Article - Plant Management Network

Plant Health Research
Phytophthora nicotianae Can Cause Both Crown Rot and Foliage
Blight on Phlox paniculata in South Carolina
Daniel T. Drechsler and Steven N. Jeffers, School of Agricultural, Forest, and Environmental Sciences; and William C. Bridges,
Department of Mathematical Sciences, Clemson University, Clemson, SC 29634
Accepted for publication 1 September 2014. Published 1 November 2014.
ABSTRACT
Drechsler, D. T., Jeffers, S. N., and Bridges, W. C. 2014. Phytophthora nicotianae can cause both crown rot and foliage blight on Phlox paniculata in
South Carolina. Online. Plant Health Progress doi:10.1094/PHP-14-0020.
Phytophthora nicotianae is a common pathogen of many herbaceous
perennial plants, and this pathogen has been found causing disease on
garden phlox (Phlox paniculata) in wholesale nurseries in South Carolina
for a number of years. However, the relationship between P. nicotianae
and garden phlox has not been studied or reported previously. Using
Koch’s postulates and standard inoculation methods for Phytophthora
spp., P. nicotianae was found to cause crown rot on P. paniculata when
potting medium was infested with colonized vermiculite and to cause
foliage blight when aerial parts of the plant were inoculated with an
aqueous suspension of zoospores. Foliage blight was more similar to
symptoms we observed on garden phlox plants in wholesale nurseries,
but crown rot also has been observed previously on plants in these
nurseries. The cause of these two diseases was confirmed, but
reproduction of Phytophthora foliage blight under experimental
conditions was inconsistent. Thus, other factors not yet identified may
play a role in the development of Phytophthora foliage blight on garden
phlox in nurseries in South Carolina.
INTRODUCTION
There are numerous species of Phlox (family Polemoniaceae)
used as ornamental plants, and Phlox paniculata (common names:
garden phlox, summer phlox, or fall phlox) is one of the most
common (1,16). This species, which is native to the eastern
United States, is an upright, herbaceous perennial plant that is
grown primarily for its colorful flowers that are produced
throughout the summer (1,16). Garden phlox can be grown in
USDA Plant Hardiness Zones 4 through 8
(http://planthardiness.ars.usda.gov/PHZMWeb), so plants are best
adapted to the moderate temperate climates of central and
northern states (1). However, it often is produced in wholesale
nurseries in South Carolina, found in Plant Hardiness Zone 8a, for
the large landscape plant market on the east coast (S. N. Jeffers,
personal observation).
Phytophthora nicotianae causes several types of disease,
primarily root and crown rots and blight of aerial parts of plants,
on a wide range of host plants (6,7,10). It is the most common
species of Phytophthora attacking herbaceous ornamental plants
in South Carolina and the southeastern United States (5,6,15; S.
N. Jeffers, personal observation), where the warm, humid climate
favors growth and reproduction of this species (6). In 2003, P.
nicotianae was documented as a causal agent of root rot on Phlox
paniculata based on two samples submitted to the Clemson
University Plant Problem Clinic from a wholesale nursery in
South Carolina—root rot on a sample in 1997 and stem rot on a
sample in 2000 (5); however, this pathogen and host combination
had not been reported previously (4,6,7,9,10,11). Since 2003, two
more samples of P. paniculata with stem rot were submitted to
the clinic—one in 2005 from the same wholesale nursery that
submitted samples in 1997 and 2000, and one in 2011 from a
different wholesale nursery that produced large numbers of P.
paniculata plants. In 2011 and 2012, we observed diseased P.
paniculata plants growing in pots at the wholesale nursery that
had submitted three of the samples to the Clemson Plant Problem
Clinic between 1997 and 2005. These plants had symptoms of
foliage blight that became progressively more severe over the
summer months. The initial symptom observed on plants in the
nursery at the beginning of the growing season was discrete
lesions on succulent young leaves (Fig. 1, top); however, by the
end of the season, widespread blight on all aerial parts of affected
plants was observed (Fig. 1, bottom). Garden phlox plants at the
other wholesale nursery in South Carolina that submitted a
sample in 2011 had symptoms of both Phytophthora crown rot
and Phytophthora foliage blight. P. nicotianae was isolated
consistently from plants with foliage blight and crown rot
symptoms, and identity of this pathogen was confirmed by
morphological characters unique to this species of Phytophthora
(6) and by ITS-RFLP fingerprints (2,5). The objective of this
study was to use Koch’s postulates to confirm that P. nicotianae is
pathogenic to P. paniculata and can cause both crown rot and
foliage blight.
