Rootstock and Scion Varieties
Julian W. Sauls
Professor & Extension Horticulturist
Texas AgriLife Extension
The choice of a rootstock and scion variety is still a fairly easy proposition in Texas citrus, primarily because of the traditional nature of the Texas industry--ours is a fresh fruit industry based on high quality, red-fleshed grapefruit, complemented with a very few orange varieties. At present, grapefruit accounts for about 72 percent of the estimated 34,000 acres of Texas citrus, of which about 75 percent is the deep red Rio Star grapefruit. Because of the long-term nature of citrus orchards, changes in the varietal makeup of an industry normally occur slowly until a major disaster forces extensive orchard removal and replanting. Historically, severe freezes have periodically crippled the Texas citrus industry, which afforded growers the opportunity to replant with newer, redder grapefruit varieties which were developed in Texas. So it was that Ruby Red supplanted white and pink varieties of grapefruit during the 1950's and 1960's. In turn, Ruby red gave way to other, redder varieties following two major freezes of the 1980's.
The dominance of sour orange rootstock and its susceptibility to citrus tristeza virus have always been a minor concern to Texas growers. However, the arrival of the brown citrus aphid into Florida and its ability to transmit severe strains of the tristeza virus have spurred the development of a budwood certification program to provide disease-free budwood for the future. Initiated in the mid-1990's, the program has advanced to the point of planting a foundation block and the establishment of nursery increase blocks of all major varieties and a large number of non-commercial varieties of citrus. Limited numbers of certified buds were provided for the propagation of rootstock trials in 1999; general availability of certified budwood of major commercial varieties for Texas citrus nurseries is expected in 2000.
A wide variety of citrus rootstocks are available, each having desirable attributes. The success of a rootstock is determined by its tolerance to prevailing conditions of soil, climate and disease, while still producing high yields of good quality fruit. A rootstock for Valley citrus must be adapted to alkalinity, salinity and calcareous soils, should be resistant to Phytophthora and virus diseases, provide some measure of cold tolerance and produce good yields of high-quality fruit.
Sour orange is the standard rootstock in Texas, being generally well adapted to the different citrus soils in the Valley. It is somewhat tolerant to salinity, alkalinity and less than optimal drainage, and is relatively tolerant to cold, cotton root rot and Phytophthora. It is susceptible to citrus nematode and citrus tristeza virus. Grapefruit and orange yields on sour orange are moderate, with average fruit size and good quality. Sour orange accounts for at least 95 percent of Valley orchards.
Cleopatra mandarin, or Cleo, is generally adapted to Valley conditions. It is more cold tolerant than sour orange and tolerant to tristeza, but is less tolerant to alkalinity. Trees on Cleo are slower to bear and yields, fruit size and quality are all poorer than those obtained on sour orange. Cleo is not being propagated currently.
Swingle citrumelo is a trifoliate hybrid that reportedly equals or exceeds sour orange in most characteristics. It is resistant to nematodes and Phytophthora, tolerant to tristeza and produces a vigorous tree with excellent yields and excellent fruit size. It is reported as tolerant to xyloporosis and exocortis, but some stunting has been observed with old-line budwood having those viruses. Swingle citrumelo is intolerant of poor drainage and exhibits severe chlorosis in heavy soils, so it should be limited to well-drained, very deep sandy soils. An estimated 15 percent of Valley citrus soils are suitable for Swingle citrumelo but very few orchards are planted on this rootstock.
Carrizo and Troyer citranges are sibling trifoliate hybrids that are intermediate between sour orange and Swingle citrumelo in terms of vigor, fruit size, total yield and soil adaptability. The citranges are tolerant to tristeza but susceptible to exocortis, citrus nematode and soil alkalinity and exhibit less cold hardiness. Perhaps a third of Valley citrus soils are suitable for the citranges, but their use has been very limited.
Numerous rootstocks were tested in Texas in years past, but none proved superior to sour orange. The better rootstocks
from those tests, plus some newer ones, are being re-evaluated on a range of citrus soils. Until a better rootstock than sour
orange emerges or until the dual threat of citrus tristeza virus and the brown citrus aphid appears eminent in Texas, there is
not likely to be any significant change in rootstocks in Texas citrus.
The Valley's reputation for high quality grapefruit is unsurpassed by other citrus-producing areas, a reputation that is based upon seedless, red-fleshed fruit high in sugars and low in acids. The principal varieties grown in Texas are commercially seedless, have red flesh, normally achieve legal maturity in October-November and hold well on the tree into April-May.
The Ruby-Sweet varieties include Ruby Red and similar selections as well as the redder Henderson and Ray Ruby varieties. Together, they account for no more than about 25 percent of Texas grapefruit acreage. Several hundred acres have been removed in the last couple of years, with no new plantings.
Ruby Red or Redblush was the leading red grapefruit in Texas for nearly four decades and it is the variety on which Texas' reputation for quality is based. It is commercially seedless, may have a red blush on the rind and has excellent quality. The early redness of the flesh gradually fades to pink by midseason and buff by spring. Ruby Red is well-suited for gift fruit, fresh market and processing.
Henderson and Ray Ruby apparently are indistinguishable cultivars discovered in the early 1970s, Henderson as a limb
sport of an Everhard strain red grapefruit tree originally planted in 1945 and Ray as four separate young trees planted in a
Ruby Red orchard. Both are commercially seedless, of excellent quality and acceptable for gift, fresh and processed
markets. Both have more rind blush and two to four times redder flesh color than Ruby Red, with flesh color holding well
into late season.
Ruby Red grapefruit.
