Proc. Fla. State HorL Soc. 107:21-26. 1994.

CONTROL OF BLACK ROOT ROT ON CITRUS SEEDLINGS IN PEAT-BASED MEDIA1

J. H. Graham

University of Florida, IFAS

Citrus Research and Education Center

700 Experiment Station Road

Lake Alfred, FL 33850

Additional index words. Thielaviopsis basicola, Cleopatra manda

rin, greenhouse production, fungicides, biological control,

cultural practices

Abstract. Cultural, biological, and fungicidal measures for con

trol of Thielaviopsis basicola, the cause of black root rot of cit

rus seedlings in commercial greenhouse operations, were

investigated in peat-based soilless media. Six media, five with

bark amendments, were surveyed to determine whether bark

was suppressive to the pathogen. Before planting, populations

of T. basicola in the infested media ranged from 3.13 to 3.78 log

cfu per cm3 medium. Eight weeks after transplanting sweet or

ange seedlings into media, populations ranged from 3.03 to

3.58 log cfu per cm3 in artificially- infested media, and from 2.96

to 3.16 log cfu per cm3 in naturally-infested, noninoculated me

dia. None of the media suppressed root rot development. Sul

fur amendment to reduce pH of a bark-based medium

(Metromix 500®) decreased black root rot on Cleopatra manda

rin seedlings when pH was lowered at least 1.0 unit compared

to nonamended medium. Post-infestation drenches with

benomyl, myclobutanil, and thiophanate methyl were more ef

fective for root rot control than pH adjustment. Mycostop®, a

biocontrol agent (Streptomyces griseoviridis strain K61), pro

vided control post-infestation only when high rates and vol

umes of soil drench were used. None of the soil treatments

consistently reduced populations of T. basicola in artificially-

or naturally-infested media. Cultural practices such as use of

bark to maintain proper air-filled porosity of the medium, judi

cious water management and properly disinfested plastic con

tainers are recommended to reduce the establishment of 7.

basicola when inoculum sources of the fungus are present in

the vicinity of the greenhouse.

Thielaviopsis basicola (Berk. & Broome) Ferraris (synan-

amorph = Chalara elegans) causes a widespread root rot prob

lem on several greenhouse crops including poinsettia,

geranium, pansy, petunia, holly (Wills and Lambe, 1978) and

citrus (Tsao and Van Gundy, 1962; Timmer, 1988). Recently,

the fungus was identified in Florida greenhouses as the cause

of black root rot affecting seedlings of all commercial citrus

rootstocks, especially Cleopatra mandarin (Graham and Tim

mer, 1991; Tsao, 1963). The fungus forms small, well-defined,

brown to black lesions on citrus fibrous roots where black

chlamydospores occur in the cortical tissue. Lesions at the

root tip cause sloughing of the cortex that exposes the stele

and gives the root system a dry, stringy appearance. Leaf

symptoms are veinal chlorosis typical of nonspecific nutrition-

Florida Agricultural Experiment Station Journal Series No. N-01024.

'This research was supported through unrestricted funds and donations

by The Scotts Company, Rohm and Haas Company, AgBio Development,

Inc., Fafard, and Lykes Citrus Management Division. I thank N. H. Timmer

and John Gose of Lykes Citrus Management Division for technical assistance.

al deficiency due to root loss. T. basicola can be introduced

into nurseries with contaminated peat-based media (Graham

and Timmer, 1991). Once established, phialoconidial spores

of the fungus become airborne and infest the entire green

house (Graham and Timmer, 1991). Damaging levels of the

pathogen range from 3.0 to 4.0 log colony-forming units

(cfu) per cm3 of potting medium. Black root rot is favored by

low temperatures and high soil moisture prevalent in the win

ter and severely limits production of Cleopatra mandarin

seedlings. Pathogen populations drop to low levels (< 1.0 log

cfu per cm3 soil) in the summer. Nevertheless, the pathogen

over-summers in the greenhouse and can infest subsequent

crops.

A survey of commercial greenhouse nurseries from 1989

through 1992 revealed that the most severe epidemics oc

curred when water-saturated, cool conditions persisted in

peat-vermiculite based media with poor drainage characteris

tics (Graham and Timmer, 1991). When conditions favorable

for root growth improved in the spring, root rot decreased

but not before the crop was delayed up to 2 months before

reaching suitable size for transplant (J. H. Graham, personal

observation). Seedlings grew slowly when transplanted into

larger containers in the greenhouse or transplanted into the

field nursery. Although T. basicola survived in field soils, pop

ulations dropped to nondamaging levels (< 2.0 log cfu per

cm3 soil) in field nurseries (J. H. Graham, unpublished data).

