10/18/2019
Citrus Rootstocks
Ute Albrecht [email protected]
Southwest Florida Research and Education Center UF/IFAS, Immokalee, FL
PLP 5115C – Citrus Pathology Fall 2019
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Outline
I. Grafting II. Rootstock propagation III. Importance of rootstocks IV. History of rootstocks V. Rootstocks and HLB
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Definitions
. “Stionic” interactions = interactions between the stock and the scion. Scion . “Stion” = any plant or tree composed of a Graft union stock and a scion growing in combination. (Webber, 1932) Rootstock
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I. Grafting
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Grafting The process by which part of one plant (the scion) is inserted into another (the rootstock or stock) so that they unite and form a single plant. Budding A type of grafting with the scion consisting of a single bud.
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Citrus nursery program . Propagation material is obtained from the Division of Plant Industry (DPI), Bureau of Citrus Budwood Registration. . The Citrus Budwood Registration Program was started in 1953 to prevent diseases and ensure a healthy commercial citrus industry. . New material is cleaned up and tested if true-to- type before being released to growers.
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https://www.freshfromflorida.com/Divisions-Offices/Plant-Industry/Agriculture- Industry/Citrus-Health-Response-Program/Citrus-Budwood-Program
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Budwood trees
M. Zekri
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Budwood
(Webber 1946)
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Inverted T budding method
(Reuter 1973)
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Inverted T budding method
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Inverted T budding method
Inserting and wrapping the bud
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Bending of rootstock liners to push bud growth
M. Zekri
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Bud growth Finished trees
M. Zekri
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Graft union
Source: Webber (1948) The smoothness of the bud union was (is) thought to be indicative of the compatibility between rootstock and scion.
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Graft union
A B C
D EF G
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II. ROOTSTOCK PROPAGATION
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Rootstock propagation is traditionally by seed.
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Nucellar embryony Nucellar embryos. Embryos derived from nucellar tissue. Apomixis. Development of an embryo without fusion of female and male gametes. Polyembryony. Formation of more than one embryo in a seed.
. Vegetative (≠ zygotic) embryos. . Clones of the mother tree. . Start to develop earlier than zygotic embryos. . Different rootstocks have different degrees of nucellar embryony. YourArticleLibrary.com
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Implications of nucellar embryony
. True-to-type propagation of rootstocks. . Elimination of viruses. . Difficulty of breeding citrus as only few seedlings will be genetically variable.
(Koltunow et al 1993)
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Rootstock fruits and seeds Sour orange Trifoliate orange Carrizo citrange
Swingle citrumelo Cleopatra mandarin Valencia orange
http://www.citrusvariety.ucr.edu/citrus
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Alternative propagation methods
. Clonal propagation is also possible through use of cuttings or tissue culture (vegetative propagation).
. Both methods will yield genetically uniform plants.
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Alternative propagation methods
. Clonal propagation is also possible through use of cuttings or tissue culture (vegetative propagation).
. Both methods will yield genetically uniform plants.
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Seed source trees
Exposure to disease
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Seed source trees
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Seed source trees Diseases: HLB, citrus scab, citrus canker, etc.
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III. IMPORTANCE OF ROOTSTOCKS
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Why use rootstocks?
Problems with seedling trees . Long juvenility . Thorniness . Soil-related
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Diseases
. Phytophthora . Diaprepes . Viruses . Blight . Nematodes
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Phytophthora (P. nicotianae, P. palmivora)
Damping-off of seedlings, foot rot, crown rot, fibrous root rot.
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Diaprepes root weevil (Diaprepes abbreviatus)
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Phytophthora Diaprepes (PD) complex
Root injury due to feeding of Diaprepes larvae predisposes citrus roots to Phytopthora infection.
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Abiotic factors
. Cold/Frost . Drought . Flooding . Salinity . pH
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Horticultural traits
. Tree size . Yield . Fruit quality
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Rootstock effect on tree size
Low vigor
Three-year-old Valencia trees
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Rootstock effect on tree size High vigor
Three year-old Valencia trees
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Requirements of a good rootstock . Reduction of juvenility. . Induce high fruit yield. . Enhance fruit quality of the scion (size, shape, brix, acids). . Adaptation to different soil types. . Resistance to abiotic stresses (salinity, drought, flooding, pH). . Resistance to pests and diseases.
