PEST MANAGEMENT: DISEASES
Role of Soil Salinity in Rice Seedling
Disease Severity Caused by Pythium Species
M.A. Eberle, C.S. Rothrock, N.A. Slaton, and R.D. Cartwright
ABSTRACT
Pythium spp. are the most common seedling disease pathogens isolated from
rice in producers’ fields in Arkansas. Pythium arrhenomanes and P. irregulare are the
most frequently isolated and virulent of the Pythium species in Arkansas. Non- or less
virulent Pythium species include P. catenulatum, P. torulosum, and P. diclinum. This
study examined the role of soil salinity on seedling disease severity using P. torulosum.
Soil salinity was shown to be an important factor in rice stand establishment in the
presence of P. torulosum. Damage from P. torulosum increased dramatically at salinity
levels that would not normally cause stand losses or reductions in growth. In addition,
for electrical conductivity treatments greater than 2022 μS/cm, soil salinity reduced
root and foliar weight of seedlings and increased leaf necrosis. This research suggests
that the interaction between seedling disease pathogens commonly found in soils and
salinity may result in greater stand losses.
INTRODUCTION
Stand establishment problems consistently cause significant production losses
and management problems in Arkansas rice fields. The causes of stand problems are
often difficult to determine; thus practices that would eliminate or reduce the amount
of losses are not able to be implemented. Stand problems have been associated with
environmental and soil factors, herbicides, insects, and seedling diseases (Rush, 1992).
Pythium spp., especially P. arrhenomanes and P. irregulare, are often the most important
seedling pathogens on rice and damage is increased under cold soil temperatures (Cother
and Gilbert, 1993; Eberle et al., 2008; Rush, 1992). Other less virulent Pythium species
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isolated from rice include P. catenulatum, P. torulosum, and P. diclinum. Research,
funded by the Rice Research and Promotion Board, has identified cold-tolerant Pythiumresistant rice genotypes that hold the promise for more reliable rice stand establishment
in Arkansas under marginal planting environments (Rothrock et al., 2004, 2005, 2006).
Soil salinity is another soil factor that may affect rice stand establishment. Rice is very
sensitive to increasing soil salinity levels (Maas and Hoffman, 1977; Shannon et al.,
1998), and the seedling stage is more sensitive than other growth stages (Heenan et al.,
1988; Kaddah, 1963; Lutts et al., 1995; Pearson and Bernstein, 1959).
The objective of this study was to examine the role of soil electrical conductivity (salinity) in rice stand establishment and the development of rice seedling disease
caused by Pythium torulosum.
PROCEDURES
The importance of soil salinity on seedling disease caused by Pythium spp. was
examined in an experiment using two infestation treatments, noninfested and infested,
and five salinity treatments in a factorial arrangement. Soil from a field near Lake Hogue
in Poinsett County, Ark., was pasteurized at ~70°C for 30 minutes to remove soilborne
plant pathogens and 375 g of soil, equivalent oven dry weight, was placed in styrofoam
containers (115 mm x 75 mm). Inoculum of P. torulosum was grown on sand-corn meal
medium for 10 days prior to adding to soil for the infested soil treatments. Electrical
conductivity (EC) was adjusted with a 1 M calcium chloride (CaCl2) solution. The range
of EC treatments used were based on ranges found in Arkansas from soil samples taken
previously (400 to 5000 μS/cm) (data not shown). Five EC treatments were established
by adding 1 M CaCl2 solution to the soil in each pot: 18.75 mL, 12.5 mL, 6.25 mL, 3.13
mL, and 0 mL per pot, respectively. The 1 M CaCl2 solution was added with enough
water to saturate the soil and each container was placed in a saucer to prevent loss of
CaCl2 during the duration of the experiment. Soil EC was measured in each pot using
a 1:2 soil weight:water volume mixture. Six seed of the cultivar ‘Wells’ were planted
in each pot and pots were arranged in a randomized complete block design with four
replications. The experiment was conducted in the greenhouse, with an average temperature of 24°C. Containers were watered with deionized water when the soil matric
potential reached levels between -10 J/kg and -30 J/kg.