Corresponding author: Steven N. Jeffers. Email: sjffrs@clemson.edu
doi:10.1094 / PHP-RS-14-0020
© 2014 The American Phytopathological Society
STATISTICAL ANALYSES
Each of the three experiments in this study was conducted
twice in independent trials. All statistical analyses for these
experiments were based on a full-factorial analysis of variance
(ANOVA)—as one-way, two-way, or three-way ANOVAs—of the
main effects and the interactions between the main effects (in
two- and three-way ANOVAs). A type I error level (α) of 0.10
was used for the hypothesis tests in each experiment because of
the variability that consistently occurred with this host-pathogen
system and the exploratory nature of establishing pathogenicity.
PLANT HEALTH PROGRESS  Vol. 15, No. 4, 2014  Page 159
All statistical assumptions were checked and maintained
throughout the analyses, and all analyses were conducted using
JMP ver. 10 statistical software (SAS Institute Inc., Cary, NC).
CROWN ROT
Small (i.e., 10 to 20 cm in height), immature plants of P.
paniculata ‘Flame Purple’, ‘Flame Pink’, ‘Flame Light Pink’, and
‘Flame White Eye’ were received from a local wholesale nursery,
transplanted into 1-liter pots containing a soilless peat-based
potting medium (Fafard 3B; Sun Gro Horticulture, Agawam, MA)
and grown for 4 weeks to acclimate to greenhouse conditions
(i.e., 14-h photoperiod, 100 W/m2 minimum light intensity, and
25°C average temperature). Plants were fertilized weekly with
Peters Professional 20-10-20 Peat-Lite Special water-soluble
fertilizer (Everris NA, Inc., The Netherlands) at a rate of 13.75
ppm.
V8-vermiculite inocula for individual isolates of P. nicotianae
recovered from diseased garden phlox plants were prepared
following standard methods (5,17). Three isolates of P.
nicotianae—Flil (isolated from cultivar ‘Flame Lilac’), Fpur
(isolated from cultivar ‘Flame Purple’), and Flpk (isolated from
cultivar ‘Flame Light Pink’)—were grown on 10% clarified V8
juice agar (cV8A) (8), and then agar plugs of each isolate were
added to separate flasks of fine-textured, horticultural-grade
vermiculite moistened with 10% V8-juice broth (2:1, respectively,
by volume). Infested V8-vermiculte was placed at 25°C in the
dark for 14 days and gently shaken periodically; inoculum in each
flask then was tested for purity and thorough colonization of
vermiculite particles by P. nicotianae before being used (5,17).
Six replicate plants of each of the four cultivars of P. paniculata
were inoculated with each isolate of P. nicotianae. Plants were
inoculated by sprinkling 5 ml of inoculum on the surface of the
potting medium in each pot and then covering the inoculum with
a 1-cm-deep layer of moist potting medium; plants were watered
thoroughly to incorporate the inoculum and prevent desiccation.
Another six plants of each cultivar received only sterile V8vermiculite and served as controls. Each plant was rated weekly
for symptoms for 6 weeks using a scale from 0 to 4 based on
percentage of the aboveground portion of the plant that had
symptoms: 0 = 0–10%, 1 = 11–40%, 2 = 41–60%, 3 = 61–90%,
and 4 = 91–100%. At the end of 6 weeks, isolation of the
pathogen from symptomatic tissues was conducted using PARPHV8 selective medium (8,12,13). Colonies suspected of being P.
nicotianae were subcultured onto fresh PARPH-V8 and grown at
25°C in the dark for 7–10 days; identity then was confirmed by
distinctive morphological characters (6).
