Notes on Rootstock Traits
Florida Citrus Rootstock Selection Guide, 3rd Edition. William S. Castle, Kim D. Bowman, Jude W. Grosser, Stephen H. Futch and James H. Graham
The last edition of this guide was published in 2006. Much has changed since then in the Florida citrus industry mostly because of the discovery and spread of Huanglongbing (HLB) or greening disease. This update considerably expands the list of rootstocks to include many that have not yet undergone the usual extensive field and commercial evaluation in Florida.
General. The rootstock information provided is a broad-based compilation of Florida information collected from field trials and commercial situations. In rating rootstocks, the differences for a particular factor are sometimes based on a qualitative comparison and in other cases a more quantitative basis. Tree height, e.g., is essentially a relative rating based on the standard of comparison. A tree on a particular rootstock rated as “Large or L” would be equivalent in height to a mature tree on rough lemon. On the other hand, a rootstock’s rating regarding citrus nematode or Phytophthora tolerance is fundamentally based on quantitative screening trials plus some commercial experience.
The information is necessarily general and is essentially “average” rootstock behavior across a range of conditions related mostly to scion variety and site conditions. The quality of information varies due primarily to the time period of evaluation. Thus, the rootstocks have been divided into three groups:
- COMMERCIAL. Time-honored rootstocks in terms of their use, thus, the ones with the most reliable descriptions;
- MINOR COMMERCIAL. Rootstocks such as Smooth Flat Seville that have been in minor commercial use for a while and others like rough lemon that were prominent at one time, but their importance has faded as newer rootstocks were introduced and adopted commercially. The information presented for most of the rootstocks in this category is reliable, but is sometimes not as fully developed as with the commercial rootstocks; and,
- RECENTLY RELEASED. Rootstocks created in breeding programs and under evaluation in Florida for only a few years. They were selected from field trials and small commercial cooperative trials with limited commercial experience.
Year of first commercial availability is when the rootstock first appeared in the nursery use records of the FL. Dept. Agric. & Consumer Serv., Div. Plant Industry, Bureau of Citrus Budwood Registration. Rootstock use: Information on rootstock use can be found in the annual reports of the Bureau available at this site: http://www.freshfromflorida.com/Divisions-Offices/Plant-Industry/Agriculture-Industry/Citrus-Health-Response-Program/Citrus-Budwood-Program/Annual-Reports
- Ease of seed propagation. If a rootstock produces seed fruit with many polyembryonic seeds that germinate well, it is rated Good or “G.” Some rootstock seed trees produce lots of fruit, but usually only a few to no seeds and are rated “G;FS.” Rootstocks like Smooth Flat Seville produce lots of seed fruit with seeds that germinate well, but the seedlings are highly variable and 50% or more are discarded as off-types. They are rated as “G; sdlg var.”
- Tree size is the size of the canopy of a mature tree. The ratings indicate relative tree vigor. A tree on a particular rootstock would be rated large or “Lg” if it was comparable in vigor and size to one on Cleopatra mandarin or rough lemon, i.e., perhaps 14 – 20 ft tall. A small tree, rated “Sm”, would be less than 8 ft tall at maturity and an intermediate tree like one on C-35 citrange would be rated “I.”
- Suggested in-row spacing. The appropriate spacing without excessive crowding given the expected vigor and growth to maturity of common commercial scion varieties. Spacing would actually change according to the particular scion. Between-row spacing would be dictated mostly by cultural and harvesting machinery used.
- Yield/tree is the amount of fruit on an individual mature tree at a recommended spacing, but recognizing that the bearing habits of different scion-rootstock combinations vary. For example, some are more precocious (early –bearing) than others. Comparisons of rootstocks for effects on tree yield should consider canopy size. Small trees usually produce less fruit/tree than larger trees, but the smaller trees can be planted at higher densities.
- Yield/acre. Yield/tree x number of trees/acre. Generally, the relationship between these two variables (yield/tree and yield/acre) is directly proportional. However, there are situations where a tree has only intermediate yield/tree because of a smaller canopy, but yield/acre is high because more trees of smaller stature can be planted/acre.
- Juice quality is a Brix/acid ratio rating.
- Fruit size is a relative rating based on legal standards.
