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SUSCEPTIBILITY TO SCLEROTIUM ORYZAE AND RHIZOCTONIA SPP.
OF
URUGUAYAN RICE CULTIVAR
RS
S. Martínez A and F. Escalante A
A
INIA Treinta y Tres, CP 33000, Treinta y Tres, Uruguay. Email: smartinez@tyt.inia.org.uy.
ABSTRACT. Eight common Uruguayan lines and rice cultivars were tested for susceptibility to Sclerotium oryzae and
Rhizoctonia spp. in orderr to establish a baseline of knowledge about
the response of these cultivar to these pathogens. The
evaluation
was conductedd in two experiments at two times of inoculation and two disease rating times. The experiments were
conductedd in plastic pots
under glasshouse with controlled conditions. Tillers were inoculated with agar discs containing
active mycelia and sclerotia of S. oryzae (ten strains) and Rhizoctonia spp. (eleven
strains). In the first experiment, tillers
were inoculated 75 days after sowing and disease severity rated at 6 weeks. In the second experiment, tillers weree inoculated
4 weeks after sowing and disease severity rated 3 month after inoculation. All the
cultivars weree susceptible to
S. oryzae
independently of the time of inoculationn and disease rating time. High disease ratings were found at 6 weeks as the disease
progress fast after inoculation being CL212 and CL243
the less susceptible. CL146 appeared as the
most susceptible cultivar
showing the highest ratings in both experiments. Cultivars showed low diseasee rating 6 weeks after inoculation with
Rhizoctonia spp. with only minor differences in susceptibility. INIA Tacuarí and Parao showed the
lowest diseasee rating at 3
months after inoculation.
INTRODUCTION
Sclerotium
oryzae, causal agent of stem rot of rice, and
Rhizoctonia spp., causal agents of sheath spot of rice, are
some of the most important diseases of
rice in Uruguay. The
sclerotia produced by these fungi servee as primary inoculum
by floating on the water and infecting rice stems at the
waterline. Stem rot progresses into the inner leaf sheaths
causing rot of the inner
stem tissue while Rhizoctonia spp.
infect the rice penetrating the outermost sheaths and
colonising
the culm of rice plant (2, 3). The objective is to
establish a baseline of the individual cultivar response to
these pathogens of the principal Uruguayan rice cultivars.
disease progressed but only to some extent since
the tillers
began
to die early after the disease progress. Although at
MATERIALS AND METHODS
Eleven strains of Rhizoctonia spp. and ten of S. oryzae were
obtained from diseased d plants or rice
soils. Ten seeds of
each cultivar were planted in 125-mm diameter plastic pots
placed in metal trays with 5 cm deep water in glasshouse (25
± 5 ºC and RH 80%) and thinned to four seedlings per pot.
Tillers were inoculated
with agar disc (6 mm diameter)
containing mycelia of the pathogen obtained from 10-day
old colonies of the pathogen and wrapped with Parafilm.
The inoculation was made in plants 75 days after sowing
and rated
6 weeks after inoculation (Experiment 1) or 4
weeks after sowing and rated after 3 months (Experiment 2).
Severity index was according with SES evaluation (1).
Figure 1. Severity
index for the
cultivars tested. Above,
Experiment 1. Below, Experiment
2. Green, S. oryzae; red,
Rhizoctonia spp. Columns with the same letters are not
statistically different (Tuckey=0,05) for the same pathogen
species.
Statistical
analysis was performed with SAS®.
different
degree, all the cultivars
studiedd
showed
RESULTS
Experiment 1 (Ex1). The plants evaluated for severity
susceptibility to this
disease as observed in field conditions.
CL146 appeared as the most susceptible cultivar to S. oryzae
ratings after inoculation with S. oryzae showed all a in both
experiments.
Rhizoctonia
showed a more
different degree of susceptibility. CL146 was the
more
susceptible cultivar without statistical differences with
INIA
Tacuarí, Parao, El Paso 144, INIA Olimar and CL244.
CL212 and CL243 were the less susceptible and this
differencee supported statistically. Cultivar susceptibility for
Rhizoctonia spp. was higher for INIA
Tacuarí. CL212 and
CL243 were a second group without statistical differences.
Cultivars CL146, CL244B, El Paso 144, INIA Olimar and
pronounced progress in the expression of symptoms with
ratings low at 6 weeks after inoculation and
no clear
distinction in susceptibility between cultivars. Previously
was demonstrated that this pathogen can penetrate the
outermost sheaths asymptomatically (2) until the
tillers are
killed. INIA Tacuarí
showed the lowest disease rating after 3
month, a striking result since it is the most susceptible
cultivar to Rhizoctonia in field conditions (2).
Parao, were the less susceptible with statistical differences
only with
INIA Tacuarí. Experiment 2 (Ex2). No statistical
REFERENCES
differences were found between cultivars inoculated with S. 1. IRRI. 2002. Standard
Evaluation
System for Rice.
oryzae. For Rhizoctonia
spp. Parao and INIA Tacuarí were (International Rice Research Institute, Manila, Phillipines).
the less susceptible cultivars and these differences supported
2. Lanoiselet, V. M., Cother, E. J. and
Ash, G. J. (2007). Crop
statistically (Figure 1).
Protection 26:799–808.
3. Ou, S. .H. (1985). Stem Rot. In ‘Rice Diseases’, pp. 247-262.
DISCUSSION
(Commonwealth Mycological Institute, Kew, UK).
Differences were found between experiments for S. oryzae
in some cultivars, more
disease severity was found
when
tillers were inoculated close to internode elongation (3). The
7th Australasian Soilborne Diseases Symposium
64