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. 2016}b; 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. 2016}a; 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. 2016}a; 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.