- Salinity. Salty waters in Florida containing high levels of NaCl are problematic because rootstocks vary in their absorbance and translocation of the Na and Cl ions. High pH. Rootstocks vary in their tolerance of calcareous soils mostly because of the CaCO3 in the soil and/or the irrigation water. Such conditions commonly lead to calcium-induced Fe chlorosis. New evidence suggested that HLB-infected trees may also suffer stresses related to bicarbonate in the irrigation water.
- Clay soil refers to soils with horizons containing >20% clay or loamy materials, and especially soils where such horizons are close to the soil surface. These soils are generally unsuitable for Swingle citrumelo, Carrizo citrange, and other citrumelo, citrange, and trifoliate orange rootstocks.
- Wet soil (flooding). Wet soils are poorly drained, chronically wet, or subject to extended periods of flooding. For additional soils information, see Field Guide to Soil Identification for Florida’s Citrus-Growing regions, SP 362 (http://ifasbooks.ifas.ufl.edu/p-266-field-guide-to-soil-identification-for-floridas-citrus-growing-regions.aspx).
- Drought. With the advent of generally practiced irrigation, drought is no longer considered an important rootstock factor.
- Freezes. The threat of a seriously damaging cold event remains. Few stressful freeze events have occurred since the decade of the 1980s. Thus, the tolerance of many of the minor and recently released rootstocks has not been determined.
Pests and Diseases
- HLB incidence in bearing trees. Observations are accumulating that the incidence of HLB is less among trees on some rootstocks especially new rootstocks included in various field trials. This rating reflects those observations; however, the ratings are quite tentative and subject to change. The interpretation of the observations is speculative and highly subject to revision over time. Survey results from a grower-scale observation of HLB incidence among commercial scions and rootstocks are available by visiting http://www.crec.ifas.ufl.edu/extension/pdf/hlb_scion_survey.pdf.
- Blight. As with HLB, rootstock tolerance to blight, a disorder of unknown cause, is based on field observation in research trials and commercial groves. It is a combined rating for overall incidence and rate and time to first losses. Thus, Cleopatra mandarin is rated “S-T” because while the incidence of blight is low among young trees, substantial losses can occur when the trees reach 12 to 15 years of age.
- Phytophthora nicotianae ratings are a combination of foot and root rot tolerances which are similar within a rootstock, but some differ, e.g., sour orange has good foot rot but mediocre root rot tolerance.
- P. palmivora/Diaprepes weevil complex. Rootstocks vary in their tolerance to P. nicotianae and P. palmivora, but when P. palmivora is present, it is not ordinarily problematic unless Diaprepes weevils are also present (Phytophthora/Diaprepes Complex). Most rootstocks are susceptible to this complex, but the ratings may vary depending on soil type.
- Burrowing nematodes. All rootstocks are susceptible except for selections of Carrizo citrange, ‘Ridge Pineapple’ sweet orange, Milam lemon and Kuharske citrange.
- Citrus nematodes. Trifoliate orange has tolerance to citrus nematodes. That trait is inherited by many of its hybrids including Swingle citrumelo. STING NEMATODE is a pest in some instances especially in soils with high sand content. There are no known tolerant rootstocks.
- Xyloporosis and Exocortis. These virus and viroid diseases, respectively, can be problematic for trees on certain rootstocks. However, they have been virtually eliminated from Florida because of clean budwood programs. Therefore, they are not included in this guide.
- Tristeza. This disease is caused by an aphid-transmitted virus. Rating rootstocks is a bit complicated because there are strains and mixtures of strains of this virus that cause a range of symptoms leading to tree decline or in some cases, very mild effects.
- Incompatibility. There are just a few situations where a particular scion-rootstock combination, like Murcott budded to Carrizo citrange or Swingle citrumelo, declines from an apparent incompatibility that manifests itself at the budunion. Other problematic combinations were described by Garnsey et al. [see rootstock database].
- WARNING! Mandarins are perhaps the most sensitive scions to incompatibility especially with trifoliate orange-hybrid rootstocks. Nurserypersons and growers should be alert to the fact that new releases of mandarin scion types have generally not been tested for compatibility with a broad range of rootstocks.
Key to Symbols: G – Good; H – High; I – Intermediate; L – Low; Lg – Large; P – Poor; S – Susceptible; Sm – Small; T – Tolerant; * Blank space – Rating unknown; [ ] – Any symbol in brackets indicates a probable or expected rating; ? – Undetermined; +/- Relative ranking; and UFR – University of Florida Rootstock.