The rootstock information provided is a broad-based compilation of Florida information collected from field trials and commercial situations. The information is general in nature as it represents 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 ofevaluation. Thus, the rootstocks have been grouped accordingly into three categories: 1) MOST USED, which are commercial rootstocks with a long history of use and are the ones for which the descriptions are the most reliable(highlighted in blue); 2) MINOR USE rootstocks, which are ones that have been in minor commercial use for a while and others that were prominent at one time but whose importance has faded as newer rootstocks were introduced and adopted commercially (highlighted in green). The information presented for most of the rootstocks in this category is reliable but is sometimes not as fully developed as with the most used commercial rootstocks; and 3) RECENTLY RELEASED rootstocks, which were more recently created in breeding programs and have been under evaluation in Florida for only a few years (highlighted in yellow), eventually selected from field trials and small commercial cooperative trials. Recently released rootstocks have limited commercial experience.
It is also important to note that in rating rootstocks, the differences for a stated factor are sometimes based on a quasi-qualitative comparison and in other cases a more quantitative basis. Tree height, for example, is essentially a relative rating based on the standard of comparison: a tree on a rootstock rated as “Large” would be equivalent in height to a mature tree on rough lemon rootstock. On the other hand, a rootstock’s rating regarding citrus nematode or Phytophthora tolerance is fundamentally based on quantitative screening trials plus commercial experience.
The year of first commercial availability is when the rootstock first appeared in the nursery use records of the Florida Department of Agriculture & Consumer Services, Division of Plant Industry, Bureau of Citrus Budwood Registration. 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/Bureaus-and-Services/Bureau-of-Budwood-Registration."
Germination is the growth of a seed into a young plant or a seedling. This parameter is important for plant propagation in the nursery. Some rootstocks, like Rusk Citrange and 1584, have a high seed germination rate (80%–90%) but usually produce fruit with few to no seeds.
This variable indicates the percentage of true-totype seedlings. Most common citrus rootstocks produce polyembryonic seed that yields true-to-type plants, that is, seedlings that are the same type of plant as the original plant. However, a few rootstocks will differ based on the parental pedigree used for crossings. Rootstocks like Smooth Flat Seville produce lots of seeds, but the seeds do not germinate well, and the seedlings are highly variable, with 50% or more discarded as off-types. UFR-3, UFR-15, and UFR-16 present 80%–90% germination, but seedlings are also highly variable.
Tree size refers to the size of the canopy of a mature tree. The ratings indicate relative tree vigor. A tree on a selected rootstock would be rated large[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 [Sm] would be less than 8 ft tall at maturity, and an intermediate tree [I] would be like one on C-35 citrange and range in height from 8 to 14 ft tall.
This is the appropriate spacing without excessive crowding given the expected vigor and growth to maturity of common commercial scion varieties. Spacing would change according to the selected scion/ rootstock combination. Between-row spacing would be dictated mostly by cultural and harvesting machinery used.
This term is related to 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 per tree than larger trees, but the smaller trees can be planted at higher densities.
Yield per tree times the number of trees per acre will ultimately determine the yield per acre. Generally, the relationship between these two variables (yield per tree and yield per acre) is directly proportional. However, there are situations where a tree has only intermediate yield per tree because of a smaller canopy but yield per acre is high because more trees of smaller stature can be planted per acre.
Brix/acid ratio rating has been determined from juice samples from various rootstock and scion combinations and compared to industry averages.
Refers to a relative rating based on US market size standards.
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.
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.
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.
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)
With the advent of generally practiced irrigation, drought is no longer considered an important rootstock factor.
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.
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.
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.
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.
All rootstocks are susceptible except for selections of Carrizo citrange, ‘Ridge Pineapple’ sweet orange, Milam lemon and Kuharske citrange.
Trifoliate orange has tolerance to citrus nematodes. That trait is inherited by many of its hybrids including Swingle citrumelo.
A pest in some instances especially in soils with high sand content. There are no known tolerant rootstocks.
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.
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.
There are just a few situations where a scion/rootstock combination, like Murcott budded to Carrizo citrange or Swingle citrumelo, declines from an apparent incompatibility that manifests itself at the bud union. Other problematic combinations were described by Garnsey et al. and are listed in the citations on the interactive website.
Mandarins are perhaps the most sensitive scions to incompatibility especially with trifoliate orange-hybrid rootstocks. Nursery managers 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.
G - Good; P – Poor; H – High; I –Intermediate; L – Low; Lg – Large; R – Resistent; S – Susceptible; Sm – Small; T – Tolerant; Blank space – Rating unknown or undertermined; [ ] – Any symbol in brackets indicates a probable or expected rating; Relative ranking; UFR – University of Florida Rootstock; and US - United States Department of Agriculture (USDA)