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