PEST MANAGEMENT: DISEASES
Relationship of Cold-Tolerance and
Pythium Resistance to Rice Stand Establishment
C.S. Rothrock, R.L. Sealy, F.N. Lee, J. Gibbons, and R.D. Cartwright
ABSTRACT
Stand problems consistently cause significant production losses and management
problems in Arkansas rice fields. Previous research, funded by the Rice Research and
Promotion Board, found that Pythium species played an important role in stand establishment, especially under cool soil temperatures. Assays to screen genotypes reported
to be cold-tolerant and thus adapted to these soil temperatures found about 8% of the
cold-tolerant genotypes had at least moderate resistance to the pathogenic Pythium
isolate used. Another 9% of the genotypes tested were moderately susceptible, and
81% of the genotypes studied had extremely low stand counts in the infested treatment,
indicating a high degree of susceptibility. Controlled environmental studies screening
for Pythium resistance with cold-tolerant Pythium-resistant genotypes demonstrated
negligible impact on stand density with the use of fungicides. In trials at Stuttgart, Colt,
and Keiser in 2005, stand counts for the untreated seed and the relative stand counts for
the Allegiance/untreated seed treatments support the growth-chamber studies. Pythiumand cold-tolerant genotypes identified in these studies demonstrate that reliable stand
establishment is possible in marginal planting environments.
INTRODUCTION
Stand problems consistently cause significant production losses and management problems in Arkansas rice fields. Pythium species play an important role in stand
establishment, especially under cool soil temperatures (Rothrock et al., 2003). Genotypes of rice have been recently identified that have the ability to germinate and grow
at colder temperatures than current cultivars, i.e. cold-tolerance. Early results show
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that these cold-tolerant genotypes differ in resistance to Pythium-incited diseases and
that this character will need to be separately screened for as cold-tolerant cultivars are
developed (Rothrock et al., 2004). The objective of this project was to screen genotypes of rice that have been evaluated for cold-tolerance for their level of resistance to
Pythium seed rot and damping-off. Selected genotypes were rated for cold tolerance
and Pythium resistance in the field in 2005 to validate the screening procedure for Pythium resistance. Preliminary results of the screening procedure have been presented
(Rothrock et al., 2005).
PROCEDURES
Three hundred forty-six rice genotypes from Dr. Gibbons’ program with demonstrated cold tolerance were selected for this study. The pathogen used in this study
was a Pythium species isolated from rice seedling roots taken from an Arkansas rice
field. Previous work in this laboratory has demonstrated this isolate to be virulent to
rice seeds and seedlings at various soil temperatures. The pathogen was grown in
homemade corn meal broth, amended with <1 g/L of commercially available potato
dextrose broth powder (Difco Laboratories, Detroit, Mich.), by inoculating flasks with
plugs from cultures grown on CMA and incubating at room temperature on a shaker
for 8 to 12 days. Inoculum was prepared by harvesting the mycelium by filtration
through a sterile Buchner funnel with suction, washing the mycelial mat twice with
sterile distilled water (SDW), resuspending in SDW and blending briefly (<10 sec) in a
sterile Waring blender, and diluting to approximately 20 g of hyphae/L in SDW. About
12 g of hyphae were mixed in sterilized vermiculite growth medium in each tray for
the infested treatment. Ten seeds of each genotype were planted in an individual plot
in the growth trays. Infested treatments had three replications; the noninfested treatments had two replications. In each growth chamber, three standard genotypes of rice
(controls) were included; PI560281 (cold tolerant and moderately resistant to Pythium),
Kaybonnet (cold tolerant, but susceptible to Pythium), and Lemont (neither cold tolerant
nor resistant to Pythium). After planting, trays were placed in a growth chamber kept
at a constant temperature of 59°F with a 13-hr light/11-hr dark period. Data, consisting of stand counts, were collected five weeks after planting. Data were analyzed as a
percentage of the stand for a plot compared to the mean of the noninfested controls for
that genotype. Germination studies were done with each genotype by placing 20 seeds
of each genotype in a sterile Petri dish containing a sterile filter paper disk moistened
with SDW.
