on Swingle citrumelo (Egel et al., 1991; Graham et al.,
1990; Graham and Gottwald, 1990). In a recent outbreak,
we observed substantial leaf-spotting and stem necrosis on
'Henderson' and 'Flame' red grapefruit budlings that were
exposed to the moderately aggressive strain from adjacent
rows of lightly-infected Swingle citrumelo (unpublished
observations). In some cases, infected stems of grapefruit
budlings were severely weakened by necrosis and broke
off at the base.
Currently, Florida citrus packinghouses are required
to surface-disinfest all fruit shipped from citrus canker
quarantine areas in the state (Anonymous, 1987). As previ
ously shown with the surrogate, X. campestris pv. vesicatoria
(Brown and Schubert, 1987), SOPP added during the
washing of the fruit was very effective in reducing the
number of viable bacteria on the fruit surface to a low level
if not in eradicating X. c. citrumelo. Brown and Schubert
(1987) discussed the advantages of adding the disinfectant
during the washing process as the physical action of the
brushes disrupt and remove surface organic matter that
improves the exposure of the fruit to the disinfectant. We
found that the washing alone removed over 99% of the
bacteria but did not possess the eradicant action of SOPP.
Brown and Schubert (1987) also demonstrated that treat
ments of SOPP applied during a 30 sec period were as
effective as longer exposures, and therefore would not dis
rupt the orderly flow of fruit through the packinghouse.
Also, SOPP is a proven fungicide (Eckert and Sommer,
1967) and could be used for the dual purpose of decay
control and bacterial eradication.
Based on current knowledge, fruit treatment for X. c.
citrumelo is unnecessary for several reasons. CBS has never
been found on commercial citrus fruit cultivars, only on
the rootstock cultivar 'Flying Dragon' trifoliate orange in a
nursery (Gottwald et al., 1988). Fruit cultivars develop a
resistant reaction when the fruit rind is artificially inocu
lated with X. c. citrumelo (Graham et al., 1992). The bac
terium does not survive in the lesions more than 30 to 60
days and, therefore, are not present on the fruit at harvest
(Graham et al., 1992).
Literature Cited
Anonymous. 1987. Citrus canker action plan for the state of Florida. Fla.
Dept. Agr. Consumer Serv., Div. Plant Ind. and USDA/APHIS. 151 pp.
Brown, G. E. and T. S. Schubert. 1987. Use of Xanthomonas campestris pv.
vesicatoria to evaluate surface disinfectants for canker quarantine treat
ments to citrus fruit. Plant Dis. 71:319-323.
Eckert, J. W. and N. F. Sommer. 1967. Control of diseases of fruits and
vegetables by postharvest treatment. Annu. Rev. Phytopathol. 5:391-
432.
Egel, D. S., J. H. Graham, and T. D. Riley. 1991. Population dynamics
of strains of Xanthomonas campestris differing in aggressiveness on
Swingle citrumelo and grapefruit. Phytopathology 81:666-671.
Goto, M. 1972. Survival of Xanthomonas citri in the bark tissues of citrus trees.
Gottwald, T. R. and J. H. Graham. 1990. Spatial pattern analysis of citrus
bacterial spot epidemics in Florida citrus nurseries. Phytopathology
80:181-190.
Gottwald, T. R., J. H. Graham, and S. M. Ritchie. 1992. The relationship
of leaf surface populations of strains of Xanthomonas campestris pv.
citrumelo to development of citrus bacterial spot and persistence of
disease symptoms. Phytopathology 82 (accepted).
Gottwald, T. R., J. C. Miller, R. H. Brlansky, D. W. Gabriel and E. L.
Civerolo. 1988. Analysis of the spatial distribution of citrus bacterial
spot in a Florida citrus nursery. Plant Dis. 73:297-303.
Graham, J. H. and T. R. Gottwald. 1990. Variation in aggressiveness of
Xanthomonas campestris pv. citrumelo associated with citrus bacterial spot
in Florida ciltrus nurseries. Phytopathology 80:190-196.
Graham, J. H. and T. R. Gottwald. 1991. Research perspectives on eradi
cation of citrus bacterial diseases in Florida. Plant Dis. 75:1193-1200.
