root system development in field- and container-grown young citrus trees

should theoretically be a more sensitive technique. How ever, saprophytic organisms often quickly colonized the leaf pieces in flooded soils and probably outcompeted the Phytophthora thus causing failures in detection. Further more, although leaf piece baiting is a simpler technique it is not quantitative. Therefore, where laboratory facilities are adequate, the selective medium would be the method of choice. Since all the isolates recovered were P. parasitica, which has a high optimum temperature for growth, little or no damage should occur in the winter months. Therefore, fungicide applications should be concentrated in the sum mer months. Since both mating types are present in some nurseries, oospores may be produced. Although the role of the oospore in the disease cycle is unclear, oospores may serve as an overwintering structure and increase the potential for variation in the fungus. The predominance of the Al mat ing type of P. parasitica, observed in this study, also has been observed in citrus orchards in Taiwan and Texas (1, 9). The reason for the lack of a 1:1 ratio of mating types is unclear. Only a few sites in Florida citrus nurseries were found where Phytophthora was causing serious problems. Produc tion of stock in soilless media has eliminated many of the problems in greenhouse-grown citrus. We believe that pro duction of liners in soilless media in the greenhouse rather than in field seedbeds has also helped reduce the incidence of Phytophthora in field nurseries.

Literature Cited 1. Ann, P. J. 1984. Species, mating types and pathogenicity of Phytophthora distributed in citrus orchards in Taiwan. Trans. Brit. Mycol. Soc. 82:631-634. 2. Barrett, J. T. 1948. Induced oospore production in the genus Phytophthora. Phytopathology 38:2. (abstr). 3. Cohen, M., G. R. Grimm, and F. W. Bistline. 1964. Root rot in young groves. Proc. Fla. State Hort. Soc. 77:45-52. 4. Grimm, G. R. and A. F. Alexander. 1973. Citrus leaf pieces as traps for Phytophthora parasitica from soil slurries. Phytopathology 63:540541.

5. Kannwischer, M. E. and D. J. Mitchell. 1978. The influence of a fungicide on the epidemiology of black shank of tobacco. Phytopathology 68:/l760-1765. 6. Ko, W. H. 1972. Heterothallic Phytophthora: Evidence for hormonal regulation of sexual reproduction. J. Gen. Microbiol. 107:15-18. 7. Newhook, F. J., G. M. Waterhouse, and D. J. Stamps. 1978. Tabular key to the species of Phytophthora DeBary. Mycological Papers: 143. Commonwealth Mycological Institute, Kew, England. 8. Ridings, W. H., N. C. Schenck, R. R. Snell, W. M. Keen, and J. A. Cornell. 1977. Reinvasion of methyl bromide treated soil by soilborne fungi and their subsequent effect on citrus seedling growth. Proc. Fla. State Hort. Soc. 90:70-74. 9. Timmer, L. W. 1973. Characteristics of Phytophthora isolates from Texas citrus orchards. J. Rio Grande Valley Hort. Soc. 27:44-48. 10. Timmer, L. W., H. A. Sandier, J. H. Graham, and S. E. Zitko. 1987. Sampling of citrus orchards for estimation of Phytophthora parasitica populations. Phytopathology (In press). 11. Tsao, P. H. and G. Ocana. 1969. Selective isolation of species of Phytophthora from natural soils on an improved antibiotic medium. Nature 23:636-638.

Proc. Fla. State Hort. Soc. 100:85-89. 1987.

ROOT SYSTEM DEVELOPMENT IN FIELD- AND CONTAINER-GROWN YOUNG CITRUS TREES William S. Castle

University of Florida, IFAS Citrus Research and Education Center 700 Experiment Station Road Lake Alfred, FL 33850

Additional index words, container-bound, bare-root.

Abstract. The effect of propagation and planting method on the

root system

development of young

citrus trees was

studied. Observations were made of container-grown nursery

trees planted in field trials and subsequently removed after 1.5 to 3.5 yr; and of field-grown nursery trees transplanted bare-root into commercial orchards and then removed after 3 to 6 yr. The root systems of the young citrus trees were com

pared to that of a carefully planted 6.5-yr-old field tree. Dif ferences in the number and distribution of major woody roots were observed among the field-grown trees but they seemed unrelated to planting method; however, trees planted such that 'J' shaped roots were formed showed restricted scion and root system development up to 6 yr. Container-grown trees

developed normal root systems unless restricted by being conFlorida Agricultural Experiment Station Journal Series No. 8623.