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Rootstock testing (short-term)
Greenhouse screening for disease tolerance.
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Rootstock testing (long-term)
Field trials in commercial settings to assess tree growth and size, compatibility, time to bearing, etc.
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Rootstock testing (long-term)
Field trials to assessing fruit quality and yield.
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Other requirements . Compatibility with the scion. . Combination must be long-lived. . Ease of propagation in the nursery. . Should produce many fruits and seeds. . High degree of nucellar polyembryony.
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IV. HISTORY OF ROOTSTOCKS
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Origin of grafting . Origin of grafting is uncertain. . Budding is said to have been practiced in China before A.D. . Budding or grafting of citrus was already common in the Mediterranean area in the 5th century and standard practice in the 16th and 17th centuries (orangeries).
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Origin of grafting
. In Spain and Italy, mostly seedling trees were planted in the 18th century. . Advent of Phytophthora foot rot (gummosis) forced European growers to bud on resistant rootstocks. . The universally favored rootstock was sour orange.
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Florida . Florida’s citrus industry grew out of 16th Century Spanish introductions of plants and seeds of sour orange, lemon, lime, and citron. . Many wild “groves” became established. . Citrus was spread further by Indians and pioneers who settled in north Florida and on the east coast. . There was limited commercial cultivation. . Once better transportation was available, interest in improving horticultural practices began.
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Florida . Budded trees came into use around 1830, when wild groves of sour orange were topworked with sweet orange (Citrus sinensis). . Goal was to reduce losses from foot rot, but also to modify tree growth and improve production. . Sour orange was the favored rootstock. . Large-scale nursery propagations of budded trees started in the second half of the 19th century. . In California, foot rot was not much of a problem and the use of budded trees instead of seedling trees was debated longer.
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Sour orange Citrus aurantium . Good cropping and good fruit quality. . Cold hardy. . Tolerant to Phytophthora foot rot. . Tolerant of blight. . Susceptible to citrus nematode and burrowing nematode. . Adaptable to a wide range of different soil conditions (highly tolerant of calcareous soils). . But…Highly susceptible to the citrus tristeza virus (CTV) in combination with most scions.
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New problems . CTV → stem pi ng and quick decline of sour orange in combination with most scions except lemons.
→ Rough lemon Brown citrus aphid Volkamer lemon Cleopatra mandarin Sweet orange . Rough lemon dominant until the 1970s. . Rangpur lime dominant in Brazil (drought tolerance).
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Cleopatra mandarin Citrus reshni/reticulata
. Produces large trees. . Small fruit of good quality. . Can be slow to come into production. . Tolerant to CTV. . Cold tolerant. . Salt tolerant. . Adaptable to a wide range of soils. . Suitable for tangerines, tangelos, oranges, and grapefruit.
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Rough lemon Citrus jambhiri . Produces large trees. . High yield and large low-quality fruit. . Suitable for orange and grapefruit. . Poor cold hardiness. . Requires less irrigation. . Susceptible to wet soils. Best on deep sandy soils. . Susceptible to foot rot. . Tolerant to CTV. . But…Highly susceptible to blight (young tree decline).
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More problems
. Blight (young tree decline) first described in 1894. Prevalence increased in the 1970s with the decline of sour orange and use of rough lemon. → Trifoliate orange rootstocks Carrizo citrange Troyer citrange Swingle citrumelo
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Trifoliate orange Poncirus trifoliata . Produces small trees. . High yield and high-quality fruit. . Used mostly for mandarins, oranges, and kumquats. . Good cold hardiness. . Susceptible to highly calcareous soils. . Sensitive to high salt. . Tolerant to foot rot. . Tolerant to CTV. . Highly susceptible to blight.