Graham, J. H., T. R. Gottwald, and D. Fardelmann. 1990. Cultivar-spe-
cific interactions for strains of Xanthomonas campestris from Florida
that cause citrus canker and citrus bacterial spot. Plant Dis. 74:753-
756.
Graham, J. H., T. R. Gottwald, T. D. Riley, and M. A. Bruce. 1992.
Susceptibility of citrus fruit to citrus bacterial spot and citrus canker.
Phytopathology 82 (in press).
Muraro, R. P. 1989. Potential economic benefits of defoliation vs. com
plete destruction for the eradication of citrus canker infected trees.
Food and Resource Economics EN-11, Univ. Fla., IFAS, Gainesville.
Pohronezny, K., M. A. Moss, W. Dankers, and J. Schenk. 1990. Dispersal
and management of Xanthomonas campestris pv. vesicatoria during thin
ning of direct-seeded tomato. Plant Dis. 74:800-805.
Timmer, L. W. 1988. Evaluation of bactericides for control of citrus
canker in Argentina. Proc. Fla. State Hort. Soc. 101:6-9.
Timmer, L. W., T. R. Gottwald, and S. E. Zitko. 1991. Bacterial exudation
from lesions of Asiatic citrus canker and citrus bacterial spot. Plant
Dis. 75:192-195.
Proc. Fla. State Hort. Soc. 104:173-178. 1991.
RELATIONSHIP OF CITRUS ROOTSTOCK TO PHYTOPHTHORA ROOT ROT AND
POPULATIONS OF PHYTOPHTHORA PARASITICA
L. W. Timmer, J. P. Agostini, J. H. Graham,
and W. S. Castle
University of Flordia, IFAS
Citrus Research and Education Center
700 Experiment Station Road
Lake Alfred, FL 33850
Additional index words.
Abstract. Inoculations of citrus rootstocks with chlamydospores
Florida Agricultural Experiment Station Journal Series No. N-00472.
This research was supported in part by Ciba-Geigy Corp.,
Greensboro, NC 27419. We gratefully acknowledge the excellent techni
cal assistance of S. E. Zitko and H. A. Sandier.
Proc. Fla. State Hort. Soc. 104: 1991.
of P. parasitica in the greenhouse produced the most fibrous
root rot on sweet orange (SwO), sour orange (SO), Carrizo
citrange (CC), and Cleopatra mandarin (CM), less on Vol-
kamer lemon (VL) and least on trifoliate orange (TO) and
Swingle citrumelo (SC). Propagule densities from rootstock
seedlings grown in pots of infested soil were greatest on SwO
and SO, less on CM, and least on TO and SC. The effects of
inoculum density and metalaxyl treatment were evaluated in
a pot test on SwO, SO, and SC budded with 'Pineapple' sweet
orange. Inoculation of SC with P. parasitica produced little
root rot and had no effect on growth. Fungicide treatment did
not affect growth of trees on this rootstock. On SwO and SO,
root rot increased and growth decreased as inoculum density
increased. Metalaxyl treatment reduced root rot and increased
growth of trees on these 2 rootstocks. In field rootstock trials
173
in Avon Park and St. Cloud, propagule densities of the fungus
were highest on Palestine sweet lime, SwO, and VL; lower on
CM; and lowest on TO and SC. Most of the common rootstocks
used in Florida with the exception of sweet orange are toler
ant to bark infection; however, all except SC and TO are sus
ceptible to fibrous root rot.
Root rot, caused by Phytophthora parasitica Dast., is a
common problem in Florida citrus groves and is occasion
ally severe in young plantings. Most of the commonly used
rootstocks in Florida are tolerant to bark infection whereas
most scion varieties are moderately to highly susceptible
(Castle et al., 1989). Bark infection of citrus is seldom seen
below the budunion except where sweet orange is used as
a rootstock. The degree of susceptibility of most commer
cial rootstocks to root rot is well-established (Castle et al.,
1989; Graham and Timmer, 1991).