Field studies were initiated in 2005 to screen selected genotypes for cold-tolerance
and Pythium resistance and to validate the controlled-environment screening procedure.
Three planting date studies at three locations in Arkansas were conducted. Plantings
were mid-February, mid-March, and mid-April. The test locations were Pine Tree
Branch Experiment Station (Colt), Northeast Research and Extension Center (Keiser),
and Rice Research and Extension Center (Stuttgart) representing the White River, Delta,
and Grand Prairie ecosystems, respectively. Each trial included 42 genotypes with the
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following seed treatments; 1) not treated 2) Allegiance, 1.5 oz/cwt (metalaxyl), or 3)
Dynasty 100 FS, 0.5 oz/cwt(azoxystrobin) + Apron XL, 0.32 oz/cwt (metalaxyl-M) +
Maxim 4 FS, 0.04 oz/cwt (fludioxonil). Each test was a split-plot design with genotype
as the main plot and fungicide treatment as the subplot. A plot was one row of 40 viable
seed. Stand was taken for each plot approximately four weeks after planting. Analyses
included stand, relative stand compared among sites, and relative stand between the
fungicide and nontreated seed treatments.
RESULTS AND DISCUSSION
Growth chamber studies involving potting media infested with a virulent Pythium
isolate showed all genotypes screened to date had some reduction in stand in the presence of Pythium. Only 8% of the genotypes screened had stand counts in the infested
treatment comparable to or better than those of the resistant control, indicating at least
moderate resistance. Another 9% of the genotypes tested had stand counts exceeding
those of the susceptible and cold-resistant standard, but less than those of the resistant
control, indicating some degree of resistance to the pathogen. The other 81% of the
genotypes studied had extremely low or no stand counts in the infested treatment,
indicating a high degree of susceptibility to this pathogen. A few genotypes evaluated
under these conditions had extremely low stand counts in even the noninfested treatments, indicating that they may not possess the ability to reliably germinate and grow
at cold temperatures.
Analyses of the 42 genotypes in eight field trials at Stuttgart, Colt, and Keiser
indicated that the genotypes differed in stand establishment and fungicide response.
Mean daily soil temperatures for the first four weeks after planting the eight trials in
February, March, and April were Keiser 55°F, 59°F, and 64°F; Stuttgart 52°F, 57°F,
and 63°F; and Colt 50°F, 57°F, and no trial, respectively. The Keiser April planting date
had the warmest soil temperatures and highest mean stand. Therefore, all of the dates
and locations were compared to this trial. Relative mean stand of a trial (mean stand of
trial/mean stand of Keiser April trial) for all genotypes over all treatments varied from
0.34 for the Keiser February planting to 0.97 for the Stuttgart March planting. Relative stand for a genotype (mean genotype stand for a trial/mean genotype stand for the
Keiser April trial) over trials varied from 1.00, indicating the genotype had the same
stands over all trials, to 0.34, indicating poor cold tolerance. Seedling disease pressure
based on the Allegiance response (Allegiance stand/nontreated stand) for the trials
ranged from a relative stand of 0.99 for the Colt March trial to 1.46 for the Stuttgart
March trial. For all fungicide treatments, disease pressure (fungicide stands/nontreated
stand) ranged from a relative stand of 1.14 for the Colt March trial to 1.62 for the
Stuttgart February trial. The improvements in stand with the complete seed treatment
(Dynasty+Apron+Maxim) compared to Allegiance indicated pathogens in addition to
Pythium spp. may be important in stand establishment.