Proc. Fla. State Hort. Soc. 100: 1987.

tainer-bound. Trees produced in a 2-yr propagation cycle in 2 gal round pots were apparently container-bound. They

showed little root system expansion after 3 yr in the field. Trees raised in smaller volume containers were also subject to poor root system growth when the root ball was undisturbed at planting. Planting citrus trees is a horticultural practice for which there are many methods and recommendations based largely on experience. In Florida, the typical citrus nursery tree is field-grown and transplanted bare-root. Millions of such trees have been used successfully in Florida and else where for many yr. It would not appear that nursery tree characteristics or planting technique were particularly im portant except that no systematic study has been made of these factors and their relationship to field tree survival and growth. Furthermore, it is usually not known if root system development is being retarded or restricted. Af fected trees may be slow-growing for several yr in the or chard but otherwise seem normal. The general satisfaction with field-grown citrus nursery trees, regardless of planting method, has not been com pletely shared by those planting container-grown trees. Prior to 1978, nursery trees raised in 2- or 3-gal round pots were used to a small extent primarily as replacement 85

mercial rootstocks with more than 85% on Carrizo citrange [C. sinensis x Poncirus trifoliata (L.) Raf.]. Diseases, soilborne pests and management practices were eliminated to the extent possible as contributing to the smaller tree size. All trees were confirmed to be field-grown. Container-grown trees. The trees were all obtained from the 3 field experiments described in Table 1.

trees in established orchards; however, since then the con tainerized citrus nursery business has expanded and in volves new techniques and materials. Perhaps 10% of all nursery trees for orchard use are being produced in con tainers. Most of these trees are grown in organic media with a high water-holding capacity and in different shaped and smaller-sized containers (4, 5). Changes in production procedures coupled with the usually smaller caliper of con tainer-grown trees and conflicting reports of their field performance have heightened concern about the proper handling and planting methods for containerized citrus

Results and Discussion

The root system of the carefully planted 6.5-yr-old 'Valencia' tree on Cleopatra mandarin (Cleo) rootstock (Fig. 1) is typical for a Florida citrus tree grown in a deep

nursery trees.

The objective of this study was to determine from field observation if propagation system and planting method affected the root system development of young citrus trees.

sand soil.

Such a root system consists of about 8 to 12 large woody roots distributed evenly around the trunk. Interspersed among these roots are numerous long slender lateral roots from which arises most of the fibrous root system. The pattern of lateral and vertical root development of the tree on Cleo was similar to that of a 'Valencia' tree on rough lemon (C. jambhiri Lush) rootstock excavated from the same soil (2). Severing roots during digging in the nur sery did not impede root growth in the field as replace

Materials and Methods

Observations regarding root system development were obtained as follows: Typical root system. An experiment consisting of fieldgrown, bare-rooted 'Valencia' orange [Citrus sinensis (L.) Osb.] trees on 7 rootstocks was planted at a spacing of 15 x 20 ft in a deep sandy central Florida soil. The trees were carefully planted so that roots were spread apart and oriented as they existed in the nursery. The planting was irrigated with a low volume, under-tree system. After 6.5 yr, a commercial tree remover was used to lift the trees from the soil for root study. A representative tree (Cleopatra mandarin, C. reshni Hort ex Tanaka) was selected for root system comparisons. Bare-root trees. Twenty-six citrus trees about 2- to 6 yrold and growing in a deep, sandy soil, were selected in 8 different orchards located throughout central Florida. Each tree was judged to be healthy but at least 25% smaller in canopy size (primarily height) than nearby healthy trees of similar age. The smaller tree and an adjacent, averagesized tree were removed in each orchard, either by hand excavation or with commercial tree removal equipment, to examine and compare their root systems. Additional adja cent trees were examined when possible. Any obvious dif ferences in major, woody root development were noted. The trees were primarily sweet oranges, but included a few grapefruit (C. paradisi Macf.), on the common com

ment roots (1) were apparent (Fig. 1). The tree on Cleo had about 8 large roots distributed around and originating at the tree crown. These roots de scended in a short distance to a depth of 12 to 18 inches and then extended radially to 8 ft and beyond. Mingled among these roots was a large number of smaller, but very long woody roots that were markedly uniform in diameter along their length. Non-woody or fibrous roots were at tached primarily to the smaller woody roots forming a dense, shallow mat beneath the entire tree canopy. Several relatively thin tap roots were also present but they were broken off at 6 ft when the tree was removed. They prob ably extended at least another 2 or 3 ft. Several lateral roots had developed along the tap roots. The major root system characteristics described above are stable under Florida conditions and are normally welldeveloped by the time field-grown nursery trees are ready for transplanting (3). Any variation in these traits, resulting e.g., from planting technique, should be easily observed. The only marked environmental modification would be the absence of vertical root development in shallow, poorly drained soils.

Table 1. Description of the field experiments involving container-grown trees that were used for root development observations.

Expt.

No.

1.