Thus, T. basicola is not considered a serious root rot pathogen

in field soils in Florida, although it may be of significance in

California citrus soils (Tsao, 1962).

Reduction of media pH from 6.5 to 5.5 (Merrill et al.,

1986) and drench applications of benomyl (Manning et al.,

1970) were reported to reduce black root rot damage on oth

er greenhouse crops in soilless media. Bark-amended media

have been observed to reduce root rot in nursery crops (J.

Knauss andj. H. Graham, personal observations).

The studies reported here evaluate cultural, fungicidal,

and biological control measures that offered potential for

control of black root rot damage on citrus seedlings. The

overall objective is to develop integrated pathogen manage

ment practices for T. basicola since few fungicides are regis

tered for use in Florida citrus greenhouse nurseries. These

fungicides are of limited usefulness as curatives after black

root rot on citrus crops develops (J. H. Graham, unpublished

data).

Materials and Methods

Inoculum and inoculation methods.

The isolate of T. basicola (Tri) used for all studies was ob

tained from infested Canadian sphagnum peat moss and de

termined to be pathogenic on seedlings of all commercial

cultivars of citrus rootstocks tested (Graham and Timmer,

1991). Agar disks of the fungus growing on V8® juice agar

plates (Ribeiro, 1978) were used to inoculate 2-liter flasks

containing 1.5 liters of sterile vermiculite mixed with 250 ml

of half-strength V8 juice broth (Ribeiro, 1978). After coloni

zation (2 to 4 weeks), one part vermiculite inoculum was

mixed with 9 parts of the appropriate potting medium to

yield at least 3.0 log cfu per cm medium. Noninfested V8-ver-

Proc. Fla. State HorL Soc. 107: 1994. 21

miculite medium was mixed similarly for control treatments.

To determine propagules in the growing medium, 10 cm3 of

medium was mixed with 90 cm3 of sterile 0.25% water agar.

After an additional 1:10 dilution with water agar, 1-ml ali-

quots of each dilution level were spread onto the surface of 5

petri plates of T. basicola-carrot- etridiazol-nystatin (TB-CEN)

agar (Specht and Griffin, 1985). After spreading, the plates

were inverted and the treated agar surface was relocated with

a spatula face down in the top of the petri plate. After 7 to 10

days, T. basicola was seen as dark brown colonies with black

chlamydospores present growing up through the agar out

onto the nontreated surface.

Control with bark-amended media.

Five commercial potting media containing mixes of

milled pine bark with Canadian sphagnum peat were exam

ined in comparison to a standard Canadian peat-vermiculite-

perlite mix (Promix-BX®, Premier Brands, Inc., New Roch-

elle, NY). Metromixes (300, 500 and 554) were supplied by

The Scotts Company (Marysville, OH) and Citrus Mix B and

Mix #4 supplied by Fafard (Springfield, MA). Each medium

with and without infestation with T. basicola inoculum was as

sayed for inoculum density as described above (3.50 log cfu

per cm3 medium). Ten-month-old sweet orange seedlings were transplanted in 150 cm3 containers (Stuewe, Inc., Cor-

vallis, OR) containing approximately 125 cm3 of each infested

and noninfested medium (7 replicate containers per seed

ling) . The transplants were placed in a greenhouse with a nat

ural infestation of T. basicola on 6 February 1991, fertilized

weekly with 20-10-20 Peters Peat-Lite Special® (The Scotts

Company) and maintained at 18 to 25°C for 8 weeks. The

treatments were located in a randomized split-block design

with T. basicola inoculation as the spit-plot. At harvest, the me

dium from each treatment was combined and mixed for de

termination of T. basicola propagule density. Root systems

were rated by grouping seedlings from all treatments into 5

categories of root symptoms. Root systems showing very few

or no black lesions or sloughing fibrous roots were rated 1,

and root systems with few or no remaining roots intact were

rated 5. Root rot ratings were subjected to an analysis of vari

ance (AOV) using the General Linear Models procedure

(SAS, Cary, NC) and differences among potting media infest

ed with T. basicola were tested using Student-Newman-Kuels

multiple range test.