Analyses of all trials (Stuttgart, Colt, and Keiser) suggest that stand counts for
the untreated seed and the relative stand for the Allegiance/untreated seed treatments
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support results from the controlled environmental studies screening for Pythium resistance. A relative stand for a genotype of 1.0 for the Allegiance treatment when divided
by the mean stand of the nontreated seed for that genotype would indicate it had good
resistance to Pythium spp. because a fungicide active against Pythium spp. gave no
stand improvement. Pythium-resistant genotypes based on the growth-chamber assays
generally demonstrated little improvement with the use of Allegiance, giving a relative
stand close to 1.0 (Table 1).
These studies have identified specific genotypes with cold tolerance and Pythium
resistance demonstrating reliable stand establishment can occur in Arkansas rice fields.
Results from field studies in 2005 were generally in agreement with the results from the
controlled environmental disease-screening procedure showing the value of screening
for Pythium resistance.
SIGNIFICANCE OF FINDINGS
These studies have identified specific genotypes with cold tolerance and Pythium
resistance holding the promise for more reliable stand establishment for rice in Arkansas
under marginal planting environments. The research also points to the value of screening
rice for seedling-disease resistance as new cultivars are being developed..
ACKNOWLEDGMENTS
This research was conducted with the support of the Rice Research and Promotion Board.
LITERATURE CITED
Rothrock, C.S., R.L. Sealy, F.N. Lee, M.M. Anders, and R.D. Cartwright. 2003.
Managing seedling disease problems on rice through fungicides, adapted cultivars, and cropping systems. In: R.J. Norman and J.-F. Meullenet (eds.). B.R.
Wells Rice Research Studies 2002. University of Arkansas Agricultural Experiment Station Research Series 504:237-241. Fayetteville, Ark.
Rothrock, C.S., R.L. Sealy, F.N. Lee, M.M. Anders, and R.D. Cartwright. 2004.
Reaction of cold-tolerant adapted rice cultivars to seedling disease caused by Pythium species. In: R.J. Norman, J.-F. Meullenet, and K.A.K. Moldenhauer (eds.).
B.R. Wells Rice Research Studies 2003. Univeristy of Arkansas Agricultural
Experiment Station Research Series 517:207-210. Fayetteville, Ark.
Rothrock , C.S., R.L. Sealy, F.N. Lee, M.M. Anders, and R.D. Cartwright. 2005.
Reaction of cold-tolerant rice genotypes to seedling disease caused by Pythium
species. In: R.J. Norman, J.-F. Meullenet, and K.A.K. Moldenhauer (eds.). B.R.
Wells Rice Research Studies 2004. University Arkansas Agricultural Experiment
Station Research Series 529:120-124. Fayetteville, Ark.
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Table 1. Field and growth chamber evaluation of
selected genotypes for resistance to Pythium species in 2005.
Genotype
STG 03P-09-058
STG 03-PR-02-096
STG 03AC-07-026
STG 03AC-07-066
STG 03-AC-07-070
Kaybonnet
Wells 97P40 STG 03P-03-042
STG 03PR-11-010
STG 03PR-03-048
STG 03PR-07-055
CyBonnet
STG 03PR-07-066
STG 03F5-12-033
STG 03PR-03-037
STG 03P-07-048
PI 597085
STG 03PR-10-127 z A value
Relative field stand
(fungicide/untreated)z
2.03 a 1.38 b
1.37 bc 1.31 bcd
1.29 bcd
1.29 bcd 1.28 bcd
1.27 bcd 1.26 bcde
1.25 bcde
1.25 bcde 1.21 cdef
1.20 cdef
1.18 def
1.17 def
1.13 def
1.08 ef
1.07 f
1.05 f
Disease reaction
(growth chamber)
S
S
R
S
S
S
S
R
S
R
R
R
R
R
R
R
R
R
R
of 1 indicates the fungicide did not improve stand over the nontreated seed and thus
the genotype has a higher level of resistance than a genotype with a relative stand >1. Means
followed by the same letter are not significantly different, protected LSD (P=0.05).
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