5. Bellinder, R. R., K. K. Hatzios, and H. P. Wilson. 1985. Mode of
action investigations with the herbicides Hoe 39866 and SC-0224.
Weed Sci. 33:779-785.
6. Kapusta, G. 1981. Hoe-661: A new herbicide for the control of veg
etation in no till fields. Proc. North Centr. Weed Contr. Conf. 36:92.
7. Labrada, R., J. Hernandez, and J. Baez. 1987. Evaluation of her
bicides for Cyperus rotundus control. Weed Abstr. 36:159.
8. Langeluddeke, P., W. Bubl, H. P. Huff, U. Kotter, and F. Wallmuller.
1985. Glufosinate-ammonium (Hoe 39866): New results on weed
control and crop tolerance in orchards. Proc. British Crop Prot. Conf.
Weeds (1985). 3:1047-1057.
9. Lawson, H. M. and J. S. Wiseman. 1985. Evaluation of glufosinateammonium for runner control in strawberries. Proc. British Crop
Prot. Conf. Weeds (1985). 3:1081-1085.
10. Twitrosoedirdjo, S. 1984. The conversion oilmperata dominated veg
etation into productive systems. Trop. Newsletter 45:6-7.
11. Wilson, H. P., T. E. Hines, R. R. Bellinder, and J. A. Grande. 1985.
Comparisons of Hoe-39866, SC-0224, paraquat and glyphosate in
no-till corn (Zea mays). Weed Sci. 33:531-536.
Proc. Fla. State Hort. Soc. 100:61-64. 1987.
GROWTH OF YOUNG 'HAMLIN' ORANGE TREES USING STANDARD
AND CONTROLLED-RELEASE FERTILIZERS
T. E. Marler, J. J. Ferguson, and F.S. Davies
University of Florida, IFAS
Fruit Crops Department
Gainesville, FL 32611
Abstract. Controlled-release fertilizers were compared with
a standard fertilizer on 1- and 2-year 'Hamlin' orange
[ Citrus sinensis (L.) Osb.] on sour orange (C. aurantium
L.) trees planted on double-row beds. All fertilizers con
sisted of an 8N - 2.6P - 6.6K - 2Mg - 0.2Mn - 0.12Cu 0.2Zn - 1.78Fe formulation. No differences were found in
tree growth over two years in two separate experiments
for comparisons between the following treatments: (A)
isobutylidene diurea (IBDU) and Wonder Gro (WG)
applied twice per year (year 1, 2 lb./tree/application; year
2, 4 lb./tree/application); (B) standard fertilizer applied
four times/year (year 1, 1 lb./tree/application; year 2, 2 lb./
tree/application). In a separate study, standard fertilizer
was applied four times/year in one experiment and five
times/year in another experiment at 0.5, 1.0, or 1.5 lb. of
material/tree/application (middle rate equivalent to aver
age recommended rate). No differences were found in
growth, suggesting that optimum fertilizer rates may be
lower in some cases than those currently recommended
for young citrus trees.
Efficiency of fertilizer use can be expressed as the per
centage of applied nutrients recovered by the crop. Nitro
gen is the most important nutrient in a citrus fertilization
program and the nutrient with the most variability in effi
ciency of recovery. Nitrogen losses due to erosion, leach
ing, denitrification, and volatilization reduce N availability
for plant uptake. Sandy soils and heavy rainfall in Florida
are frequently associated with substantial N losses, espe
cially through leaching. The problem is greater in areas
where high water tables limit rooting depth. Concerns over
energy conservation and ground water pollution, com
bined with the competitive pressure to reduce production
costs in the Florida citrus industry (2) make reduction of
applied fertilizer losses desirable.
Controlled-release fertilizers potentially reduce N los
ses, improving efficiency of plant recovery (6, 10). Fewer
applications are needed (4, 7, 10), which reduces labor and
equipment costs and soil compaction by equipment.
Florida Agricultural Experiment Station Journal Series No. 8559.