Phytophthora parasitica also causes fibrous root rot which
affects most citrus rootstocks. Traditionally, this has been
considered primarily a problem in seedbeds and nurseries
where frequent irrigation and high planting densities
create favorable conditions for the disease. Fibrous root
rot also occurs in bearing orchards but its importance there
has been difficult to assess. Long-term fungicide treat
ments have reduced Phytophthora populations, increased
fibrous root densities, and in some instances, increased
yield, and the size and juice content of fruit (Sandier et al.,
1989; Timmer et al., 1989). Thus, in some situations, fibr
ous root losses caused by Phytophthora may be significant.
Evaluations of the susceptibility of citrus species and
hybrids to fibrous root rot caused by Phytophthora have
been conducted in the past (Carpenter and Furr, 1962;
Grimm and Hutchison, 1977; Smith et al., 1987;
Whiteside, 1974). Many of the techniques used produced
severe infection which served to eliminate susceptible en
tries in breeding programs; however, ratings often did not
correspond well to field experience.
The purpose of the studies reported herein was to as
sess the susceptibility of commercial rootstocks to fibrous
root rot using greenhouse, screenhouse, and field experi
ments. Portions of these studies have been previously pub
lished in greater detail (Agostini et al., 1990; Graham,
1990).
Materials and Methods
Greenhouse inoculation. Seedlings of the following
rootstocks were grown for 6 months in soilless medium:
trifoliate orange (TO) (Poncirus trifoliata (L.) Raf), Ridge
Pineapple sweet orange (SwO) (Citrus sinensis (L.) Osb.),
Carrizo citrange (CC) (C. sinensis x P. trifoliata), Swingle
citrumelo (SC) (C. paradisi Macf. x P. trifoliata), sour orange
(SO) (C. aurantium L.), Cleopatra mandarin (CM) (C. re-
ticulata Blanco), and Volkamer lemon (VL) (C. volkameriana
Pasq.). For inoculation, chlamydospores were produced by
the method of Tsao (1971) and mixed with moist, auto-
claved Candler fine sand. The inoculum mix was incubated
for several days and propagule densities determined by
plating on the selective medium, PARPH, developed by
Kannwischer and Mitchell (1978) using the methods de
scribed by Timmer et al. (1988b). The inoculum was mixed
with autoclaved Candler fine sand to achieve a density of
10 propagules/cm3.
Seven seedlings of each rootstock were transplanted to
infested soil in about 2.5-liter pots and arranged in a ran
domized block design on the greenhouse bench. Pots were
flooded 3 days each week by placing a dish under the pot
and fililng it with water. After 6 weeks, root rot was rated
on a scale of 1 = no root rot to 11 = all fibrous roots
rotted. Roots were dried to a constant weight and data
expressed as the ratio of root weight of inoculated seedl
ings to the non-inoculated controls of the same variety.
Screenhouse experiment. Seedlings of TO, SwO, SO, CM,
and SC were grown for 18 months in an artificial potting
mix. Nine seedlings of each rootstock were transplanted to
about 5-liter pots in a soil naturally infested with P.
parasitica collected from a grove near St. Cloud. Nine pots
of soil were prepared as unplanted controls. Rootstocks
were arranged in a randomized complete block design on
benches in a screenhouse and watered as needed. Soil cores
were removed from the pots monthly and assayed for
propagule densities on PARPH selective media
(Kannwischer and Mitchell, 1978; Timmer et al., 1988b).
Mean propagule densities were calculated over the 8
months of the experiment. At the end of the experiment,
the percentage of root rot was determined by counting the
number of rotted roots of 50 roots selected at random in
each quadrant of each root system.
Inoculum density and fungicide effects. This experiment
was established to determine the effects of inoculum den
sity and fungicide application on the growth of rootstocks
of differing susceptibility to fibrous root rot. It was de
signed with 3 factors each at 3 levels: rootstock—sweet
orange, sour orange, and Swingle citrumelo; inoculum
density of Phytophthora parasitica—0, 1, and 10 propagules
per cm3 soil and frequency of metalaxyl application—0, 4,
and 8 times per year.
Infested soil was collected from a citrus grove near St.