Medium pH adjustment.

Metromix® 500 was prepared without sulfur amendment

and amendment of 0.3 kg/m3, 0.6 kg/m3, and 1.2 kg/m3 of

finely-ground wettable sulfur. Additional treatments utilized

nonamended medium for benomyl (Benlate® 40 WP)

drenches and foliar sprays at 1.2 g/liter.

Ten-month-old Cleopatra mandarin seedlings, grown in

an infested greenhouse at CREC, were naturally-infested with

T. basicola at the beginning of the experiment but root rot was

minimal. Seedlings were transplanted into inoculated and

noninoculated media with vermiculite inoculum of T. basicola

prepared as described above except the inoculum was mixed

1:19 (v:v) with medium (inoculum density was 3.03 log cfu

per cm3). There were 7 inoculated and 7 noninoculated seed lings for each sulfur and fungicide treatment. Seedlings were

placed in the greenhouse on 19 February 1992. Benlate sprays

(to runoff) and drenches (50 ml per 125 cm3 medium) were

applied 20 February 1992 and seedlings harvested 25 March

1992. At harvest, seedlings were rated for root rot symptoms,

fibrous roots dried and weighed (70°C, 24 hr), and the medi

um from each treatment assayed for T. basicola populations

on TB-CEN selective medium as described above. The pH of

the medium from each treatment was measured as a saturated

paste with deionized water.

Alternative fungicides and a biological agent.

An experiment of similar design to the medium pH ad

justment experiment was conducted with disease-free Cleo

patra mandarin seedlings. T. basicola was eradicated from the

propagation greenhouse used to raise seedlings by removal of

all plant material from the house, cleaning out of all media

debris on the benches and floor. After 2 months, air samples

were collected using an Anderson air sampler and TB-CEN se

lective medium plates as previously described (Graham and

Timmer, 1991). T. basicola was not detected in air samples col

lected in March 1992. In February 1993, Cleopatra mandarin

seedlings grown for 10 months in the propagation green

house were confirmed to be free of T. basicola by sampling the

growing medium.

Drench treatments of the seedlings were Benlate (1.2 g/

liter), myclobutanil (a sterol-inhibiting fungicide, Rohm and

Haas Company) at 3 concentrations (0.064 g/liter, 0.16 g/li

ter, and 0.32 g/liter), and a biological control agent My-

costop (Streptomyces griseoviridis strain K61) as a 0.03% (0.32

g/liter) suspension in tap water. The experimental design, in

oculation, and planting methods were the same as the previ

ous experiment. The seedlings were inoculated 12 February

1993 (3.38 log cfu per cm3 medium) and placed in a green

house infested with T. basicola. Drenches of 25 ml per contain

er were applied 15 February 1993 and 2 weeks later for

myclobutanil and Mycostop, and 4 weeks later for Benlate.

On 27 March 1993, 6 weeks after inoculation, seedlings

were evaluated for the percentage of roots with black root rot

by pinching each root tip between the thumb and index fin

ger to determine whether the cortex sloughed off the stele.

Root tips that easily sloughed were scored positive for root

rot, while those that did not slough were considered healthy.

Seedling shoots, tap roots, and fibrous roots were dried and

weighed and the potting medium from each treatment as

sayed on TB-CEN.

A second experiment of similar design was conducted

with the following drench treatments: thiophanate-methyl

(Domain®) at 0.5 ml/liter, etradiazole-thiophanate methyl (Banrot® 40 WP) at 0.49 g/liter, Benlate at 1.2 g/liter, my

clobutanil at 0.36 g/liter, and Mycostop at 1.5 mg/liter and

15 mg/liter. Cleopatra mandarin seedlings were planted 12

December 1993 and drenches (25 ml per container) were ap

plied 14 December 1993 and 4 weeks later. Seedlings were

harvested on 12 January 1994 at 6 weeks after inoculation and

dry weights of fibrous roots and percentage root rot evaluated

as in the previous experiment.

Commercial citrus nursery trial.

The trial was designed to include best management prac

tices for disease control in an endemic pathogen situation.