Stand counts were taken at 2 (emergence) and 5 weeks (final stand). At the
termination of the experiment, seedlings were removed and leaf number, root weight,
root discoloration, percent leaf necrosis, and aboveground seedling dry weight were
recorded. Root discoloration and leaf necrosis were assessed on a 1 to 5 scale with 1 =
none, 2 = 1 to 10%, 3 = 11 to 25%, 4 = 26 to 50%, and 5 = 51 to 100% discoloration
or necrosis. Isolation was done by harvesting the seedlings from the soil and rinsing them for 20 minutes under tap water. After rinsing, the above- and below-(roots)
ground plant parts were cut apart and the roots were disinfested in 0.5% NaOCl for 1.5
minutes, blotted dry, and plated on water agar for isolation of the pathogen. Analysis
was done by GLM using SAS for the main effects of infestation and soil salinity and
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main effects and interactions examined as appropriate. Means were separated using a
protected Fisher’s LSD.
RESULTS AND DISCUSSION
Electrical conductivity levels in the experiment averaged 428 μS/cm for the field
soil. Soil EC levels for the other treatments receiving increasing amounts of CaCl2 were:
1144 μS/cm, 2022 μS/cm, 3543 μS/cm, and 4862 μS/cm. Seedling emergence after
two weeks averaged 4.2 plants of the 6 seed planted for soil infested with P. torulosum
and 3.7 plants for the non-infested control across salinity treatments. Emergence was
reduced in soil having an EC >2022 μS/cm (Table 1). There was a significant salinity
by infestation interaction for final plant stands at five weeks after planting (p = 0.0493),
indicating the effect of P. torulosum on rice was dependent on soil salinity. In the presence of P. torulosum, stands were reduced at salinities as low as the 1144 μS/cm salinity treatment, but differences between infested and non-infested treatments were most
apparent in the 2022 μS/cm salinity treatment. For soil having an EC of 2022 μS/cm,
stands for the infested treatment significantly differed from those of the non-infested
treatment and the P. torulosum 428 μS/cm salinity treatment (Table 1). In soil that did
not have any CaCl2 solution added, P. torulosum had a stand of 5.2 compared to 4.2
for the non-infested control suggesting that this Pythium species is not important in
rice stand establishment under conditions when rice is not under another stress. Results
of this study suggest that in fields having soil with moderate salinity problems, damage from Pythium species will be more severe. Stand losses due to salinity were not
significantly different from the control for the non-infested treatments until a salinity
treatment of 3543 μS/cm. The salinity effect is most likely producing a stress on the
plant increasing its susceptibility to Pythium spp. rather than salinity increasing the
activity of the pathogen.
In addition to stand losses, soil having ECs ≥ 2022 μS/cm increased root discoloration compared to the control (428 μS/cm) (Table 2). Root and shoot weights were
decreased and leaf necrosis increased at 3543 μS/cm compared to the control. This
research is similar to other research showing soil salinity is important in rice development (Heenan et al., 1988; Kaddah, 1963; Lutts et al., 1995; Pearson and Bernstein,
1959; Shannon et al., 1998).
SIGNIFICANCE OF FINDINGS
Field and controlled environmental studies to date have examined the importance
of different seedling pathogens and soil and environmental factors on establishment of
rice. Five different Pythium species have been identified and their importance characterized; Pythium arrhenomanes, P. irregulare, P. torulosum, P. catenulatum, and P. diclinum.
Soil salinity is an important soil factor in rice stand establishment and was shown to
increase the virulence of P. torulosum on rice seedlings. Additional research needs to
be done on different pathogenic species of Pythium to better define the importance of
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soil salinity levels on stand establishment. It is likely that the more virulent species will
cause even more stand losses and an increase in disease symptoms at moderate soil
salinities. The research suggests that salinity may be a significant factor affecting rice
stand density as a result of its interaction with seedling disease pathogens commonly
found in soils. The knowledge of salinity and its effects on the virulence of different
pathogens could be a useful tool to assist producers in determining environmental conditions that may limit stands and seedling development in the field and help to select
appropriate management practices.
ACKNOWLEDGMENTS
This research was conducted with the support of the Arkansas Rice Research
and Promotion Board.
LITERATURE CITED
Cother, E.J. and R.L. Gilbert. 1993. Comparative pathogenicity of Pythium species
associated with poor seedling establishment of rice in Southern Australia. Plant
Pathology. 42:151-157.