P. nicotianae caused disease on all four cultivars of P.
paniculata, resulting in extensive rotting of the root crown but
little to no damage to the roots (Fig. 2). Occasionally, aerial
hyphae (Fig. 3, left) and cankers (Fig. 3, right) were evident at the
base of diseased plants as the pathogen moved up the main stem
from the surface of the potting medium, causing stem rot and
wilting (Fig. 4). Isolations from diseased tissues consistently
confirmed the presence of P. nicotianae. A two-way analysis of
variance (ANOVA) of the main effects of isolate and cultivar
showed that disease incidence and severity differed significantly
(Table 1). Disease incidence was similar for all three isolates on
the four cultivars combined, and area under disease progress
curve (AUDPC) values revealed similar results. Disease was most
severe on plants inoculated with isolates Fpur and Flil and was
FIGURE 1
Symptoms of Phytophthora foliage blight on Phlox paniculata,
caused by Phytophthora nicotianae, at a nursery in South Carolina.
Localized, discrete lesions on leaves and petioles of P. paniculata
were visible in May—early in the growing season (top); by August,
foliage blight had become severe on these plants (bottom).
FIGURE 2
Phytophthora nicotianae caused severe crown rot (arrow) on Phlox
paniculata, but roots remained relatively intact and appeared
healthy.
PLANT HEALTH PROGRESS  Vol. 15, No. 4, 2014  Page 160
FIGURE 3
As Phytophthora crown
rot, caused by Phytophthora nicotianae,
progressed on Phlox
paniculata plants, aerial
hyphae (left) and cankers
(right) were evident at the
base of many inoculated
plants.
least severe on plants not inoculated. Some plants of each of the
four cultivars of P. paniculata became diseased by the end of the
6-week experimental period. However, not all cultivars were
equally affected; disease severity on cultivar ‘Flame Pink’ was
significantly greater than that on the other two cultivars (Table 1).
There are two key results from this study. First, isolates of P.
nicotianae recovered from diseased P. paniculata plants in a
South Carolina nursery were capable of causing crown rot on P.
paniculata plants when Koch’s postulates were followed.
Previously, this pathogen was shown to cause root rot on garden
TABLE 1
Main effects from a two-way analysis of variance
(ANOVA) for disease incidence and area under the
disease progress curve (AUDPC) when plants of four
cultivars of Phlox paniculata were grown in a greenhouse for 6 weeks after the potting medium was not
infested (control) or was infested with each of three
isolates of Phytophthora nicotianae.
Factor
Level
Disease
incidence (%)w
Isolatew
Control
Flpk
Fpur
Flil
LSD (P = 0.10)
Cultivar
Flame Light Pink
Flame Pink
Flame Purple
Flame White Eye
LSD
2-way ANOVAz
Isolate
Cultivar
Isolate × cultivar
7.3
31.3
49.0
54.2
28.7
22.9
72.9
14.6
28.8
ns
df
3
3
9
by
ab
a
a
P>F
0.088
0.121
0.211
AUDPCx
1.0
17.9
24.9
31.9
17.7
11.4
43.9
7.7
12.8
21.8
df
3
3
9
by
ab
a
a
b
a
b
b
P>F
0.084
0.079
0.320
w Mean percentage of plants (n = 12) in two trials that exhibited
symptoms over the 6-week period.
x AUDPC values were determined by monitoring disease severity weekly
FIGURE 4
When potting medium was infested with Phytophthora nicotianae,
many Phlox paniculata plants expressed symptoms of overall wilting
and decline that started at the base of the plant.
over 6 weeks. Disease severity was evaluated on a scale of 0 to 4 based
on percentage of above-ground plant tissues with symptoms: 0 = 0–
10%, 1 = 11–40%, 2 = 41–60%, 3 = 61–90%, and 4 = 91–100%.
y Means within a column for each level of a factor that have a letter in
common are not significantly different based on Fisher’s protected least
significant difference (LSD) with α = 0.10; ns = not significant.
z Two-way ANOVA for the main effects of isolate and cultivar: df =
degrees of freedom and P > F is the probability of a greater F value
occurring.
PLANT HEALTH PROGRESS  Vol. 15, No. 4, 2014  Page 161
phlox (5). Second, results demonstrated that there are differences
in susceptibility among cultivars of P. paniculata to isolates of P.
nicotianae recovered from this host plant.