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Carrizo citrange C. sinensis x P. trifoliata . Produces standard size trees. . High yield and large good-quality fruit. . Moderately cold tolerant. . Tolerant to foot rot. . Low salt tolerance. . Resistant to burrowing nematode. . Adapted to a wide range of soils, except highly calcareous soils. . Tolerant to CTV. . But … Susceptible to blight
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Swingle citrumelo C. paradisi x P. trifoliata
. Produced in 1907, but not released until 1974. . Cold tolerant. . Trees vary in vigor depending on the scion. Valencia trees are average. Grapefruit trees are larger. . Not suitable for heavy clay (flatwoods) or highly calcareous soils. . Moderately salt and drought tolerant. . Resistant to the citrus nematode. . Tolerant to CTV and blight. Walter T. Swingle
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1953-1974 Sour orange
1974-1988 Rough lemon
Carrizo 1988-2015
Swingle
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Modern rootstocks UF/IFAS breeding program UFR-1, UFR-2, UFR-3, UFR-4, UFR-5, UFR-6, UFR-7, UFR-8, UFR-9, UFR-10, UFR-11, UFR-12, UFR-14, UFR-15, UFR-16, UFR-17, …
USDA breeding program US-802, US-812, US-897, US-942, US-1279, US-1281, US-1282, US-1283, US-1284, US-1516, SS1, SS2, SS3, …
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https://www.freshfromflorida.com/
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2017-2018
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V. ROOTSTOCKS AND HLB
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Huanglongbing (HLB)
Asian citrus psyllid
InfectedHealthy Candidatus Liberibacter asiaticus (CLas)
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Definitions
Pagán and García-Arenal (Int J. Mol. Sci. 2018)
The two major mechanisms of plant defense against pathogens are: 1) resistance (the host’s ability to limit pathogen multiplication) and 2) tolerance (the host’s ability to reduce the effect of infection on its fitness regardless of the level of pathogen multiplication).
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HLB field tolerance of rootstock trees
US-897 US-942
Citrus reticulata × Poncirus trifoliata
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Greenhouse studies
Control Infected ControlInfected Control Infected
Cleopatra US-897 US-942
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Grafting
HLB-susceptible!
Grafting removes many rootstock-specific leaf biochemical compounds that may be associated with HLB tolerance.
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How do we define rootstock tolerance in a commercial grafted tree? The ability to maintain tree health and productivity despite infection. This is best measured relative to some other “standard” rootstock under the same conditions.
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Field observations . None of the rootstock cultivars prevent a grafted susceptible scion from becoming infected with CLas. . But some rootstocks enable the grafted tree to better cope with HLB. WHY?
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Metabolomics Relative concentration Relative concentration
The rootstock can influence the scion metabolically
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Rootstock can influence the scion metabolic response to HLB
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Reciprocal influence of rootstock and scion challenged with CLas
Albrecht and Bowman (2019) Scientia Horticulturae 254
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Disease progression
Scion/rootstock combination 2 mai 4 mai 6 mai 9 mai 12 mai Cleopatra/US-802 1.0 2.4 2.8 2.0 3.4 Cleopatra/US-897 1.0 1.8 2.1 1.8 2.8 Cleopatra/US-942 1.0 1.8 2.1 2.4 3.1 Cleopatra/Cleopatra 1.0 2.3 2.6 2.3 3.0 US-802/Cleopatra 1.0 1.0 1.0 1.1 1.3 US-897/Cleopatra 1.0 1.0 1.1 1.0 1.0 US-942/Cleopatra 1.0 1.0 1.0 1.0 1.1 US-802/802 1.0 1.0 1.1 1.0 1.1 US-897/US-897 1.0 1.0 1.1 1.0 1.3 US-942/US-942 1.0 1.0 1.2 1.1 1.1 Valencia/US-802 1.0 1.4 2.2 2.5 3.3 Valencia/Cleopatra 1.0 1.0 1.7 2.5 2.7 Trees with a tolerant cultivar in the scion position were less damaged by CLas infection and became infected at a lower rate than trees with a susceptible cultivar in the scion position.
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What does this mean?
. Results indicates a greater influence of the scion cultivar than rootstock cultivar on tree tolerance to infection with CLas.
. This does NOT indicate the rootstock plays no role in tree tolerance to HLB!
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Other factors . Rootstock vigor. . Better ability to cope with other soil-borne stresses and diseases under the weakened condition caused by HLB.
. Rootstock-specific architectural and anatomical traits that are associated with efficient uptake and transport of nutrients and water.
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Root structural differences
Swingle US-897 US-942
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Summary Rootstock influences a grafted citrus tree in many ways and is very important for the success of a citrus operation. The success of a rootstock depends on the genetic potential and on the interaction with climate, soil, pests and diseases, and cultural practices. Rootstocks can be used to reduce the negative impacts of HLB. Even when a rootstock is highly tolerant to a specific stress or disease, it will not thrive in the presence of unfavorable conditions unrelated to that specific stress or disease.
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Ute Albrecht [email protected]
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