Proc. Fla. State Hort. Soc. 100: 1987.
Controlled-release fertilizers have been used on many
horticultural crops (10), including citrus. These sources in
creased fruit production on mature citrus (7) and growth
of young containerized citrus (3, 6) when compared to
more soluble fertilizer sources. In contrast, growth of
young 'Orlando' tangelo trees was comparable for control
led-release sulfur-coated urea and soluble sources, but fre
quency of application was reduced by 50% (4). Neverthe
less, acceptance of controlled-release fertilizers by the
Florida citrus industry has been limited (5), primarily be
cause of higher fertilizer costs and lack of grower experi
ence with these materials.
Current fertilizer recommendations (8) for young cit
rus trees have been based on previous studies (1, 11), ob
servations, and industry trends. Recommendations call for
0.06 to 0.10 lb. N/tree (average = 0.08 lb.) for newlyplanted citrus, applied five to six times per year (8). Fer
tilizer is usually broadcast evenly in a 3 ft diameter circle,
which translates to ca. 3,000 lb. N/year per treated acre.
Young citrus trees require an adequate supply of nutrients
to optimize growth; however, this rate is 10 times more
than recommended levels for mature trees and may be
excessive
under some
circumstances.
Rasmussen and
Smith (11) also expressed concern that young trees were
being over-fertilized and recommended reduced applica
tion frequency and rates.
Objectives of this study were to compare the effects of
commonly available controlled-release and standard fer
tilizers on growth of young citrus trees, and to determine
the effects of three rates of application of standard fer
tilizer on leaf nutrient levels and tree growth.
Materials and Methods
Four field experiments were conducted at the Horticul
tural Unit NW of Gainesville, Florida, using 'Hamlin'
orange on sour orange rootstock. Double row beds, 55 ft
wide and 2-2.5 ft in height, were constructed in March,
1985. Soil type was Kanapaha sand (loamy, siliceous,
hyperthermic, Grossarenic, Paleaquult) underlain by a
hardpan. Two tree rows 25 ft apart were used on each bed
with trees set 11 ft apart. Irrigation was applied by 90
degree, 10 gallons-per-hour microsprinklers located ca.
3.25 ft NW of tree trunks. Available soil moisture was
maintained at optimum conditions (20% soil moisture de
pletion; Marler, T.E., Univ. of Florida, unpublished).
61
when measured as trunk cross sectional area, canopy vol
ume, fresh and dry weight, new root growth, and total
shoot length. Trunk cross sectional area averaged 0.66 and
2.29 inch2 after 8 and 20 months, respectively.
Smaller trees were obtained for experiment two with
initial trunk cross sectional area averaging 0.41 inch2. This
initially smaller size was reflected in ultimate tree size after
8 and 18 months when compared to experiment one
(Table 1). Again, no differences among fertilizer sources
were found. Trunk cross sectional area averaged 0.54 and
1.44 inch2 after 8 and 18 months, respectively. Measure
ments of canopy volume, fresh and dry weight, new root
growth, and total shoot length followed a similar pattern
to experiment one, with no differences among fertilizers.
Isobutylidene diurea and Wonder Gro may be used to
reduce application frequency by 50% without decreasing
growth. Sulfur-coated urea has been used on young citrus
trees with similar results (4). The feasibility of using controlled-release fertilizers in a young tree care program
should be determined on a case-by-case basis, since controlled-release materials are more expensive than more soluble
fertilizers. However, reduction in application frequency
and costs could be realized for replants in bearing groves.
Fertilizer rates
Application of 0.5, 1.0, or 1.5 lb. of fertilizer per tree
four times throughout the season resulted in no difference
in growth of the bare-rooted trees in experiment one
(Table 2). Trunk cross sectional area increased from 0.17
to 0.84 inch2 from May to December, 1985. Canopy vol
ume, fresh weight, and dry weight averaged 19.53 ft3, 5.17
lb., and 1.98 lb., respectively. Fertilizer rate had little influ
ence on leaf analyses, as no consistent relationship among
treatments existed for all elements (Table 3). Levels of
most elements were in the optimum or high range (8) in
all cases except potassium for the lower two rates and zinc
for all three rates. Leaf N was optimum for the lower two
rates, and ranged between optimum and high for the 1.5
lb./tree rate.