Cloud and a portion of it was autoclaved. Propagule deter
minations in infested soil were made by plating on PARPH
and batches of soil with the desired propagule densities
prepared by mixing sterilized and infested soil. Fifty-four
uniform, small seedlings of each rootstock were selected
and one-third planted in each inoculum density in 15-cm
diameter pots. One-third of each group was then not
treated or received metalaxyl every 6 weeks (8 times/yr) or
every 12 weeks (4 times/yr) using a solution of 50 mg/liter
and about 100 ml/pot. Treatments began 10 days after the
seedlings were potted. Six single-plant replicates of the in
dividual treatments were used in a 3 x 3 x 3 factorial ar
rangement.
The experiment was established in June 1989. Plants
with different propagule densities were placed on separate
benches in a screenhouse to avoid cross contamination and
rootstock and metalaxyl treatments were randomized on
each bench. Pots were flooded for 2 days immediately after
planting. In Apr. 1990, all seedlings were budded with
Tineapple' sweet orange and the seedling tops lopped off,
dried, and weighed. When the experiment was terminated
in Sept. 1990, the percentage of rooted roots was deter
mined by counting the number of tips rotted on 100 roots
per plant. Shoots and roots were collected, oven-dried, and
weighed. Determinations of propagule densities were
made by collecting 4 1-cm diameter soil cores from each
pot and plating on PARPH in Nov. 1989 and in Mar. and
Sept. 1990. Since propagule densities were still low in Mar.
174 Proc. Fla. State Hort. Soc. 104: 1991.
Table 1. Effect of greenhouse inoculation of citrus rootstock seedlings
with chlamydospores of Phytophthora parasitica on root rot severity and
root growth.
Rootstock
Sour orange
Carrizo citrange
Cleopatra mandarin
Sweet orange
Volkamer lemon
Swingle citrumelo
Trifoliate orange
Root rot
rating2
9.1 a
8.7 a
8.0 ab
7.9 ab
6.3 be
4.9 c
2.4 d
Reduction
in rootwt (%)
69
78
47
65
66
38
2
zRoot rot on a scale of 1 = healthy to 11 = all roots rotted; means
separated by Duncan's multiple range test, P ̂ 0.05.
Table 2. Propagule densities and root rot on citrus rootstock seedlings
potted in a field soil naturally infested with Phytophthora parasitica.
Rootstock
Sweet orange
Sour orange
Cleopatra mandarin
Trifoliate orange
Swingle citrumelo
Unplanted control
Propagules2
cm3
102 a
87 a
49 b
14 c
14 c
7 c
Root
rot (%)
74 a
55 b
26 c
13 c
13 c
-
zPropagule densities are the means of 7 sampling dates from June 1988
to Mar. 1989; mean separation in columns by Duncan's multiple range
test, P < 0.05.
1990, all plants were flooded for 2 days every 2 weeks
from Apr. to Aug. 1990.
Field studies of rootstock effects on propagule densities. Two
citrus rootstock experiments with 12-yr-old 'Valencia'
sweet orange on various rootstocks in Avon Park and St.
Cloud, Florida were selected as study sites. Both experi
ments were designed as split plots with preplant fumiga
tion as the main plot treatment in Avon Park and irrigation
as the main plot treatment in St. Cloud. Fumigation and
irrigation had only minor effects on populations of
Phytophthora (Agostini et al., 1990), and are not considered
in this report. There were 4 replications of the main plots
with 6 and 2 trees per subplot (rootstock) in Avon Park
and St. Cloud, respectively. Four soil cores were collected
from each of 2 trees per replicate, the 8 cores combined,
and a single determination of propagule density made for
each replication in Avon Park. In St. Cloud, 8 cores were
collected from each of the 2 trees per replication, compo
sited, a single determination made for each tree, and the
mean of the 2 trees calculated for the replicate mean. In
Avon Park, rootstocks sampled were: TO, SC, SwO, CM,
and Palestine sweet lime (PSL) (C. limettioides Tanaka) and
in St. Cloud they were: TO, SC, CM, PSL, and VL. Samples
were collected 5 times from Mar. 1988 to Feb. 1989 in
Avon Park and from Dec. 1987 to Dec. 1988 in St. Cloud.
Results
Greenhouse inoculation. When rootstock seedlings were
inoculated with 10 propagules/cm3 in the greenhouse, SO,
CC, CM, and SwO had the most severe root rot (Table 1).