The trial was conducted in a commercial greenhouse with

roll-up side curtains, next to a greenhouse of the same design

containing a heavily diseased seedling crop which acted as a

source of inoculum. The containers were injection molded

22 Proc. Fla. State Hort. Soc. 107: 1994.

plastic with 96 cells/tray (100 cm3 cells). The trays were ar

ranged in rows of 13 trays (2.78 m2) which acted as a nonrep-

licated treatment unit (1,248 seedlings) for ease of

retreatment applications. Trays of Metromix 500 were seeded

with Cleopatra mandarin in early October 1993. When seed

lings were at the 2 to 4 true leaf stage, drenches began 19 No

vember 1993 and were repeated on a monthly basis (4 times).

The treatments were as follows: nontreated control, sulfur-

amended at 0.9 kg/m3 medium, Banrot 40 WP (24 g/41.6 li

ters/2.78 m2), myclobutanil (7.2 g/41.6 liters/2.78 m2), and Mycostop (0.6 g/41.6 liters/2.78 m2). On 30 March 1994, the

height of 50 randomly selected seedlings from each treat

ment was measured to the nearest 0.5 cm. Medium from five

randomly selected locations in each treatment block were

combined and assayed for T. basicola propagules.

Results

Bark-amended media.

Six peat-based media, five with bark amendments, were

surveyed to determine whether bark was suppressive to the

pathogen and/or black root rot. Before planting, T. basicola

populations in the infested media ranged from 3.13 to 3.78

log cfu per cm3 medium. Eight weeks after transplant of sweet

orange seedlings into media, populations ranged from 3.03

to 3.58 log cfu per cm3 with the highest population in the

peat-based medium without bark. In naturally-infested, noni-

noculated media, the populations ranged from 2.96 to 3.28

log cfu per cm3, nearly as high as in infested media. None of

bark-amended media suppressed root rot in either naturally-

infested or inoculated media compared to the medium with

out bark (data not shown).

Medium pH adjustment.

At the end of the experiment, the pH of Metromix 500 in

the nonsulfur amended treatments ranged from 5.76 to 5.84.

Sulfur treatments of 0.3, 0.6, and 1.2 kg/m3 dropped pH ap

proximately 0.4, 0.8, and 1.4 units, respectively (Table 1).

On the naturally-infested seedlings, moderate black root

rot developed in all treatments except the Benlate drench.

The drench treatment, but not the foliar spray, prevented

root rot, reduced medium populations of T. basicola, but did

not significantly affect fibrous root weight (Table 1). After in

oculation with T. basicola, moderate disease levels resulted ex

cept in the 1.2 kg/m3 sulfur rate and the Benlate drench

treatments where disease ratings were significantly lower than

the nontreated control. Fibrous root weight was significantly

increased by the Benlate drench treatment. Populations of T.

basicola were no higher in the inoculated than in the nonin-

oculated treatments and none of the treatments reduced

populations in the inoculated medium compared to the non-

treated control.

Alternative fungicides and a biological agent.

In the 1993 experiment, the nonamended Metromix had

a starting pH of 6.14. Sulfur amendments of 0.3, 0.6, and 1.2

kg/m3 dropped pH approximately 0.3, 0.6, and 1.5 units, re

spectively (Table 2).

A low level of root rot developed in all treatments of non-

inoculated seedlings infested by naturally occurring popula

tions in the greenhouse. Although there were no significant

reductions in root rot, most treatments provided for slight

nonsignificant increases in fibrous root, shoot, and total dry

weight. Populations of T. basicola in the medium were one log

unit lower in the Benlate and myclobutanil treatments (Table

2). Inoculation with T. basicola again produced higher levels

of root rot in nontreated controls than in the naturally-infest

ed treatments. Percentage root rot was significantly reduced

by treatments with fungicides, Mycostop, and the 0.6 and 1.2

kg/m3 rates of sulfur. The 0.3 sulfur rate had no effect on de

velopment of black root rot. None of the treatments substan

tially reduced populations of T. basicola in the medium.

Because of the short duration of the test (6 weeks), there was

insufficient time for substantial plant growth responses to oc

cur as a result of root rot control. However, nonsignificant

growth responses were observed for shoots in the Benlate

Table 1. Effect of sulfur amendments and Benlate on development of root rot on Cleopatra mandarin seedlings infested with Thielaviopsis basicola.

Treatment

Medium

pH

Root rot

rating (l-5)x

Fibrous wt.