Scion rootstock Hamlin

Type of container Trees grown in a

Tree age (yr)z

Planting method

1.5

Undisturbed

Carrizo citrange

cone-shaped container and transplanted into a 24 gal fabric bag placed in the ground

2.

Valencia Carrizo citrange

2 gal round pot

2.5

3.

Star Ruby grapefruit Carrizo citrange

0.9 gal plastic bag

1.5

Type of site

Irrigation

Poorly drained,

Overhead

high water table

sprinklers

Undisturbed root ball

Deep sand

Low volume

Undisturbed or slightly broken apart root ball

Nursery trees were transplanted into 10 gal containers using Candler fine sand soil

Manually with a hose

zAge at time of root system examination.

86


Fig. 1. The root system of a 6.5-yr-old 'Valencia' orange tree on Cleopatra mandarin rootstock grown in deep sandy soil with microsprinkler irrigation. Fibrous roots were removed to show the major (m), slender lateral (1) and replacement (r) roots.

The manner in which a citrus tree is planted can affect tree growth. Among the 26 bare-rooted field trees examined, 18 had some deformation or deficiency in the root system as compared with the typical root system. The reason for small size among the remaining 8 trees was not apparent. One possible explanation is that these, or any of the smaller trees, had been propagated on a zygotic (non nucellar) rootstock seedling. Root development of the small trees was most com monly either eccentric or deficient. Many plants had an uneven distribution of major roots and thus, fibrous roots, or, there were gaps in the root system. It is likely that woody roots did not develop evenly around the trunk while a tree was in the nursery. These spaces were not filled by adventitious or adjacent root growth in the field, leaving unexplored areas in the soil surrounding the tree. Nevertheless, it did not appear that the failure of the root system to completely exploit the available soil volume was contributing significantly to small tree size. Some of the average-sized trees of the same age in an orchard had a similar root pattern. Furthermore, among the small trees, those with this type of root system morphology tended to differ the least in size with their adjacent, average-sized

Fig. 2. The 'J' roots of a 'Murcott' tree on Carrizo citrange rootstock at age 7 yr.

planted, roots may be clipped, bent, or bunched as neces sary to fit the tree into the available space. Roots placed into unnatural positions (e.g. 'J' roots) can limit root system development. It is for this reason that I conceived and

mates.

The root system trait clearly related to the manner of planting and tree growth was the formation of 'J' roots (Fig. 2). There were 7 trees among the 18 noted above with several major roots bent upward during planting. The iy root condition persisted resulting in a root system that was shallow, with fewer large woody roots and little vertical development. These trees were the oldest (5 or 6 yr) ones examined and had the greatest disparity in size (about 50%) with the nearby average-sized trees. The prolonged effect of 'J' roots is undesireable because retarded tree de velopment decreases yield potential. In young trees, yield is closely related to canopy size (6). The planting of field-grown nursery trees often in volves digging a hole manually or using a mechanical de vice that forms a water ring and simultaneously bores about a 12 x 12 inch conical-shaped hole. The usual plant ing hole does not match the natural morphology of the field-grown nursery tree root system. When these trees are

Proc. Fh. State HorL Soc. 100: 1987.

Fig. 3. A) A hole-making implement for planting field-grown citrus nursery trees. B) The hole formed is 30 wide by 15 inches deep with a platform 4 inches below the soil surface.

87

The advantage of this implement for setting field-grown trees, particularly when they are being planted rapidly, is that the hole is self directing for root placement. Container-grown citrus nursery trees have the capacity to develop a typical root system as exemplified by a tree from Expt 1 (Table 1). This tree was raised in a coneshaped container and transplanted into a fabric bag using the soil removed when the bag was set in the ground. The root ball was left intact. The tree was excavated after 18 months and the root system appeared normal (Fig. 4). Sub stantial root growth had occurred away from the root ball. Therefore, the poor root development noticed in the con tainerized trees of other experiments was not the result of the container per se. It is more likely that some trees were container-bound which restricted root system expansion.

Fig. 4. Root system development in a container-grown nursery tree 18 months after being transplanted into a larger fabric bag previously set in the ground.

Scott Hedden (U. S. Dept. of Agriculture, retired) de signed and built an implement that forms a hole providing a central area for placing the tap root(s) and a platform 4 inches below the soil surface for the lateral roots (Fig. 3).

Container binding easily occurs when roots develop in a circular pattern in round pots. A tree from Expt 2 (Table 1) illustrates a severe and classical instance (Fig. 5). The root ball was not disturbed during planting. Many roots continued to thicken after transplanting but there was al most no root system expansion. The round, rigid pot has been replaced in the Florida citrus nursery industry with a square, rigid container, and plastic bags. Each of these containers is about 4 to 5 inches in diameter and 12 to 16 inches deep with a nominal vol ume of 0.5 to 0.8 gal. These containers can bind a root system and their relatively small volume probably reduces

Fig. 6. Root system development of a 'Star Ruby' grapefruit tree on Carrizo citrange rootstock. The tree was grown in a 0.9 gal plastic bag Fig. 5. Root system development 3.5 yr after field planting of a se

verely

container-bound

rootstock.

