RECyT
Year
24 / No 38 / 2022 /
DOI: https://doi.org/10.36995/j.recyt.2022.38.010
Stem rot (Sclerotium oryzae) in rice crops on
the southern region of Paraguay
Pudrición
del tallo (Sclerotium
oryzae) en cultivos de arroz en la región sur de Paraguay
Podridão
do caule do arroz (Sclerotium oryzae) em lavouras de arroz no Paraguai
L., Quintana1, *; S.
A., Gutiérrez2
1- Laboratorio de Fitopatología.
Facultad de Ciencias Agropecuarias. Universidad Nacional de Itapúa. Abog.
Lorenzo Zacarías y Ruta VI. Encarnación, Paraguay.
2- Facultad de Ciencias
Agrarias. Universidad Nacional del Nordeste. Corrientes, Argentina. Juan
Bautista Cabral 2131. Corrientes, Argentina.
* E mail: lviedmaq@gmail.com
Received:
21/01/2021; Accepted: 23/03/2022
Abstract
The expansion
of rice cultivation in Paraguay observed in the last five agricultural seasons,
together with the new technologies applied, has been accompanied by an increase
in the presence of diseases affecting rice leaf sheaths and stems. In view of the scarcity of information in the
country on some of these diseases, this study was carried out to characterise
rice stem rot. Samples of symptomatic plants were collected from commercial
lots located in the southern region of the country, and routine techniques in
plant pathology for the study of mycoses were applied, such as macro and
microscopic observations, in vitro sowings, and the study of morphological and
cultural characteristics of the fungus. The pathogenicity of the fungus was
determined by inoculations on healthy 50-day-old rice plants. Associated with
the symptoms observed on rice pods and stems, the fungus Sclerotium oryzae Catt
was identified as causing irregular black lesions on the leaf sheaths at the
water line. As the disease progressed, it caused lesions on the sheaths and
leaf blades, and finally stem rot with the formation of sclerotia inside the
affected stems. The symptomatology observed in the rice plants and the cultural
and morphometric characteristics of the pathogen Sclerotium oryzae were
consistent with those reported in the bibliographical references analysed.
Key words: Oryza sativa, Mycosis, Soil fungus.
Resumen
La expansión del cultivo de arroz en Paraguay
observada en las 5 últimas campañas agrícolas, junto a las nuevas tecnologías
aplicadas, ha sido acompañada por un aumento en la presencia de enfermedades
que afectan vainas foliares y tallos de arroz. Ante la escasez de
información en el país sobre algunas de ellas, se realizó este trabajo a fin de
caracterizar a la
pudrición del tallo de arroz. Se colectaron muestras de plantas sintomáticas de
lotes comerciales localizados en la región sur del país, y se aplicaron
técnicas de rutina en fitopatología para el estudio de micosis como
observaciones macro y microscópicas, siembras in vitro, y estudio de
características morfológicas y culturales del hongo. La patogenicidad del hongo
se determinó mediante inoculaciones en plantas sanas de arroz de 50 días de
edad. Asociado a los síntomas observados en vainas y tallos de arroz, se
identificó al hongo Sclerotium oryzae Catt causando
lesiones irregulares de color negro, en las vainas foliares a la altura de
la línea de agua. Con el avance de la enfermedad, se produjo la lesión de las vainas y láminas foliares, y finalmente la
pudrición del tallo con formación de esclerocios en el interior de los tallos
afectados. La sintomatología observada en las plantas de
arroz y las características culturales y morfométricas del patógeno Sclerotium
oryzae, fueron coincidentes con los reportados por las referencias
bibliográficas analizadas.
Palabras clave: Oryza sativa, Micosis, Hongo de suelo.
Resumo
A expansão da cultura do
arroz no Paraguai tem sido acompanhada por um aumento na presença de doenças
que afetam as vagens das folhas e os colmos do arroz. Dada a escassez de
informações no país sobre alguns deles, este trabalho foi realizado para
caracterizar a podridão do colmo do arroz. Foram coletadas amostras
sintomáticas de plantas nas principais regiões produtoras do país e aplicadas
técnicas de rotina em fitopatologia para o estudo da micose, como observações
macro e microscópicas, semeaduras in vitro e estudo das características
morfológicas e culturais do fungo. A patogenicidade do fungo foi determinada
por inoculações em plantas de arroz saudáveis com 50 dias de idade. Associado
aos sintomas observados nas panículas e caules do arroz, o fungo Sclerotium
oryzae Catt foi identificado causando lesões pretas irregulares nas bainhas das
folhas na linha de água, morte gradual das vagens com suas lâminas foliares e
finalmente a apodrecimento do caule com formação de esclerócio preto e
superfície lisa nos tecidos afetados. Os sintomas observados e as
características culturais e morfométricas do patógeno Sclerotium oryzae foram
consistentes com os relatados nas referências bibliográficas analisadas.
Palavras-chave:
Oryza sativa, Micose, Fungo do solo.
Introduction
In recent years, the rice
(Oryza sativa L.) production area of Paraguay has experienced a large growth in
the sowing of the crop, due to the expansion of productive areas and increased
yields (Enciso, 2020). This situation was also accompanied by the inclusion of
crop management technologies such as fertilisation, planting dates, new rice
varieties, etc. However, an increase in the presence of different fungal
diseases that cause diverse symptomatologies in different organs of the rice
plant was also observed (Quintana et al. 2016, 2017). Among these pathologies,
those caused by soil fungi were the ones that have given rise to numerous
consultations from producers and technicians, due to the incidence values of
these diseases in the different growing regions.