Eberle, M.A., C.S. Rothrock, and R.D. Cartwright. 2008. Pythium species associated
with rice stand establishment problems in Arkansas. In: R.J. Norman, J.-F. Meullenet, and K.A.K. Moldenhauer (eds.). B.R. Wells Rice Research Studies 2007.
University of Arkansas Agricultural Experiment Station Research Series 560:5763. Fayetteville.
Heenan, D.P., L.G. Lewin, and D.W. McCaffery. 1988. Salinity tolerance in rice
varieties at different growth stages. Aust. J. Exp. Agric. 28:343-349.
Kaddah, M.T. 1963. Salinity effects on growth of rice at the seedling and inflorescence stages of development. Soil Sci. 96:105-111.
Lutts, S., J.M. Kinet, and J. Bouharmont. 1995. Changes in plant response to NaCl
during development of rice (Oryza sativus L.) varieties differing in salinity resistance. J. Exp. Bot. 46:1843-1852.
Maas, E.V. and G.J. Hoffman. 1977. Crop salt tolerance-current assessment. J. Irrig.
Drain. Div., ASCE 103 (IR2):115-134.
Pearson, G.A. and L. Bernstein. 1959. Salinity effects at several growth stages of
rice. Agron. J. 51:654-657.
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. University of Arkansas Agricultural
Experiment Station Research Series 517:207-210. Fayetteville.
Rothrock, C.S., S.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. M Meullenet, and K.A.K. Moldenhauer (eds.). B.R. Wells
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Rice Research Studies 2004. University of Arkansas Agricultural Experiment Station Research Series 529:120-124. Fayetteville.
Rothrock, C.S., R.L. Sealy, F.N. Lee, J. Gibbons, and R.D. Cartwright. 2006. Relationship of cold-tolerance and Pythium resistance to rice stand establishment. In:
R.J. Norman, J.-F. Meullenet, and K.A.K. Moldenhauer (eds.). B.R. Wells Rice
Research Studies 2005. University of Arkansas Agricultural Experiment Station
Research Series 540:138-142. Fayetteville.
Rush, M.C. 1992. Fungal diseases; seedling diseases. Pages 12-13 In: R.K. Webster
and P.S. Gunnell (eds.). Compendium of Rice Diseases. APS Press.
Shannon, M.C., J.D. Rhoades, J.H. Draper, S.C. Scardaci, and M.D. Spyres. 1998.
Assessment of salt tolerance in rice cultivars in response to salinity problems in
California. Crop. Sci. 38:394-398.
Table 1. Effect of soil salinity and Pythium torulosum on
emergence, averaged across soil infestation, and final rice
seedling stands as affected by the interaction of salinity and soil infestationz.
Soil electrical
Plant stand
conductivity
(μS/cm) Emergence
Non-infested
P. torulosum
4.2 abc
4.8 ab
3.5 c
1.2 d
0.5 de
5.2 a
4.0 bc
0.5 de
0.5 de
0.0 e
428
1144
2022
3543
4862
4.9 ay
5.5 a
4.8 a
3.0 b
1.6 c
Emergence (2 weeks) and stand (5 weeks) from 6 seed per container with 4 replications.
Means for emergence or stands, followed by the same letter are not significantly different,
Fisher’s protected LSD (p=0.05).
z
y
Table 2. Effect of soil salinity on plant development in noninfested soil.
Electrical
conductivity
(μS/cm)
428
1144
2022
3543
4862x
Leaf
number
Root
discolorationz
Root fresh
weight
Leaf
necrosisz
Foliar dry
weight
(g)
(g)
3.3 ay
3.3 a
3.4 a
2.6 a
--
9.9 c
15.8 bc
39.2 ab
58.0 a
--
0.247 a
0.220 a
0.150 ab
0.056 b
--
0.2 a
5.3 a
9.7 a
28.5 a
--
0.041 a
0.039 a
0.034 a
0.014 b
--
Average percentage of root discoloration or leaf necrosis (mid-percentile values) for the scale:
1 = none, 2 = 1 to 10%, 3 = 11 to 25%, 4 = 26 to 50%, and 5 = 51 to 100% discoloration or
necrosis.
y
Means in a column followed by the same letter are not significantly different, Fisher’s protected
LSD (p=0.05).
x
Means are not given as a result of limited number of plants and replications for analysis as a
result of plant death.
z
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