88

'Valencia'

orange

tree on

Carrizo citrange

and then transplanted into a 10 gal container using Field soil. The mat

(m) of roots present at the time of transplanting showed little expansion after 1 yr.


the time before this condition develops. An example of undesireable root development is the thin mat of roots which formed at the base of the plastic bag used in Expt 3 (Table 1). These roots were not removed or disturbed dur ing transplanting. When the trees were examined later, this mat was still intact (Fig. 6). Some roots grew from the mat, but new growth often came from adventitious roots which emerged along the trunk above the root ball. Re moval of the mat at planting would probably enhance root system development. The new containers seem to encourage fibrous root growth rather than woody roots (Fig. 7) as compared with field-grown trees which have the typical root system de scribed earlier. The fibrous to woody root ratio may change with plant age while in the nursery and differ ac cording to the scion/rootstock combination and other fac tors. In the containerized nursery, if plant age and con tainer volume affect the "type" of nursery tree root system, then this might help to explain some of the variation re ported in the behaviour of container-grown trees in com mercial orchards. The successful establishment and subsequent field growth of citrus nursery trees, regardless of production method, requires root system expansion and development. From the limited observations reported herein, tree growth would generally be regarded as satisfactory except under extreme circumstances of root system mishandling because of poor planting or the failure to treat a containerbound condition. Literature Cited 1. Bevington, K. B. and W. S. Castle. 1982. Development of the root system of young 'Valencia' orange trees on rough lemon and Carrizo citrange rootstocks. Proc. Fla. State Hort. Soc. 95:33-37.

Fig. 7. Typical root system development of a citrus nursery tree grown

in a 0.5 gal plastic bag using a medium with 50% peat.

2. Castle, W. S. 1978. Citrus root systems: Their structure, function and relationship to tree performance. Proc. Intern. Soc. Citriculture 1:6269.

3. Castle, W. S. and C. O. Youtsey. 1977. Root system characteristics of citrus nursery trees. Proc. Fla. State Hort. Soc. 90:39-44.

4. Castle, W. S. and J. J. Ferguson. 1982. Current status of greenhouse and container production of citrus nursery trees in Florida. Proc. Fla. State Hort. Soc. 95:42-46.

5. Castle, W. S., W. G. Adams, and R. L. Dilley. 1979. An indoor, con tainer system for producing citrus nursery trees in one year from seed. Proc. Fla. State Hort. Soc. 92:3-7. 6. Wheaton, T. A., W. S. Castle, D. P. H. Tucker, and J. D. Whitney. 1978. Higher density plantings for Florida citrus-concepts. Proc. Fla. State Hort. Soc. 91:27-33.

Proc. Fla. State Hort. Soc. 100:89-93. 1987.

GROWTH OF BARE-ROOTED AND CONTAINER-GROWN 'HAMUN' ORANGE TREES IN THE FIELD Thomas E. Marler and Frederick S. Davies

of media removal on growth over one season. Bare-rooted

University of Florida, IFAS Fruit Crops Department

trees were significantly larger than container-grown trees 8

Gainesville, FL 32611

of more uniform size were used, bare-rooted trees were sig nificantly larger than container-grown trees 8 months after planting. After 18 months, trunk cross sectional area of barerooted trees remained significantly larger, but canopy volume was similar. Removal of medium from container-grown trees improved growth the first season, especially root growth, suggesting that it is important to select large nursery trees

Additional index words. Citrus sinensis, nursery. Abstract. Bare-rooted and container-grown 'Hamlin' orange trees [ Citrus sinensis (L.) Osb.] on sour orange rootstock (C. aurantium L) were planted in double-row beds to compare

and 20 months after planting in experiment one. When trees

growth for the first 2 years under the same edaphic, cultural

with healthy root systems and to break up the root ball prior

and environmental conditions. In experiment one, standard nursery trees were used, with bare-rooted trees being larger than container-grown trees at planting time. In a second ex periment, trees of more uniform size were used. In a third experiment, container-grown trees were planted after remov ing all, 1/2, or no medium prior to planting to study the effect

to planting to achieve optimum growth for container-grown

Florida Agricultural Experiment Station Journal Series No. 8558.


trees.

Florida nurserymen have been producing bare-rooted citrus trees in field nurseries for many years. Recently, however, citrus trees have also been produced in various types of containers in the greenhouse (2). Advantages of greenhouse systems include greater control over the pro89