(g)

Population

(log cfu/cm3)

Control

0.3 Sulfur*

0.6 Sulfur

1.2 Sulfur

Benlate spray

Benlate drench

5.78

5.41

5.02

4.37

5.84

5.76

- Naturally Infested -

2.2 abz

2.0 ab

2.0 ab

1.9 ab

2.9 a

1.1 b

Control

0.3 Sulfur>

0.6 Sulfur

1.2 Sulfur

Benlate spray

Benlate drench

5.74

5.41

4.80

4.34

5.82

5.84

- Inoculated —

3.60 ab'

3.00 ab

2.79 b

1.57 c

4.14 a

1.57 c

0.12 a

0.13 a

0.15 a

0.12 a

0.10 a

0.16 a

0.08 b

0.11 b

0.08 b

0.11b

0.06 b

0.18 a

4.06

4.06

4.61

4.63

3.12

2.89

4.31

4.14

4.10

3.39

4.10

4.48

"Root rot ratings vary from 1 to 5 where 1 = few black and sloughing roots and 5 = complete fibrous root loss.

ykg/m^ sulfur per volume medium.

'Means (n = 7) followed by different letters are significantly different at the 0.05 level by least significant difference (LSD).

Proc. Fla. State Hort. Soc. 107: 1994. 23

Table 2. Effect of sulfur amendments and drenches of Mycostop, and three rates of myclobutanil in comparison to the nontreated control and Benlate

drench treated on development of root rot on Cleopatra mandarin seedlings infested with Thielaviopsis basicola.

Treatment

Control (6.14)x

0.3 Sulfury (5.88)

0.6 Sulfur (5.50)

1.2 Sulfur (4.67)

Mycostop

Myclobutanil-low

Myclobutanil-med

Myclobutanil-high

Benlate

Control (6.14)x

0.3 Sulfury (5.88)

0.6 Sulfur (5.50)

1.2 Sulfur (4.67)

Mycostop

Myclobutanil-low

Myclobutanil-med

Myclobutanil-high

Benlate

Percentage —

root rot

13 az

11 a

15 a

10 a

13 a

10 a

14 a

6 a

9 a

28 a'

27 a

11 be

14 be

13 be

12 be

15 be

8 c

11 be

Dry weight (g)

Fibrous

0.07 b

0.08 ab

0.08 ab

0.07 b

0.10 ab

0.09 ab

0.08 ab

0.10 a

0.08 ab

0.08 ab

0.08 ab

0.10 ab

0.09 ab

0.07 ab

0.08 b

0.08 b

0.12 a

0.08 ab

Shoot

[

0.54 a

0.64 a

0.58 a

0.62 a

0.69 s

0.63 s

0.61 a

0.64 a

0.63 a

0.58 a

0.61a

0.59 a

0.51 a

0.60 a

0.55 a

0.57 a

0.67 a

0.69 a

Total

0.78 a

0.90 a

0.80 a

0.84 a

0.96 s

0.91s

0.86 a

0.90 a

0.88 a

0.83 a

0.86 a

0.86 a

0.74 a

0.84 a

0.77 a

0.81 a

0.96 a

0.93 a

— Population

(log cfu/cm3)

3.61

3.41

3.44

3.00

3.47

2.25

0.60

2.38

2.60

4.00

3.53

3.74

3.34

4.10

3.79

3.81

3.34

3.82

"Medium pH at start of trial.

ykg/m3 sulfur per volume medium.

'Means (n = 9) followed by different letters are significantly different at the 0.05 level by least significant difference (LSD).

treatment and fibrous roots and shoots in the myclobutanil-

high rate. In the latter case, seedlings had increased root

branching and fibrousity, and the foliage was darker green in

appearance. These plant growth regulator-like effects of high

rates of Myclobutanil confirmed similar observations in an

earlier preliminary test conducted in December 1992 (data

not shown).

In the 1993-94 trial, noninoculated seedlings did not be

come infested by airborne conidia except at a very low level in

the sulfur-amended treatment (data not shown). Conse

quently, root rot was low (background) and there were no sig

nificant effects of treatments on fibrous root growth and total

dry weight (data not shown). Inoculation with T. basicola re

sulted in the greatest fibrous root rot on seedlings in the sul

fur- treated and nonamended medium control. Most

treatments with fungicides showed lower levels of root rot,

but only Banrot significantly reduced rot compared to the

control (Table 3). Fresh amendment with sulfur did not ade

quately reduce the pH (6.48 to 6.02), therefore, did not con

trol root rot. Mycostop used at lower rates and drench volume

than in the previous trial was ineffective. Fibrous root weight

results paralleled root rot. The control, sulfur-amended and

Mycostop treatments had the lowest root weights, while Do

main had significantly higher root weights than those treat

ments. Total seedling weights showed similar trends but were

not significantly different among treatments. Levels of T. ba

sicola in the medium were highest in the control, sulfur- treat

ed, and Mycostop treatments, the treatments with the highest

levels of root rot (Table 3).