Symptoms on diseased plants that were inoculated by infesting
potting medium were not the same as the symptoms observed on
diseased plants in the wholesale nursery. Inoculated plants
became chlorotic, wilted, and died from the potting medium
surface up the main stem, and leaves did not develop discrete
lesions and blighting characteristic of diseased garden phlox
plants in the nursery. Roots on inoculated plants were not
obviously affected by P. nicotianae; instead, roots appeared to be
healthy even in dead and dying plants. Most plants with evidence
of dramatic health decline had severely diseased crowns and, in
several instances, crowns were completely rotted. Therefore, roots
probably are not a primary infection court for P. nicotianae on P.
paniculata, but the crown does appear to be a primary infection
court under certain circumstances.
FOLIAGE BLIGHT
Small (i.e., 10 to 20 cm in height), immature plants of P.
paniculata ‘Flame Purple’ and ‘Peacock Cherry Red’ were
received from a local wholesale nursery. Plants were maintained
in the same greenhouse described above and were grown in a
similar manner before inoculation. The method for preparing
zoospore suspensions of P. nicotianae was adapted from the
protocol used routinely in the Jeffers Lab at Clemson University
and reported previously (14). Three 5-mm plugs were removed
from a colony of P. nicotianae isolate Fpur growing on cV8A,
placed in a 100-ml disposable petri plate, and then flooded with
15 ml of 10% clarified V8 broth (cV8B = cV8A without agar).
Cultures were incubated for 72 h at 25°C in the dark, and then the
cV8B was decanted and mycelium mats were rinsed twice with
15 ml of distilled water. A 20-ml aliquot of 1.5% non-sterile soil
extract solution (NSSES) (12) then was added to each plate.
Cultures were returned to 25°C in the dark for 48 h to allow
sporangia to form. To stimulate zoospore production and release,
colonies were placed at 15°C for approximately 15 min and then
moved to room temperature (22–25°C) for approximately 45 min.
The density of zoospores in suspension was determined using a
hemacytometer, and then distilled water was used to make a 100ml stock suspension that had a final density of 3 × 104
zoospores/ml.
Three plants of each cultivar were sprayed to drip with distilled
water using a finger-pump spray bottle and served as controls. Six
plants of each cultivar were sprayed to drip with the zoospore
suspension. All plants were placed in the dark at near-100%
relative humidity (RH). Half of the inoculated plants were
removed after a 24-h incubation period, and the remaining
inoculated and non-inoculated plants were removed after a 72-h
incubation period. All plants then were grown in the greenhouse
for 5 weeks following inoculation, and disease incidence on each
plant was evaluated weekly. When symptoms developed, diseased
tissues were plated onto PARPH-V8, and all resulting colonies
suspected of being P. nicotianae were subcultured, grown, and
identified as described above.
Plants that were inoculated with the zoospore suspension
developed symptoms that were very similar to those observed
previously in the wholesale nursery. Initially, water-soaked
lesions were visible along leaf margins and on succulent shoots.
Over time, many of the discrete initial lesions coalesced into
larger lesions that resulted in overall blighting of foliage; once
established within the petioles, infection spread into the main
stem and resulted in chlorosis, wilting, and death of distal foliage.
Although this progression of symptoms was observed on some
plants, lesion margins often were halted at nodes on other plants
(Fig. 5). Signs of P. nicotianae also were present on both cultivars
of P. paniculata: aerial hyphae were observed extending from
lesions, particularly on succulent growth and flower buds (Fig. 6).
Initial analysis by 3-way ANOVA indicated that the main
effects of treatment, cultivar, and trial were significant in their
effects on incidence of Phytophthora foliage blight on the two
cultivars inoculated with zoospores (Table 2). However, all 2-way
and 3-way interactions also were significant, so analyses were
conducted on individual cultivars within each trial. In trial 1,
disease incidence due to treatment was significantly different on
both ‘Peacock Cherry Red’ and ‘Flame Purple’ plants (Table 2);
however, a significant difference in disease incidence due to
treatment only occurred on ‘Flame Purple’ plants in trial 2
(Table 2). For both trials, foliage blight only occurred on
inoculated plants, and disease incidence occurred most
consistently when inoculated plants were exposed to a 72-h
incubation period (Table 2). The longer infection period
apparently provided the pathogen a more conducive environment
for infection and establishment within this host.