During the 2015-2018 cropping seasons, symptoms were
observed on leaf sheaths and stems of rice plants associated with a fungus of
the genus Sclerotium. The literature refers to Sclerotium oryzae Catt. as the
causal agent of the disease called stem rot, first described by Cattaneo in
1876 in Italy (present as sclerotia on rice plants, and named Sclerotium oryzae
Catt. and in its teleomorphic form on rice stubble as Magnaporthe salvinii
(Catt.) Krause and Webster (=Lepthosphaeria salvinii Catt.). In 1889, Cavara
identified the anamorphic stage in Italy, named Nakataea sigmoidea (Cav) Hara
(=Helminthosporium sigmoideum Cav.) (Ou, 1985). This disease was detected in
several rice production regions worldwide, including Japan, India, Vietnam and
the Philippines in Asia; Italy in Europe; Kenya, Madagascar and Mozambique in
Africa; Cuba in the West Indies; and Argentina, Brazil, Colombia, Guyana and
Peru in Latin America (Ou, 1985; Martinez de la Parte et al., 2014; Pramesh et
al., 2017; Aguilar-Anccota et al., 2017; Chethana and Kumar, 2019). The disease
causes significant crop damage, with yield losses of up to 22% in California,
75% in Arkansas (Webster and Gunnell, 1992) and 5 to 80% in India (Kumar et al.
2003). As far as Argentina is concerned, there are no studies related to yield
losses. However, it has 100% prevalence and incidence values varying from 5 to
80% in the growing region (Gutiérrez et al. 2017).
Studies carried out by different authors worldwide affirm that the
damage caused by this disease varies depending on climatic factors, crop
management, susceptibility of the varieties, virulence of the isolates and time
of infection (Kumar et al. 2003; Gopika et al. 2016b; Aguilar-Anccota, 2017).
Considering the regional and
economic importance of rice cultivation in Paraguay, and the scarcity of
information on this pathology, the aim of this study was to characterise the
symptomatology and aetiology of rice stem rot.
Materials and methods
During the 2015-2018 agricultural seasons, samples of diseased rice
plants of Irga 424, Irga 417, Irga 428 and Taim varieties were collected from
different districts of the departments of Itapúa, Caazapá and Misiones. In vitro isolations were made with pieces of
symptomatic rice leaf sheaths, previously disinfected with 1% sodium
hypochlorite, on potato dextrose agar medium (1.5%, pH 6) and incubated in an
oven at 25°C for 10-12 days (Alfenas and Gonçalvez Maffia, 2007). Subsequently,
the cultural characteristics of the colonies were analysed and the size of the
sclerotia was determined by measuring 100 sclerotia with an optical microscope
(400x).
The pathogenicity of the causal agent was determined by inoculations on
healthy 50-day-old rice plants, raised in pots in greenhouses, using 15-day-old
discs of culture medium containing inoculum (mycelium and sclerotia) which were
placed on the leaf sheaths and incubated at 25°C.
Results and discussion
In
samples of leaf sheaths from rice plants of Irga 424, Irga 417, Irga 428 and
Taim varieties, symptoms characteristic of rice stem rot were observed in
association with diseased tissues (Figure 1). Symptoms on the rice plants
appear towards the middle of the tillering stage as irregular, black lesions on
the leaf sheaths at the water line. As the disease progresses, lesions occur on
the sheaths and leaf blades, with sclerotia forming inside the affected stems
(Figure 2); at the end of the harvest, the sclerotia survive in the crop debris
and in the soil, constituting the source of inoculum for the next crop cycle,
mainly for no-tillage systems.
The
fungus Sclerotium oryzae Catt. was isolated and identified by means of its
cultural and morphometric characteristics, as well as the corresponding
specific literature. In culture medium, the pathogen presents colonies with
cottony aerial mycelium, greyish in colour (Figure 3; sclerotia develop around
7-9 days. Initially greyish-white, smooth, globose, or sub-globose, sometimes
covered with hyphae; finally, when they completely cover the plate, they
acquire their glossy black colour (Figure 4). Their measurements ranged from
150 to 260 µm. These cultural and morphometric characteristics were consistent
with those observed in the corresponding specific literature, allowing the
identification of the causal agent (Ou, 1985; Webster and Gunnell 1992;
Beldarrain and Avila, 2009; Gopika et al. 2016a; Gopika et al. 2016b).
The inoculations reproduced the
disease symptoms on leaf sheaths and stems of rice plants, which appeared about
10 days after the start of the tests. Re-isolation of the causal agent was
achieved.
The symptomatology observed in rice
plants and the cultural and morphological characteristics of the pathogen were
consistent with those reported in the literature (Ou, 1985; Webster and
Gunnell, 1992; Gopika et al. 2016a; Gopika et al. 2016b, Aguilar-Anccota et al.
2017).
Fig.1. Stem rot
symptoms.
Fig.2. Sclerotia formation in infected tissues.
Fig.3. Development of sclerotia on APG
culture medium.
Fig.4. Colony of
Sclerotium oryzae on PDA.
Conclusions
It was possible to confirm that the
symptomatology observed in rice plants and the cultural and morphometric
characteristics of the pathogen correspond to the fungus Sclerotium oryzae
Catt. The information generated in the present work will be useful for the
control of the disease, considering that it is a fungus that survives in the
soil and crop residues; it should also be complemented with an evaluation of
the susceptibility of the varieties planted in the country.
Acknowledgements
To the Consejo Nacional de Ciencia y Tecnología
(CONACYT) for funding this research through Project 14-INV-111
"Epidemiological knowledge of parasitic diseases of rice in
Paraguay". Res.
437/2014-Contract 013/2015
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