Commercial citrus nursery trial.

In the greenhouse adjacent to the trial, black root rot on

Swingle citrumelo was severe in peat-based media without

bark because infested styroblock containers were reused. The

Table 3. Effect of sulfur and drenches of four fungicides and Mycostop on development of root rot on Cleopatra mandarin seedlings inoculated with Thielav iopsis basicola.

Treatment

Percentage

root rot

30.1 az

38.5 a

14.6 b

17.6 ab

22.7 ab

20.1 ab

29.2 ab

31.0 ab

Dry weight (g)

Fibrous root

0.07 b

0.07 b

0.10 ab

0.18 a

0.12 ab

0.14 ab

0.07 b

0.06 b

Total

1.08 ab

0.94 ab

1.12 ab

1.12 ab

1.10 ab

1.38 a

0.93 ab

0.72 b

— Population

(log cfti/cm3)

3.86

3.86

3.29

3.08

3.20

3.30

3.95

3.90

Control

Sulfur

Banrot

Domain

Myclobutanil

Benlate

Mycostop low

Mycostop high

'Means (n = 7) followed by different letters are significantly different at the 0.05 level according to Student-Newman-Keuls test.

24 Proc. Fla. State Hort. Soc. 107: 1994.

Table 4. Effect of sulfur amendment and drenches of three fungicides, and

Mycostop on infestation of Metromix 500 with Thielaviopsis basicola and

growth of Cleopatra mandarin seedlings in a commercial greenhouse at

Ft. Bassinger, FL.

Treatments

Control

Sulfur

Banrot

Myclobutanil

Mycostop

Seedling height

Mean

37.0

35.8

39.0

38.8

39.3

NSZ

(cm)

SD

4.1

3.8

4.7

4.4

3.4

(log cfu/cm3)

0

0

0

0

0.41

'Means (n = 50) not significantly different at the 0.05 level according to Stu-

dent-Newman-Keuls test.

medium stayed very wet or dry in individual cells in the blocks

depending on the level of root rot damage. Swingle citrumelo

seedlings were stunted and had a high cull rate when harvest

ed later (personal communication with the nurseryman).

The crop was delayed about 1 month before planting into lin

er beds in the field.

In the greenhouse where the trial was conducted, black

root rot did not develop anywhere in the house, even though

airborne inoculum of T. basicola became established in the

potting medium (Table 4, Mycostop treatment). In the ab

sence of pathogen establishment, no treatment had signifi

cantly positive or negative effects on seedling growth (Table

4). Seedlings throughout the test area and the entire green

house were highly vigorous with white fibrous root systems.

Since no black root rot symptoms were evident, root rot rat

ings were not performed.

Discussion

In the greenhouse at CREC, airborne inoculum of T. basi-

co/awas repeatedly demonstrated to infest noninoculated pot

ting media whether or not they contained bark. This is

supported by a previous study citing a lack of control of black

root rot on holly cultivars with bark amendment of media

(Merrill et al., 1986). The experiments reported here were

designed to create conducive conditions for disease, so water

ing cycles were frequent enough to prevent drying of the me

dium. Although bark was not apparently biologically or

chemically suppressive to population development after nat

ural or artificial infestation, drainage was improved due to in

creased air-filled porosity of the medium compared to peat-

based medium alone. Overall, root rot damage after 6 weeks

was light to moderate and not as severe as observed in com

mercial nurseries using media without bark.

With natural infestation and inoculation of T. basicola,

Benlate, as previously reported (Manning et al., 1970), was ef

fective for reducing black root rot damage, but did not have

activity as a foliar spray. Apparently, systemic activity in the

roots was sufficient to reduce infection, even though popula

tions in the medium were often as high as in nontreated me

dium. Activity of Benlate, as well as the other fungicides,

myclobutanil, Banrot, and Domain, was limited for reducing

population of the pathogen in the medium and would likely

not reduce spread of T. basicola throughout a greenhouse op

eration. However, these alternative fungicides used as

drenches were as effective as Benlate in reducing root rot

symptoms. Although increases in fibrous roots were often

nonsignificant over the short duration of the tests, they indi

cate an ability of these fungicides to act as protectants against

infection. Mycostop was also effective in reducing disease at

high rates and drench volumes but not at lower, more eco

nomical rates. Apparently, sufficient inoculum of the S.

griseovirides strain must be dispersed throughout the rhizo-

sphere to be effective as a protectant.