Inoculation with a zoospore suspension was effective at
producing Phytophthora foliage blight on garden phlox. Discrete
lesions were observed following 24- and 72-h incubation periods.
Many of these lesions progressed into petiole and upper stem
tissues, resulting in a blighting of succulent tissues. However, the
pathogen seemed to be unable to colonize the woody stems of
garden phlox plants.
FOLIAGE BLIGHT: THE EFFECTS OF HIGH RELATIVE
HUMIDITY
Based on results from the first experiment on foliage blight,
periods of high RH appeared to be important for infection and
disease development, so the role of RH was studied further. Small
(i.e., 10 to 20 cm in height), immature plants of P. paniculata
‘Laura’ were received from a local wholesale nursery,
transplanted, and grown as in other experiments. Three sets of
experimental RH conditions were established: low, moderate, and
high. The ambient environment of a growth room (i.e., 54 ± 0.1%
RH, 25 ± 1.8°C, and a 14-h photoperiod) was used to maintain
the lowest RH level. The moderate RH level (i.e., 88 ± 13.4%)
was produced in a humidity chamber that was built within the
growth room. The humidity chamber consisted of a PVC pipe
frame (1.2 m × 1.2 m × 2.4 m) covered with a 1-mm-thick clear
plastic sheet in which an ultrasonic humidifier (PureGuardian
H4500; Guardian Technologies, Mentor, OH) was run
continuously. Vents and a small intake fan were installed to
regulate the internal temperature (28 ± 2.8°C) of the chamber and
provide adequate air circulation. High RH (i.e., 100%) was
achieved by the use of a large moist chamber placed inside the
growth room; the moist chamber was a plastic tub (45 cm ×
35 cm × 75 cm) containing approximately 2 liters of water
covered by inverting a second, similar tub. Temperature and RH
were recorded using a HOBO Micro Station data logger with a
12-bit temperature and RH sensor (Onset Computer Corporation,
Bourne, MA).
PLANT HEALTH PROGRESS  Vol. 15, No. 4, 2014  Page 162
FIGURE 5
FIGURE 6
Symptoms of Phytophthora foliage blight and advance of the
pathogen, Phytophthora nicotianae, were halted at nodes on some
garden phlox plants
Under humid conditions, aerial hyphae (circled) of Phytophthora
nicotianae extended from lesions, particularly on succulent young
shoots and flower buds of Phlox paniculata plants affected by
foliage blight.
TABLE 2
2
TABLE
Disease incidence on plants of two cultivars of Phlox paniculata that were grown in a greenhouse for
Disease
incidence
on
plants
of
two
cultivars
of
Phlox
paniculata
that were
in a greenhouse
for
5 weeks after foliage was not inoculated or inoculated with zoospores
ofgrown
Phytophthora
nicotianae
5 weeks after foliage
was
notsubjected
inoculated
inoculated
with zoospores
of Phytophthora nicotianae
and
then
toor
incubation
periods
of two durations.
and then subjected to incubation periods of two durations.
Disease incidencew (%)
Treatmentx
Trial 1
Inoculum
Incubation
period (h)
–
+
+
72
24
72
Peacock
Cherry Red
Trial 2
Flame
Purple
Peacock
Cherry Red
Flame
Purple
0.0 b
0.0 b
0.0
0.0 b
33.3 a
0.0 b
0.0
33.3 a
40.0 a
60.0 a
13.3
33.3 a
LSD
18.1
9.7
ns
0.1
1-way ANOVAy
df
P>F
df
P>F
df
P>F
df
P>F
Treatment
2
0.004
2
<0.0001
2
0.397
2
<0.0001
3-way ANOVAz
df
P>F
Treatment
2
<0.0001
Cultivar
1
0.0450
Trial
1
0.0085
Treatment × cultivar
2
0.0202
Trial × treatment
2
0.0014
Trial × cultivar
1
0.0012
Trial × treatment × cultivar
2
<0.0001
w Mean percentage of plants of each cultivar in each of two trials (n = 6) that exhibited symptoms over 5 weeks. Means within a column that have a letter in
common are not significantly different based on Fisher’s protected least significant difference (LSD) with α = 0.10; ns = not significant.
x Treatments consisted of plants that were sprayed with distilled water (– inoculum) or sprayed with a zoospore suspension of P. nicotianae isolate Fpur (3 ×
104 zoospores/ml) (+ inoculum) followed by an incubation period of 24 or 72 h during which plants were held in a moist chamber at 100% relative
humidity.
y One-way analysis of variance (ANOVA) for the simple effect of treatment on each cultivar within a trial (n = 3): df = degrees of freedom and P > F is the
probability of a greater F value occurring.
z Three-way ANOVA for the main effects of treatment, cultivar, and trial (n = 12).