All of the above suffered severe reduction in root weight
compared to the noninoculated control. VL had a lower
root rot rating than SO or CC, but still had 66% root loss.
SC had less root rot and loss than most of the others and
trifoliate orange was essentially unaffected.
Screenhouse experiment. When rootstock seedlings were
transplanted into naturally infested soil and grown for 9
months, SwO and SO maintained the highest propagule
densities and had the most root rot (Table 2). CM sup
ported intermediate propagule densities and root rot was
less than on SwO or SO. Propagule densities were no
higher on TO and SC than on nonplanted pots of soil.
Inoculum density and fungicide effects. Conditions in this
experiment were not highly favorable for disease develop
ment. Propagule densities on inoculated plants not treated
with fungicide were less than 3 per cm3 in Nov. 1989 and
about 1 per cm3 prior to budding in Mar. 1990. Biweekly
flooding after budding increased propagule densities, but
densities on inoculated, nontreated plants averaged less
than 10 per cm3 at the end of the experiment. Neverthe
less, significant treatment effects were observed in many
cases.
Rootstock and inoculum density significantly affected
most growth variables and the percentage root rot (Table
3). Metalaxyl application affected only root rot and root
dry weight. Significant interactions were observed in many
cases. The rootstock x inoculum density interactions were
probably attributable to the fact that inoculum density af-
Table
rot
Factor
3. Analysis of variance of the effect
, and final propagule densities.
of rootstock, inoculum
Seedling
dry wt
density of Phytophthora
Scion
dry wt
parasitica, and
Variable
Root
dry wt
metalaxyl application
Root
rot
on plant growth, root
Final
propagule density
Main effects
Rootstock (R)
Inoculum density (I)
Metalaxyl application (M)
Interactions
Rxl
RxM
IxM
RxIxM
***
n.s.
*
n.s.
n.s.
n.s.
***
n.s.
n.s.
n.s.
n.s.
*
n.s.
***
**
*
**
*
*
n.s.
**
***
***
*
***
**
n.s.
n.s.
***
n.s.
n.s.
n.s.
n.s.
n.s.
**** ** * _ significant atP< 0.001, < 0.01, and < 0.05, respectively or n.s. = not significant using analysis of variance of the 3 x 3 x 3 factorial experiment.
Proc. Fla. State Hort. Soc. 104: 1991. 175
Rootstockx
SwO
SO
SC
SwO
SO
SC
SwO
SO
SC
SwO
SO
SC
SwO
SO
SC
0
22.1
34.1
20.2
9.1
15.8
7.0
4.1
7.6
3.5
4.8
8.7
10.8
0.0
0.0
0.0
1
13.4
23.2
13.6
9.0
12.8
4.8
2.6
5.8
3.4
17.7
30.2
4.5
5.6
33.4
0.4
10
9.3
26.4
15.5
6.8
9.5
7.0
2.4
4.4
3.3
26.8
36.3
11.5
2.4
14.6
3.0
38.1**
0.4ns
2.6ns
10.4ns
18.5 +
2.0ns
16.5 +
26.8*
3.1ns
21.3 +
25.3*
2.6ns
0.0ns
0.0ns
13.4ns
Table 4. Effect of inoculum density of Phytophthora parasitica on growth of citrus rootstocks, root rot, and final propagule densities.
Inoculum density (propagules/cm3)z
Variabley
Seedling dry wt
(g)
Scion dry wt
(g)
Root dry wt
(g)
Root rot (%)
Final propagule
density/cm3
zData included in this table represents only plants not receiving metalaxyl to discern the effect of inoculum density on rootstocks in the absence of
fungicide application.
ySeedling dry wt of tops removed after budding; all other parameters measured when experiment terminated.
xSwO = sweet orange; SO = sour orange; and SC = Swingle citrumelo.
wCoefficient of determination for the linear regression with inoculum density; +,*,** = significant at P < 0.10, P < 0.05, P < 0.01; ns = not
significant.
fected SwO and SO but had no effect on SC (Table 4). The rot increased, but other variables were not significantly af-
rootstock x metalaxyl interaction likewise was attributable fected with increasing propagule densities. There was no
to little effect of inoculation on SC and thus little curative significant effect of inoculum density on the SC seedlings,
effect of metalaxyl on this variety (Table 5). The inoculum Propagule densities were highest on SO.
density x metalaxyl interaction was significant because On SwO, metalaxyl treatment increased seedling and
metalaxyl had no effect on non-inoculated controls root dry weight and decreased root rot but did not affect
whereas it often had significant effects on inoculated other variables (Table 5). On SO, metalaxyl applications
plants. increased scion and root weights and decreased root rot.