Fertilizer rate did not significantly affect tree growth in
experiment four (Table 2). These container-grown trees,
although originally larger in trunk diameter than the barerooted trees of experiment three, had limited growth from
March to October, 1987. This slow initial growth of some
container-grown trees was not related to fertilizer rate and
Table 2. Effects of standard fertilizer rate on growth of young 'Hamlin'
orange trees in the field/
Rate per tree
Expt
Expt three
(Bare-root)
Expt four
(Container)
Lb. fert.
per applic.x
Lb.N
TCAy
Canopy
volume
Total
fresh wt
Total
dry wt
peryr
(inch2)
(ft3)
(lb.)
(lb.)
22.97
16.87
18.74
5.29
1.94
4.71
5.52
2.12
0.5
0.16
0.80
1.0
0.32
0.48
0.85
1.5
0.5
0.20
1.0
0.40
1.5
0.60
0.26
0.25
0.26
0.86
1.88
2.03
2.00
1.95
zNo significant differences among treatments.
yTCA = trunk cross sectional area.
xLb. of 8N - 2.6P - 6.6K - 2Mg - 0.2Mn - 0.12Cu - 0.2Zn - 1.78Fe per tree
per application.
Proc. Fla. State Hort. Soc. 100: 1987.
Table 3. Influence of standard fertilizer rate on leaf analysis.
Dry wt (%)
ppm
Rate*
N
P
K
Ca
Mg
0.5
2.56
0.87
1.06
0.51
91.88
14.63 36.25
4.81
2.58
0.13
0.14
4.48
1.0
4.11
0.50
106.25
18.00 45.44
5.31
1.5
2.78
0.14
0.51
4.50
0.01
0.19
0.02
100.63
4.36
17.00 48.56
0.09
1.20
0.05
3.70
SEy
Fe
Zn
1.09
Mn
6.05
Cu
0.25
zLb. of 8N - 2.6P - 6.6K - 2Mg - 0.2Mn - 0.12Cu - 0.2Zn - 1.78Fe per tree
per application.
ySE = standard error, n = 8 samples of 15 leaves from each of two trees.
has been seen in other experiments on the same site (9).
Trunk cross sectional area increased from 0.21 to 0.26
inch2 and final canopy volume was 1.99 ft3. These data
indicate a reduction in currently recommended fertilizer
rates (8) for 1- and 2-year-old citrus may be possible in
some situations without any reduction in plant growth or
leaf nutrient status. The low rate of 0.5 lb. fertilizer/tree/
application (0.16 to 0.20 lb. N/year) was adequate under
these circumstances.
Rasmussen and Smith (11) suggested that 0.16 lb. N/
year for the first 2 years after planting was adequate for
young citrus. Their study was conducted in Lake and Pasco
Counties using large, bare-rooted trees with trunk diamet
ers of over 2 inches after 1 year. In contrast, Calvert (1)
reported that trees responded more favorably to 0.48-0.72
lb. N/year than 0.24 lb. when grown on raised beds on
marginal soil, illustrating the importance of location and
soil type in determining fertilizer rates for young trees.
In previous studies (1, 11) two to three applications of
fertilizer per season were adequate because growers were
less likely to "push" trees throughout the season. Cur
rently, however, many growers in southern locations fer
tilize and water trees continuously, choosing to overlook
potential dangers of cold damage. In this case, a 1-year-old
tree may be as large as a 2-year-old tree in more northerly
regions where fertilization is discontinued in September to
reduce the possibility of cold damage. Our findings are
more applicable to the situation for northern citrus areas,
suggesting that fertilizer rates may be reduced and controlled-release materials substituted for standard practices
particularly in scattered reset situations where application
costs are high. We are currently evaluating the effective
ness of controlled-release materials in southern flatwoods
and the Indian River areas of the state.