PLANT HEALTH PROGRESS  Vol. 15, No. 4, 2014  Page 163
A suspension of zoospores of P. nicotianae isolate Flpk
(3 × 104 zoospores/ml) was prepared as described previously and
used as inoculum. Groups of six replicate plants were sprayed
with zoospore suspension or distilled water and then exposed to
one of the three RH conditions for an initial 72-h incubation
period in the dark. Following the initial incubation period, plants
were placed at one of two RH levels (54 ± 0.1% RH or 88 ±
13.4% RH) for a 14-day post-incubation period. All plants were
evaluated at 7 and 14 days after inoculation (DAI) for disease
incidence and severity, using a scale from 0 to 5 based on the
percentage of foliage with symptoms: 0 = 0% (healthy); 1 = 1–
25%; 2 = 26–50%; 3 = 51–75%; 4 = 76–99%; 5 = 100% (dead).
Infection by P. nicotianae was confirmed by isolation on PARPHV8 medium as described previously.
Zoospores of P. nicotianae caused disease on plants of P.
paniculata ‘Laura’ when foliage was inoculated: discrete lesions
on leaves (Fig. 7, top) coalesced and moved into petioles (Fig. 7,
bottom left), resulting in blighting of succulent foliage (Fig. 7,
bottom right). Foliage blight occurred only on inoculated plants,
and disease severity was significantly greater when inoculated
plants were exposed to a 100% RH incubation period for 72 h
(Table 3).
For foliage blight to occur in the nursery, it appears that
propagules of P. nicotianae need to be disseminated to the foliage
by some means—e.g., splashed from infested soil or delivered in
contaminated water by overhead irrigation. Based on results from
the experiment with infested potting medium, it did not appear
that inoculum was splashed from the potting medium because
overhead hand-watering was used on plants in the greenhouse,
and plants did not develop foliage blight symptoms. However, in
an outdoor nursery, rain or sprinkler-applied irrigation may be
more effective at splashing inoculum onto the foliage—from the
soilless medium in the pots or from the surface on which pots are
sitting.
High RH levels were critical during the incubation period for
foliage blight to develop, but RH levels in the subsequent postincubation growth period were less important to the progression
of Phytophthora foliage blight on garden phlox. Apparently, once
infection occurred and the pathogen became established within
susceptible host tissue, disease progressed regardless of ambient
RH levels. These results demonstrate the role of moisture and RH
in creating a conducive environment—an essential component of
the disease triangle. If the incubation period was not long enough
or the RH level was not 100%, disease progress was arrested,
presumably because zoospores died or hyphae desiccated before
infection and establishment were successful.
CONCLUSIONS AND RECOMMENDATIONS
P. nicotianae is pathogenic to garden phlox, P. paniculata,
plants and can cause both crown rot and foliage blight on this
host. It is interesting that these diseases have not been reported
TABLE 3
Effect of relative humidity (RH) on severity of
Phytophthora foliage blight on plants of Phlox paniculata
‘Laura’ at 14 days after inoculation (DAI) with zoospores of
an isolate of Phytophthora nicotianae.
Treatment
72-h incubation periodw
FIGURE 7
Symptoms of Phytophthora foliage blight on garden phlox (Phlox
paniculata), caused by Phytophthora nicotianae, started as discrete
lesions on leaves (top) and petioles (bottom left) but lesions
coalesced over time and resulted in foliage blight (bottom right).