As previously shown for other greenhouse crops (Bate-

man, 1962; Merrill et al., 1986), lowering the pH one unit

from 6.5 to 5.5 or lower was beneficial in controlling black

root rot, but not as effective as fungicides. In contrast, if sulfur

was freshly added and the pH not adjusted before inoculation

with T. basicola, black root rot was not controlled. Equilibra

tion of the sulfur in the medium for 2 to 3 weeks prior to in

oculation to reduce the pH was necessary to gain activity.

Fungicides were not effective as curatives after symptoms

were well-established in the commercial nursery (Graham,

unpublished data). Diseased seedlings did not respond in the

field nursery when the curative treatments were applied one

month before transplanting of Carrizo citrange seedlings in

the spring. Regardless of fungicide treatment, seedlings grew

off slowly, but roots that emerged were healthy in appearance

by late summer. During the same time, populations of T. basi

cola dropped from 3.70 to 4.00 log cfu per cm3 medium in the

greenhouse to 1.70 to 2.40 log cfu per cm3 soil in the field 4

months later. Thus, conditions in field soils in Florida do not

appear to be conducive for black root rot, even though epi

demics continue to be prevalent on greenhouse crops.

Use of bark-amended media with proper air-filled porosi

ty, plastic containers free of contamination and judicious wa

tering practices prevented establishment of T. basicola on

susceptible Cleopatra mandarin seedlings in the commercial

greenhouse in spite of the presence of airborne inoculum in

an adjacent greenhouse. Application of fungicides and a bio

logical control agent with activity against T. basicola provided

no benefit either because the inoculum pressure was not high

enough or the conditions in the medium were not sufficiently

conducive for pathogen establishment and/or black root rot

development. By contrast, in the adjacent greenhouse, use of

styrofoam containers with residual infected roots imbedded

in the walls and poorly drained peat- based medium without

bark resulted in an epidemic of black root rot on Swingle cit

rumelo. Because root damage varied greatly among seed

lings, some cells containing larger seedlings were dry next to

heavily damaged stunted seedlings that were overly wet. Thus,

the same water management practices used in the noninfest-

ed greenhouse could not compensate for the damage due to

the disease and the poorly drained medium.

While fungicides appear to be effective for black root rot

control under conducive conditions,only Banrot is currently

registered for use in the greenhouse.Given this situation and

the effectiveness of the cultural practices implemented in the

commercial nursery trial, management of T. basicola should

be attempted through modification of seedling production

practices and reduction in repeated prophylactic application

of fungicides. Mycostop was effective at high rates, suggesting

that T. basicola is susceptible to biological control, and man

agement practices to produce a biologically suppressive medi

um might be effective provided the treatments are

economical.

Proc. Fla. State Hort. Soc. 107: 1994. 25

Literature Cited

Bateman, D. F. 1962. Relation of soil pH to development of poinsettia root

rots. Phytopathology 52:559-566.

Graham, J. H. and N. H. Timmer. 1991. Peat-based media as a source of

Thielaviopsis basicola causing black root rot on citrus seedlings. Plant Dis.

75:1246-1249.

Manning, W. F., F. J. Campbell, P. M. Papia, and P. A. Hughes. 1970. Effec

tiveness of benomyl soil drenches for control of Thielaviopsis root rot of

poinsettia. Plant Dis. Rep. 54:328-330.

Merrill, L. E., K. C. SandersonJ. C. Williams, and R. B. Reed. 1986. Response

of /kx: cultivars to media and pH on the incidence of black root rot caused

by Thielaviopsis basicola (Berk. & Br.) Ferraris. J. Amer. Soc. Hort. Sci.

111:102-105.

Ribeiro, O. K. 1978. A source book of the genus Phytophthora.]. Cramer, Hir-

schberg, Germany, 417 pp.

Specht, L. P. and G. J. Griffin. 1985. A selective medium enumerating low

populations of Thielaviopsis basicola in tobacco field soils. Can. J. Plant

Pathol. 7:438-441.