On SwO, seedling dry weight and root weight de- On SC, there was no effect of metalaxyl on any variable,
creased and root rot increased as inoculum density in- Effect of rootstock on propagule densities in the field. In the
creased (Table 4). On SO, root weight decreased and root Avon Park and St. Cloud rootstock experiments, prop-
Table 5. Effect of metalaxyl treatment frequency on the growth, root rot, and final propagule densities on citrus rootstocks inoculated with
Phytophthora parasitica.
Metalaxyl (applications/yr)z
Variabley
Seedling dry wt
(g)
Scion dry wt
(g)
Root dry wt
(g)
Root rot (%)
Final propagule
density/cm3
zData included in this table includes only inoculated plants to discern the effect of fungicide treatment only in the presence of the pathogen. Sums
of squares for plants inoculated with 1 and 10 propagules/cm3 were partitioned using orthogonal contrasts.
ySeedling dry wt of tops removed after budding; all other parameters measured when experiment terminated.
xSwO = sweet orange; SO = sour orange; and SC = Swingle citrumelo.
Coefficient of determination for the linear regression with number of fungicide application; +, *, **, *** = significant at P ̂ 0.10, P < 0.05, P ̂
0.01; ns = not significant.
176 Proc. Fla. State HorL Soc. 104: 1991.
Rootstock*
SwO
SO
SC
SwO
SO
SC
SwO
SO
SC
SwO
SO
SC
SwO
SO
SC
0
11.4
24.8
14.6
7.9
11.2
5.9
2.5
5.1
3.3
21.8
33.3
8.0
3.7
24.0
1.7
4
17.2
24.5
16.0
9.3
14.4
5.0
3.7
6.0
3.0
9.0
7.6
9.0
5.5
0.2
0.0
8
16.1
26.3
16.6
10.5
16.2
4.8
3.8
7.2
3.4
5.5
12.3
7.0
0.3
0.5
0.7
12.8*
0.4ns
1.6ns
3.3ns
9.6 +
4.1ns
15.3*
10.7*
0.9ns
20.5**
58.7***
0.2ns
0.4ns
17.4**
6.3ns
agule densities were highest on SwO, PSL, and VL (Table
6). TO and especially SC supported only low populations
in both experiments and CM supported intermediate pop
ulations.
Discussion
Similar results were obtained in the greenhouse,
screenhouse, and field evaluations of rootstock susceptibil
ity to Phytophthora root rot. SwO was quite susceptible in all
tests where it was evaluated. TO and SC were highly resis
tant. SC appeared more susceptible than TO in the
greenhouse test (Table 1), but the latter supported higher
populations in one field experiment (Table 6). Inoculated
SC did not respond to fungicide treatment (Tables 3-5).
Results with other rootstocks were less consistent with
many appearing as susceptible as SwO. VL had a lower
root rot rating than some other stocks in a greenhouse test
(Table 1), but supported high populations in one field test
(Table 6). CM was intermediate in susceptibility in most
tests, but in the field, scion cultivars on this stock are more
likely to show leaf chlorosis as a result of fibrous root loss
(Timmer, unpublished observations). In almost all cases,
SO and PSL appear to be about as susceptible as SwO to
fibrous root rot.
Results varied somewhat in the different tests, but with
the exception of SC and TO, most commercial rootstocks
must be considered susceptible to fibrous root rot. It will
be difficult to determine with precision the susceptibility
of potential new rootstocks to fibrous root rot; however, of
primary importance in rootstock selection is the tolerance
of the candidate to bark infection which results in collar
rot and frequently in tree decline and loss. Rootstocks such
as SO, CC, and CM which are resistant to bark infection
and susceptible to root rot have been grown successfully in
Florida and elsewhere (Castle et al., 1989). Some yield loss
may be incurred on these stocks (Sandier et al., 1989; Tim
mer et al., 1989), but generally tree losses are not observed.