Inoculum
RH (%)
+
+
+
+
–
–
100
100
87
54
87
54
Postincubation
RHx
%
41.9
40.3
2.2
0.0
0.0
0.0
Transformed
5.24 a
4.86 a
0.60 b
0.00 b
0.00 b
0.00 b
2.58
2-way ANOVAy
df
P>F
DAIz
1
0.3861
Treatment
5
0.0047
DAI × treatment
5
0.9124
v Disease severity was evaluated on a scale of 0 to 5 based on the
percentage of foliage with symptoms: 0 = 0%; 1 = 1–25%; 2 = 26–
50%; 3 = 51–75%; 4 = 75–99%; 5 = 100%. Data are from two trials (n
= 6) and means were calculated using the median for each range.
Datum analysis was conducted using a square-root transformation
(Transformed) of the median values. Means that have a letter in
common are not significantly different based on Fisher’s protected
least significant difference (LSD) with α = 0.10.
w Plants were sprayed with distilled water (– inoculum) or with a
suspension of 3 × 104 zoospores/ml (+ inoculum) from isolate Flpk.
Plants then were placed in a growth chamber with low ambient RH
(54.0 ± 0.1%), medium RH (87.0 ± 14.0%), or high (100%) RH for
72 h.
x After the incubation period, plants were maintained in a growth room
at ambient (54.0 ± 0.1%) or high (88.4 ± 13.4%) RH for 11 days. All
other environmental parameters (i.e., photoperiod, light intensity,
temperature, etc.) were similar for the two growing conditions.
y Two-way analysis of variance (ANOVA) for the main effects of
treatment and DAI; df = degrees of freedom and P > F is the
probability of a greater F value occurring.
z Plants were evaluated at 7 and 14 DAI, but treatments were not
significantly different at 7 DAI; therefore, only results at 14 DAI are
shown.
PLANT HEALTH PROGRESS  Vol. 15, No. 4, 2014  Page 164
High
Ambient
High
Ambient
High
Ambient
LSD
Disease
severityv
previously even though garden phlox is a very popular and widely
recommended plant for landscapes (1,16). However, Dr. Allan M.
Armitage makes this statement about P. paniculata in his book
Herbaceous Perennial Plants (1): “Plants perform better in the
North than in the deep South as they are not particularly heat
tolerant. Under prolonged hot summer conditions, plant vigor
diminishes and susceptibility to root rot organisms increases.”
These same environmental conditions that apparently stress
P. paniculata are optimum for growth and reproduction of
P. nicotianae (6), which may explain why these diseases have
been such problems in two wholesale nurseries in South Carolina,
where large quantities of P. paniculata plants are grown in
containers on nursery beds through the summer in full sun when
ambient daytime temperatures routinely range between 30 and
40°C .
The incidence and severity of foliage blight on P. paniculata
plants were dependent on high relative humidity during the initial
incubation period when infection occurred and the pathogen
became established within the host; however, disease incidence
varied considerably under the controlled conditions of the
greenhouse and growth room. This suggests that other
environmental factors besides RH may contribute to the
development of Phytophthora foliage blight on garden phlox in
the nursery, which could be exacerbated in warm, humid climates.
Therefore, nursery production of garden phlox may be better
suited to the cooler climates of more northern regions of the
country. Future research to identify these contributing or
predisposing factors will be necessary for growers in the
southeastern region to tailor their strategies to manage this
potentially devastating disease. Currently, the most effective
method by which these diseases can be managed appears to be the
exclusion of propagules of P. nicotianae from container-grown
plants and irrigation water. This can be done through proper
sanitation of materials, resources, and tools used during routine
production practices in nurseries (3,4,18).
ACKNOWLEDGMENTS
We thank L. A. Luszcz, S. R. Sharpe, C. A. Scott, and S. G. I. Schreier
for technical assistance and Stacy’s Greenhouses and Layman Wholesale
Nurseries for supplying many of the plants used in this study. This study
was supported by the United States Department of AgricultureAgricultural Research Service (USDA-ARS) specific cooperative
agreement number 58-1907-0-103 titled Tracking and Managing
Diseases of Floriculture Crops Caused by Oomycetes and Fungi, which
is part of the USDA-ARS Floriculture and Nursery Research Initiative.
Technical contribution no. 6258 of the Clemson University Experiment
Station. This material is based upon work supported by the NIFA/USDA,
under project number SC-1700445.
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PLANT HEALTH PROGRESS  Vol. 15, No. 4, 2014  Page 165