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rows of lightly-infected Swingle citrumelo (unpublished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

Agostini, J. P., L. W. Timmer, W. S. Castle, and D. J. Mitchell. 1990.

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.

Rootstocks from Florida citrus. Univ. Florida Coop. Ext. Publ. SP 41.

47 pp.

Carpenter, J. R. and J. R. Furr. 1962. Evaluation of tolerance to root rot

caused by Phytophthora parasitica in seedlings of citrus and related gen

era. Phytopathology 52:1277-1285.

Graham, J. H. 1990. Evaluation of tolerance of citrus rootstocks to

Phytophthora root rot in chlamydospore-infested soil. Plant Dis.

74:743-746.

Graham, J. H. and L. W. Timmer. 1991. Phytophthora diseases of citrus,

p. 250-269. In: A. N. Mukhopadhyay, U. S. Singh, H. S. Chaube, and

J. Kumar (eds.). Plant diseases of international importance, Vol. III.

Prentice-Hall, NJ.

Grimm, G. D. and D. J. Hutchison. 1977. Evaluation of Citrus spp., rela

tives and hybrids for resistance to Phytophthora parasitica Dastur. Proc.

Int. Soc. Citriculture 3:863-865.

Kannwischer, M. E. and D. J. Mitchell. 1978. The influence of a fungicide

on the epidemiology of black shank of tobacco. Phytopathology

68:1760-1765.

Sandier, H. A., L. W. Timmer, J. H. Graham, and S. E. Zitko. 1989.

Effect of fungicide applications on populations of Phytophthora

parasitica and on feeder root densities and fruit yields of citrus trees.

Plant Dis. 73:902-906.

Smith, G. S., D. J. Hutchison, and C. T. Henderson. 1987. Screening

sweet orange cultivars for relative susceptibility to Phytophthora foot

rot. Proc. Fla. State Hort. Soc. 100:64-66.

Timmer, L. W., J. H. Graham, H. A. Sandier, and S. E. Zitko. 1988a.

Populations of Phytophthora parasitica in bearing citrus orchards in

Florida and response to fungicide applications. Citrus Ind. 69(1 ):40-

44, 54.

Timmer, L. W., H. A. Sandier, J. H. Graham, and S. E. Zitko. 1988b.

Sampling citrus orchards in Florida to estimate populations of

Phytophthora parasitica. Phytopathology 78:940-944.

177

Timmer, L. W., H. A. Sandier, J. H. Graham, and S. E. Zitko. 1989.

Phytophthora feeder root rot of bearing citrus: fungicide effects on

soil populations of Phytophthora parasitica and citrus tree productivity.

Proc. Fla. State Hort. Soc. 102:5-9.

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cultures oi Phytophthora parasitica. Phytopathology 61:1412-1413.

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the relative susceptibility of citrus rootstocks to foot rot. Plant Dis.

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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.