Certainly rootstocks with other desirable traits should not
be discarded because of their susceptibility to fibrous root
rot. On the other hand, the general availability of
rootstocks with a wide range of desirable traits and with
the resistance of SC and TO to Phytophthora spp. would
practically eliminate yield losses due to fibrous root rot.
Rootstock is also a major consideration in decisions on
whether to apply fungicides to control fibrous root loss.
Sweet orange is highly susceptible to fibrous root rot and
to below-ground bark infection. Fungicide applications on
SwO rootstock have reduced Phytophthora populations and
increased fibrous root densities (Sandier et al., 1989; Tim
mer et al., 1989). However, fungicide applications have
failed to reverse tree decline due to bark infections on
scaffold roots (Timmer et al., 1988a; Timmer et al., 1989;
Timmer, field observations). Fungicide applications to
groves on SwO rootstock may be useful where fibrous root
rot is the only problem or to prevent development of bark
infection, but should not be made in an attempt to reverse
tree declines.
Groves on SC and TO do not support high populations
of P. parasitica, do not suffer from fibrous root rot, and
should not require fungicide application for disease con
trol.
The greatest benefits of fungicide application should
be derived on rootstocks which are tolerant to bark infec-
Proc. Fla. State Hort. Soc. 104: 1991.
Table 6. Mean propagule densities of Phytophthora parasitica in 2 rootstock
experiments with 12-yr-old 'Valencia' sweet orange.
Propagules/cm3
Rootstock
Sweet orange
Palestine sweet lime
Volkamer lemon
Cleopatra mandarin
Trifoliate orange
Swingle citrumelo
Avon Park2
14.6 ax
11.0 ab
_
8.5 be
7.0 be
5.4 c
St. Cloud*
w
32.3 ax
31.4 a
17.9 b
16.3 b
8.3 c
zMean of 5 sampling dates from Mar. 1988 to Feb. 1989.
yMean of 5 sampling dates from Dec. 1987 to Dec. 1988.
xMean separation in columns by Duncan's multiple range test, P < 0.05.
w Rootstock not included in test.
tion but susceptible to fibrous root rot. Methods for sample
collection and assay of propagule densities in citrus or
chards using selective media have been developed (Tim
mer et al., 1988b). Propagule densities of 0-5 per cm3 are
considered low, 5-15 moderate, and above 15 high (Tim
mer et al., 1988a). Groves on SO have been encountered
with average populations of 100-200 propagules per cm3,
extensive fibrous root rot, and wilt in the presence of
adequate soil moisture (Timmer, unpublished data). The
highest Phytophthora populations have been encountered
in bedded groves with seepage irrigation. Where condi
tions favor development of high Phytophthora populations
on SO, VL, PSL, CC, CM, and other stocks of similar sus
ceptibility, fungicide application may prove beneficial.
Literature Cited
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Effect of citrus rootstocks on soil populations of Phytophthora parasitica.
Plant Dis. 74:296-300.
Castle, W. S., D. P. H. Tucker, A. H. Krezdorn, and C. O. Youtsey. 1989.
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47 pp.
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Effect of fungicide applications on populations of Phytophthora
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Proc. Fla. State Hort. Soc. 104:178-180. 1991.
FREQUENCY AND DISTRIBUTION OF CITRUS BLIGHT IN A TEST OF NEW
HYBRID ROOTSTOCKS
H. K. WUTSCHER
United States Department of Agriculture, ARS
2120 Camden Road, Orlando, FL 32803
F. W. BlSTLINE
Horticultural Research Department
Coca-Cola Foods
P. O. Box 368
Plymouth, FL 32768
Additional index words, citrus blight susceptibility, distribu
tion, soil Ca and Mg.
Abstract. The number of trees affected by citrus blight in a
'Valencia' orange rootstock test planted in 1980 was recorded
in April 1991. The test was part of a commercial grove on the
lower ridge near Sebring and consisted of four 4-tree replica
tions of trees on 19 rootstocks, 7 of them new, unnamed hyb
rids. Trees on 11 of the 19 rootstocks were included in the
blight survey, a total of 176 trees, 16 trees on each rootstock.