Rio Star Grapefruit
Rio Star grapefruit are the super-red or deep-red fleshed grapefruit of Star Ruby and Rio Red varieties, which comprise about 75 percent of Texas grapefruit acreage. Very few, if any, Star Ruby orchards exist, however, so virtually all of the Rio Star grapefruit is Rio Red. Star Ruby was released by Texas A&I University in 1970, having originated from irradiated seed of Hudson grapefruit. Its primary attributes are intensely red flesh, good color retention even in late season
|Rio Red grapefruit.|
and a fairly uniform red blush on the rind. Star Ruby commanded good market acceptance and premium prices, but it is sensitive to some herbicides, frequently exhibits winter chlorosis and apparently is more susceptible to Phytophthora and cold damage than other cultivars. Star Ruby is also noted for erratic bearing. Because of its inherent production problems, the freezes of 1983 and 1989 and the introduction of Rio Red in 1984, Star Ruby has essentially disappeared from the Texas industry.
Rio Red was released by Texas A&I University in 1984 as a natural mutation on a tree produced from irradiated budwood
which came from Ruby Red seedlings. Tested by Texas A&I University as A&I-1-48S, it produces fruit with a rind color
similar to Henderson and Ray and flesh color almost as red as Star Ruby. Other fruit and tree characteristics are similar to
Ruby Red except that its deep red flesh color persists throughout the season and it has a strong tendency to sheepnosing. It
is a heavy bearer, with a slight tendency to alternation, i.e., exceptionally large crops are often followed by what would
normally be considered an average crop. Rio Red is suitable for gift, fresh and processed markets.
Most oranges grown in the Valley are characterized by low acidity, thin peel, good yellow peel color, but generally light juice color. Fruit maturity is considered as early and mid-season, navels (October-February) and late season (February-May). Total orange acreage is estimated at about 10,000, which is about 29 percent of total citrus acreage.
Navels and Early Oranges
Navel oranges comprise several selections and varieties, all commercially seedless and with a small, secondary fruit (the navel) embedded in the apex of the primary fruit. Navels normally mature in October and are shipped through January. Most produce comparatively low yields. The primary use is for gift pack and fresh market.
Everhard is a smaller fruit characterized by a very small or no navel; it is a consistent producer. N33E is a local navel selection that accounts for the majority of the estimated 2800 acres of navels in Texas. It is characterized as a large fruit with a prominent navel. N33E consistently suffers severe fruit splitting in late summer, yet production is still better than most other navel oranges.
Marrs is a navel orange budsport relatively unknown outside Texas. It is commercially seedless, but seedy fruit can occur
because of adjacent pollinizers. Marrs attains legal maturity in early October, sometimes in late September, primarily
because of its low acidity. It bears heavy crops of medium fruit size but it exhibits a tendency to alternate bearing. It is
grown for the fresh market, usually being shipped through January. Marrs comprises the vast majority of the estimated
6,000 acres of early and mid-season oranges.
N33E navel orange.
Parson Brown is a very early, seedy orange that usually matures in early September. Its fruit are of medium size, containing 10 to 20 seeds, and have a pebbly-textured peel. Fruit quality is about as good as Marrs. Parson Brown is sometimes designated as Pineapple, which is inaccurate. Only a few acres of Parson Brown exist.
Hamlin is a seedless, early orange that matures in October. Fruit size is smaller than Marrs and its quality is not so good as Marrs. Very few acres exist.
Parson Brown orange.
Pineapple matures in late November and holds into February. Fruit size, fruit quality and yields are a little better than Marrs. The fruit is seedy, having 15 to 20 seeds. It has a strong tendency to alternate bearing. Although other varieties are often designated as Pineapple, the true Pineapple orange can be distinguished by its seediness and time of maturity. Not many acres of Pineapple exist.
Jaffa and Joppa are mid-season, seedless oranges that are commonly confused with each other and the Shamouti or
Palestine Jaffa from which they apparently originated. Generally, Joppa has a smaller fruit that is also rounder than that of
Jaffa, with somewhat richer color and flavor. Jaffa reportedly does not store well on-tree and tends to alternate bearing.
Occasional blood orange flecking has been noted in a couple of smaller plantings in some seasons, although such flecking
is not reported in the literature for either variety. Acreage is very limited.
Late Season Oranges
|Olinda Valencia orange.|
Valencia, probably the premier orange variety in the world, is grown in every commercial citrus-producing area. It is a late-season orange that normally achieves maturity in late January, with shipments continuing into May. It is commercially seedless, of average size and has excellent juice quality, although yields generally are lower than early oranges. Valencia is excellent for fresh market and for processing to upgrade the quality of juice from early oranges.
Although various selections of Valencia exist, all but Olinda are simply referred to as Valencias. Olinda is more productive
than other selections in Texas, and limited new Olinda Valencia orange plantings, possibly about 200 acres, have been
made recently in orchards which were previously planted to Ruby-Sweet grapefruit varieties. Only about 1400 acres of
Valencia oranges are grown in Texas, including the recent new plantings, primarily because of lower yields and late
maturity, the latter of which puts the crop at greater freeze risk than other orange varieties.
Other Citrus Varieties
Very limited acrea of Clementine (Algerian) tangerine, Dancy tangerine and Orlando and Minneola tangelos provide fruit for gift and specialty markets. Most are more cold hardy than oranges or grapefruit, are easy to peel and have orange peel color and rich flavor.
True lemons and limes are too cold sensitive for commercial plantings in the Valley, but a few acres of Meyer lemon are grown for local use.
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This document last modified January 7, 2008.