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The evolution of rice cultivation in Paraguay
The expansion of rice cultivation in Paraguay observed in the last 5 agricultural campaigns, together with the new technologies applied, has been accompanied by an increase in the presence of diseases that affect foliar sheaths and rice stems. Given the shortage of information in the country on some of them, this work was carried out in order to characterize rice stem. Samples of symptomatic plants of the main production regions of the country were collected and routine techniques in phytopathology were applied for the study of mycosis such as macro and microscopic observations, sowing in vitro, and study of morphological and cultural characteristics of the fungus. The pathogenicity of the fungus was determined by inoculations in healthy 50 -day rice plants. Associated with the symptoms observed in pods and rice stems, the Sclerotium fungus oryzae cattStem with black sclerot formation and smooth surface inside the stem, or on the inner face of the affected leaf pods. The observed symptomatology and the cultural and morphometric characteristics of the pathogen Sclerotium oryzae, were coinciding with those reported by the bibliographic references analyzed.
Keywords: Oryza sativa, symptoms, Magnaporthe Salvinii, pathogenicity test
In recent years, the rice production zone (Oryza sativa L.) Paraguay has experienced great growth in planting crop, due to the expansion of productive areas and increased yields. Likewise, this situation was accompanied by the inclusion of technologies to crop management such as fertilization, planting dates, new rice varieties, etc. However, there was also an increase in the presence of different diseases caused by fungi that cause diverse symptomatologies in different organs of the rice plant (Quintana et al. 2016, 2017). Among these pathologies, which are caused by soil fungi were those that have originated numerous consultations from producers and technicians, due to the incidence values of these diseases in the different cultivation regions.
During the 2015-2018 agricultural campaigns, symptoms were observed in foliar pods and rice plants associated with a fungus of the sclerotium genus. The bibliographic background refer to Sclerotium Oryzae Catt as the causal agent of the disease called Stem Pudition, first described by Cattaneo in 1876, in Italy (present as sclerotes in rice plants, and called Sclerotium Oryzae Catt., and in its teleomorphic form in rice stubble as Magnaporthe Salvinii (Catt.) Krause and Webster (= Lepthosphaeria Salvinii Catt.). In 1889, Cavara identified the Anamorfo state in Italy, called Sigmoid Nakatea (CAV) Hara (= Helminthosporium sigmoideum cav) (OU, 1985). This disease was detected in several regions of world rice production, such as Japan, India, Vietnam and the Philippines in Asia, Italy in Europe, Kenya, Madagascar and Mozambique in Africa, Cuba in the Antilles and in Latin America: Argentina, Brazil, Brazil,Colombia, Guyana and Peru (OU, 1985; Pramesh et al. 2017;Martínez from the et al., 2014, Aguilar-Anccota et al., 2017, Chethana and Kumar, 2019). The disease causes important damage to the crop, having registered losses in yield of up to 22% in California, 75% in Arkansas (Webster and Gunnell, 1992) and 5 to 80% in India (Kumar et al. 2003). With regard to Argentina, there are no studies related to losses in performance, however, it presents 100% prevalence and incidence values that vary from 5 to 80% in the culture region (Gutiérrez et al. 2017).
Studies carried out by different authors worldwide, affirm that the damages caused by this disease are variable depending on climatic factors, crop management, susceptibility of varieties, of the virulence of isolates and moment of infection (Kumar etto the. 2003;Gopika et al. 2016b;Aguilar-Anccota, 2017).
Considering the regional and economic importance achieved by the crop of rice in Paraguay, and given the shortage of information about this pathology, the objective of the work was to characterize the symptomatology and etiology of the rot of the rice stem.
Results and Discussion
In samples of foliar pods of rice plants of the varieties IRGA 424, IRGA 417, IRGA 428 and TAIM were observed characteristic symptoms of rice stem rot, associated with the sick tissues. It was possible to identify and isolate the Sclerotium Oryzae Catt fungus. The symptoms in rice plants manifest towards the middle of the Macollage Stadium, such as irregular, black lesions, in the foliar pods at the height of the water line. As the disease progresses, both the leaf pods and their corresponding leaf lamina, die and finally the pudition of the stem occurs, causing the weakening of the plant with possible turn. The fungus develops mycelium and sclerotes which are visible inside the stem or in the inner face of the leaf pods;At the end of the harvest, the sclerotes survive in the remains of crops and on the ground, constituting the inoculum source for the next crop cycle, mainly for direct sowing systems.
In the middle of cultivation, the pathogen developed compact, white colonies at the beginning, then dark gray, whose sclerotes developed around 7-9 days, at the beginning of grayish white, smooth, globose, or sub gloumosewith whitish hyphae;finally acquired their bright black color. Its measures varied 150 to 260 mm.
The pathogenicity of the fungus was determined by inoculations in healthy 50 -day rice plants. Associated with the symptoms observed in foliar pods and rice stems, the Sclerotium Oryzae Catt fungus was identified, causing irregular black lesions, in the leaf pods at the height of the water line. As the development of the disease continues, the gradual death of pods is observed with their corresponding foliar sheets. Towards the final stages of the crop, there is the rot of the rice stem, with mycelium formation and pathogen sclerot;These are black, smooth surface, which can be seen inside the stem, or on the inner face of the sick foliar pods.
In pathogenicity tests, the symptoms of the disease were developed around 10 days after inoculation, forming irregular, black lesions. With the advancement of the disease, the injury of the pods and foliar sheets occurred, with sclerot formation inside the affected stems.
The symptomatology observed in rice plants and the cultural and morphological characteristics of the pathogen Sclerotium oryzae were coincident with those reported by the bibliographic references analyzed and correspond to the Sclerotium Oryzae Catt fungus (OU, 1985, Webster and Gunnell, 1992, Aguilar-Anccota et al. 2017, Gopika et al. 2016a, Gopika et al. 2016b).
The symptomatology observed in foliar pods of rice plants of the IRGA 424, IRGA 417, IRGA 428 and TAIM and the cultural and morphometric characteristics of the pathogen corresponds to the Sclerotium fungus oryzae catt, teleomorphic form magnaporthe salvinii (Catt.) Krause and Webster (= Lepthosphaeria Salvinii Catt.
The information generated in the present work will be very useful for the control of the disease, considering that S. Oryzae is a fungus that survives on the ground and cultivation remains;It must also be complemented with the evaluation of the susceptibility of the varieties sown in the country.