Eleven trees with visual symptoms of blight had been re
moved before the survey. The remaining trees were inspected
visually, and 66 trees were tested by water injection with a
syringe and analysis of the trunk wood for zinc and potas
sium. All trees with visual symptoms were tested; when all
The technical help of Ms. Susan Chalk in collecting the data is grate
fully acknowledged.
trees in a 4-tree plot were healthy, one randomly chosen tree
was tested. There were more blighted trees in replications 1
and 2 than in replications 3 and 4, where soil Ca and Mg
were lower. Trees on FF-3-16-69, a hybrid of Christiansen
trifoliate orange X Cleopatra, were most severely affected by
blight (7/16 trees), followed by trees on FF-3-15-11, Cleopatra
X Carrizo and rough lemon 76-645 (5/16 trees); FF-3-16-54,
Cleopatra X Swingle trifoliate orange, and citrangequat CPB-
48032 (4/16 trees); FF-3-15-70, Cleopatra X Swingle trifoliate
orange (3/16); FF-3-8-8, Rangpur X Swingle trifoliate orange
(2/16); and Carrizo and C54-64-8, Rangpur X Troyer (1/16).
No blighted trees were found on Flying Dragon trifoliate
orange and Swingle citrumelo.
Citrus blight, a tree decline of unknown cause, con
tinues to be Florida's and Brazil's most serious production
problem (9,10). Opinions are divided about the cause of
the decline (1,10,15), and resistant rootstocks are the only
effective defense. The blight tolerance of commonly used
citrus rootstocks is fairly well known (6,11,16). Rough and
Volkamer lemon (C. limon Burm. f.) and Rangpur lime (C.
reticulata hybrid) are very susceptible, Sunki mandarin (C.
reticulata Blanco), sweet orange (C. sinensis L. Osbeck), sour
orange (C. aurantium L.), and Swingle citrumelo (C. paradisi
Macf. X Poncirus trifoliata L. Raf.) are the most resistant
rootstocks (5,6). Cleopatra mandarin (C. reticulata Blanco)
was thought to be resistant, but recent observations show
it to be susceptible, especially when the trees get older
Table 1. Blight incidence on an eleven-year-old planting of'Valencia' orange trees on 11 rootstocks. Water absorption in syringe injection, Zn and
K in the outer trunk wood, trees removed before survey, number of trees tested, and total number of trees lost or affected by citrus blight, April
1991.z
Rootstocks
Christiansen TF x Cleopatra, FF-3-16-60
Cleopatra x Carrizo, FF-3-15-11
Rough lemon, 76-645
Cleopatra x Swingle TF, FF-3-16-54
Citrangequat, CPB-48032
Cleopatra x Swingle TF, FF-3-15-70
Rangpur x Swingle TF, FF-3-8-8
Carrizo
Rangpur x Troyer, C54-64-8
Flying Dragon TF, FF-9-12-44
Swingle citrumelo
Means
Statistical significance
Water absorption,
i ml/min
Blight Healthy
7
10
16
8
-
3
2
1
-
-
-
9
25
50
43
30
39
29
56
46
20
44
33
37
0.001
ppm
Blight
14
17
14
9
-
10
23
10
-
-
-
13
Zn
Healthy
3
3
4
4
4
4
3
3
3
3
3
3
0.001
°A
Blight
.205
.286
.267
.216
—
.271
.305
.377
-
-
—
.270
Healthy
.154
.142
.145
.167
.165
.156
.135
.151
.153
.151
.142
.156
0.001
Trees
removed
before
survey
1
3
1
0
4
1
0
0
1
0
0
NS
Trees
tested
8
4
7
11
6
6
4
4
5
6
5
affected by
affected bv
blight
7 a
5 b
5 b
4 be
4 be
3 cd
2 de
1 ef
1 ef
0 f
0 f
0.05
zTotal trees tested: 66 (21 blight positive, 45 healthy)
Total trees planted on each rootstock: 16
178 Proc. Fla. State Hort. Soc. 104: 1991.