Literature Cited
1. Calvert, D. V. 1969. Effects of rate and frequency of fertilizer appli
cations on growth, yield and quality factors of young 'Valencia'
orange trees. Proc. Fla. State Hort. Soc. 82:1-7.
2. Fairchild, G. F. and M. G. Brown. 1986. Economic factors affecting
Florida fresh fruit. Proc. Fla. State Hort. Soc. 99:78-81.
3. Fucik, J. E. 1974. Potting mixes and fertilizer tablets for containergrown citrus. J. Rio Grande Valley Hort. Soc. 28:143-148.
4. Jackson, L. K. and F. S. Davies. 1984. Mulches and slow-release fer
tilizers in a citrus young tree care program. Proc. Fla. State Hort.
Soc. 97:37-39.
5. Jackson, L. K., W. R. Summerhill, and J. J. Ferguson. 1986. A survey
of young citrus tree care practices in Florida. Proc. Fla. State Hort.
Soc. 99:44-46.
6. Khalaf, H. A. and R. C. J. Koo. 1983. The use of controlled release
nitrogen on container grown citrus seedlings. Citrus Veg. Mag.
46(9): 10,32.
7. Koo, R. C. J. 1986. Controlled-release sources of nitrogen for bearing
citrus. Proc. Fla. State Hort. Soc. 99:46-48.
63
Koo, R. C. J., C. A. Anderson, I. Stewart, D. P. H. Tucker, D. V.
Calvert, and H. K. Wutscher. 1984. Recommended fertilizers and
nutritional sprays for citrus. Fla. Agr. Expt. Sta. Bui. 536D.
Marler, T. E. and F. S. Da vies. 1987. Growth of bare-rooted and
containerized 'Hamlin' orange trees in the field. Proc. Fla. State Hort.
Soc. 100:93-95.
10. Maynard, D. N. and O. A. Lorenz. 1979. Controlled-release fertilizers
for horticultural crops. Hort. Reviews 1:79-140.
11. Rasmussen, G. K. and P. F. Smith. 1961. Evaluation of fertilizer prac
tices for young orange trees. Proc. Fla. State Hort. Soc. 74:90-95.
12. Westwood, M. N. 1978. Temperate zone pomology. W.H. Freeman
& Co. New York.
Proc. Fla. State Hort. Soc. 100:64-66. 1987.
SCREENING SWEET ORANGE CITRUS CULTIVARS FOR RELATIVE
SUSCEPTIBILITY TO PHYTOPHTHORA FOOT ROT
G. S. Smith, D. J. Hutchison, and C. T. Henderson
U.S. Department of Agriculture,- ARS
2120 Camden Road, Orlando, FL 32803
Additional index words. Citrus sinensis, Phytophthora parasitica.
Abstract. Nine sweet orange ( Citrus sinensis (L.) Osbeck) cultivars, Bedmar Vernia, Koethen, Madam Vinous, Natal, Ridge
Pineapple, Ruffert, Sanguine Grosse Ronde, Valencia, and a
sweet orange seedling, were evaluated for relative suscepti
bility to foot rot using three different inoculum methods. Two
months after inoculation, stem-girdling ratings (scale 0 to 5)
and extent of lesion development (relative lesion area) were
measured. Bedmar Vernia was rated as tolerant as Carrizo
citrange ( Poncirus trifoiiata (L) Raf. x C. sinensis), Cleopatra
mandarin (C. reticulata Blanco), rough lemon (C. limon
Burm. f.) and sour orange (C. aurantium L). Madam Vinous,
Koethen, Sanguine Grosse Ronde, Ruffert, and Ridge Pineap
ple cultivars were rated the most susceptible, while Natal,
sweet seedling, and Valencia may possess some degree of
tolerance of foot rot. A sweet orange cultivar that is less sus
ceptible to phytophthora foot rot could be useful as a poten
tial rootstock especially if it is tolerant of tristeza and blight.
