, a new release from Washington State University, may be incompatible with Mahaleb rootstocks (Figure 1). In 2001, indications that Tieton™ may be incompatible with Mahaleb rootstocks also developed. Until further research is conclusive, Chelan™ and Tieton™ are not recommended to be propagated on Mahaleb rootstocks. Other recent releases and elite selections are currently being tested for compatibility with commonly used rootstocks by Dr. Matthew Whiting, at Washington State University's Irrigated Agriculture Research and Extension Center.
Figure 1. Chelan incompatibility on Mahaleb rootstock.
In collaboration with Drs. Kyung-Hwan Han and Amy Iezzoni, we are examining the rootstock-induced changes in gene expression from stem tissue above and below the graft union of cherry trees grown on dwarfing and standard rootstocks. DNA microarray technology is used in Dr. Han's lab for this work.
Please visit the Han Laboratory website for more information about this project.
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