Timmer, L. W. 1988. Black root rot, p. 10. In: J. O. Whiteside, S. M. Garnsey,

and L. W. Timmer (eds.). Compendium of citrus diseases. American Phy-

topathological Society, St. Paul, MN.

Tsao, P. H. 1962. Prevalence of Thielaviopsis basicola in California citrus soils.

Plant Dis. Rep. 46:357-459.

Tsao, P. H. 1963. The relative susceptibility of certain varieties and hybrids of

citrus species and relatives to Thielaviopsis basicola. Plant Dis. Rep. 47:437-

439.

Tsao, P. H. and S. D. Van Gundy. 1962. Thielaviopsis basicola as a citrus root

pathogen. Phytopathology 52:781-786.

Wills, W. H. and R. C. Lambe. 1978. Pathogenicity of Thielaviopsis basicola

from Japanese Holly (Ilex crenata) to some other host plants. Plant Dis.

Rep. 62:1102-1106.

Proc. Fla. State Hort. Soc. 107:26-29. 1994.

COMMERCIAL ORGANIC CITRUS PRODUCTION IN FLORIDA1

J.J. Ferguson

Horticultural Sciences Department

IFAS, University of Florida

Gainesville, Fl 32611

M. E. SWISHER

Home Economics Department

IFAS, University of Florida

Gainesville, Fl 32611

P. MONAGHAN

Anthropology Department

University of Florida, Gainesville 32611

Additional Index Words, survey, pest management, farm size

Abstract. A survey of commercial organic citrus growers was

conducted in 1993 to characterize organic citrus growers, their

farms and production practices. Organic growers were identi

fied from lists of organic certifying agents registered with the

Florida Department of Agriculture. Using a standardized ques

tionnaire, 16 personal interviews of organic citrus growers

were conducted, 14 of which are included here. Mean grove

size was 41 acres with a range of 15 to 200 acres. Organic cit

rus acreage is concentrated in central Florida (28%) and the In

dian River Production area (46%), with the remaining acreage

in Sarasota and DeSoto counties (12%) and South Florida

(14%). Seventy-nine percent of growers used chicken manure

as their primary nutrient source; weed management was

ranked as the most important pest problem; estimated produc

tion costs were lower in organic than conventionally managed

groves but yields, profit margins and organic versus conven

tional production efficiencies were difficult to quantify.

Organic farming was first defined as a farming system pri

or to World War I by Sir Albert Howard, a British agricultural

administrator in India. His goal, primarily economic, was to

Florida Agricultural Experiment Station Journal Series No. N-00990.

'IFAS Energy Extension Service assisted with this project.

formulate the best possible system for producing food in ar

eas where neither transportation nor funding was available to

acquire production inputs from outside sources. The organic

farming movement in the United States traces its origins and

leadership to J. I. Rodale and his son who began publishing

an organic farming and gardening magazine in Pennsylvania

in 1942. Although organic farming today represents a wide

spectrum of practices and philosophies, organic farming is

now legally defined by the Florida Organic Farming and Food

Law passed in 1990 (Statute 504.21, ff) as a "food production

system based on farm management methods or practices that

rely on building soil fertility by utilizing crop rotation, recy

cling of organic waste, application of unsynthesized minerals,

and when necessary, mechanical, botanical or biological pest

control." The term "synthetic" refers to materials that have

been "manufactured chemically, by synthesis from its element

or from chemicals as compared to a material found in na

ture."

Although the Florida Department of Agriculture main

tains records of certified organic acreage, litde documented

information is available about the range of organic citrus pro

duction practices in Florida. Other surveys of the Florida cit

rus nursery industry (Castle and Ferguson, 1982; Williamson

and Castle, 1989) and weed management in southern Florida

(Spyke et al., 1977), based on personal interviews, have de

scribed current production practices and research and exten

sion priorities. The objective of this survey was to describe

organic citrus growers in Florida, their farms and production

practices and to identify research and extension needs.

Materials and Methods

A list of organic citrus growers, those already certified

and those within the 3-year certification process, was obtained

from organic certifying agents registered with the Florida De

partment of Agriculture. A total of 16 personal interviews was

conducted with organic citrus growers in 1993, 14 of which

were included in this survey. These interviews were conduct

ed using a standardized form containing questions on grower

and farm history, land preparation, soil management, fertili-

26 Proc. Fla. State Hort. Soc. 107: 1994.