Foot rot or gummosis of citrus is caused by the soil-in
habiting fungi Phytophthora parasitica Dast. and P. citrophthora (R. E. Sm. & E. H. Sm.) Leonian. Symptomatic
trunk lesions are characterized by gumming, bark splitting
and peeling, and discolored cambial tissue. Lesions may
girdle the trunk causing debilitation and tree death.
Management strategies for phytophthora foot rot rely
on host resistance in the rootstock, sound cultural prac
tices, and fungicide applications to the foliage, trunk, or
soil (7). Resistance to foot rot, however, is only one of sev
eral factors affecting the commercial utilization of a
rootstock in citrus production. Tolerance of virus and
nematode diseases, degree of cold hardiness, and horticul
tural performance are important considerations in select
ing a rootstock (4).
In the U.S. Department of Agriculture rootstock pro
gram in Florida, citrus hybrids are initially screened for
foot rot resistance primarily because of the ease of the
screening process whereby large numbers of selections can
be screened as young plant material (4). Since 1960, more
than 5,000 selections have been screened for foot rot resis
tance. Approximately 95-98% of these selections were
eliminated because they were rated as more susceptible
than Carrizo rootstock with a moderate level of resistance
to phytophthora foot rot.
64
The screening process used mycelial fragments, zoospores, or sporangia on mycelial agar disks for root and
foot rot inoculations (1, 2). The inoculum potential of zoospore suspensions and agar disks used in screening experi
ments, however, is not accurately quantifiable and may be
greater than that present in field conditions (7, 8). Thus,
the screening process may be capable of detecting only a
high degree of tolerance or resistance to foot rot.
Sweet orange, while considered highly susceptible to
foot rot, possesses tolerance of citrus tristeza virus, citrus
blight, and citrus burrowing nematode (5). Management
of these diseases requires almost exclusive reliance on to
lerant or resistant rootstocks. Foot rot, however, can be
effectively managed with cultural practices and systemic
fungicides (7, 10). Thus, selection of a sweet orange cul
tivar as a rootstock that possesses some degree of tolerance
of foot rot may be a viable alternative in certain citrus-pro
ducing regions.
This paper reports the results of a screening experi
ment to assess the relative susceptibility of nine sweet
orange cultivars to foot rot compared with Carrizo cit
range, Cleopatra mandarin, rough lemon, and sour
orange.
Materials and Methods
Plants were grown in a soilless medium in the green
house for 6 months. Soil and ambient temperatures ranged
from 22-28°C and 24-32°C, respectively. Seedlings of the
following citrus cultivars and hybrids were used in the sc
reening experiment: Carrizo, Cleopatra, rough lemon,
sour orange, Bedmar Vernia, Koethen, Madam Vinous,
Natal, Ridge Pineapple, Ruffert, Sanguine Grosse Ronde,
a sweet seedling, and Valencia.
An isolate of P. parasitica mating type A2 was obtained
from a foot rot lesion on a 3-yr-old rough lemon tree grow
ing near St. Cloud, FL. Inoculum sources consisted of: 1)
0.5-cm-diam. agar disks, 2) zoospore suspensions, and 3)
chlamydospore-infested soil. Agar disks of uniform thick
ness were cut with a #3 cork borer from the edge of 7- to
10-day-old P. parasitica cultures grown on carrot agar (5%
finely grated carrots, 1.8% agar: w/w) at 25°C in the dark.
Zoospores were produced by the method of Henderson et
al. (3). Zoospore densities were estimated with a
hemacytometer and the suspension diluted to deliver 1,000
zoospores/ml. Inoculum suspensions were serially plated
on PARPH selective medium (8) (10 mg pimaricin, 125 mg
ampicillin, 10 mg rifampicin, 100 mg pentachloronitrobenzene, and 50 mg hymexazol, 17 g corn meal agar/L) before
Proc. Fla. State Hort. Soc. 100: 1987.