The study was carried out in the lowland of the INERA/Banfora experimental site (Tannounyan region), which represents an RSNV hot spot. This site, located between the 1000 mm and 1200 mm isohyets, consists of a simple layout with compacted earth bunds. Figure 1 shows the location of the experimental site with the following geographical coordinates: longitude 4˚46'38.56'', latitude 10˚37'47.89''. The site is located in the sudanian climatic zone of Burkina Faso, characterized by an average annual rainfall ranging from 900 to 1200 mm, a rainy season lasting approximately six months (May to October), and a mean annual temperature of about 27˚C [10].

Figure 1.Varietal screening site in the presence of the RSNV.

The plant material consisted of 30 improved high-yielding rice varieties from the INERA rice and rice cultivation programme (Table 1).

Table 1.List of varieties used.

In the second ten-day period of June, 1050.5 m² (38.2 m × 27.5 m) of surface was ploughed with a disc plough and pulverized with a hoe before the basic plots (5.2 m × 1.2 m) were set up. The preparation of the seedbed was done with a hoe during the first ten days of July. The experimental device used was a three-replicate alpha lattice (Figure 2). Each repetition was composed of six blocks and each block was subdivided into five plots. The size of each plot was 5.2 m long and 1.2 m wide. The distance between the lines and between the pockets was 20 cm × 20 cm.

Figure 2.Experimental design of the RSNV trial.

For each variety, direct sowing is carried out at the rate of one (01) grain per pocket in elementary plots of 6.24 m² (5.2 m × 1.2 m) on 5 rows. The spacing between lines and between pockets is identical and corresponds to 20 cm. NPK-based base fertilizer (14-23-14) was applied at a rate of 300 kg per ha. Then, urea-based top-dressing at a dose of 100 kg/ha is divided into two applications. The first application is made at a dose of 35 kg per ha at 15 days after sowing (DAS) and the second at a dose of 65 kg per ha at the initiation of panic at about 63 DAS in accordance with research recommendations to producers. Three weddings were carried out manually by hand given the grass cover of the trial. During tillering, a 5 cm water gap (drain when the fertilisers are spread) was maintained in the plots. This water was reduced to 8 cm during the bolt-flowering phase. During the ripening-harvesting phase, water was maintained for up to 14 days before harvest.

The observations made were related to the agro-morphological characteristics of the different varieties of seven (07) traits:

The number of tillers per m² at 60 DAS to assess the tillering ability of each variety, which is one of the parameters directly influencing yield.The sowing heading cycle (SHC) 50% corresponding to the difference between the date on which half of the tillers have sprung and the sowing date on each plot.The sowing maturity cycle (SMC) 85%, corresponding to the difference between the date from which 85% of the panicles on each plot have a straw colour and the sowing date.The height of the plants measured on five (5) plants randomly chosen per elementary plot at 60 DAS during the maturity phase. The height measurement is taken from the collar of the plants to the top of the panicle leaf or at the end of the highest panicle when the latter exceeds the size of the panicle leaf.The number of panicles per m² was obtained by counting five (5) plants randomly chosen per elementary plot at 110 DAS.Yield was determined by measuring the weight of the grains in each plot. Indeed, winnowing and weighing using a scale were carried out for each variety in a given plot.

In addition to these seven (7) traits observed, the number of infected plants per variety was determined.

The incidence of the disease was calculated according to the formula described by [11]:

where I: incidence of the disease; PA: number of infected and/or dead plants (a plant was considered infected when symptoms were immediately observed); PT: total number of plants.

A slightly modified standard of the evaluation system described by Sereme etal. (2016) was used to determine the status of each variety with respect to the RSNV: highly resistant (HR) for 0% - 10% infection, resistant (R) for 11% - 20% infection, moderately resistant (MR) for 21% - 30% infection, susceptible (S) for 31% - 85% infection, and highly susceptible (HS) for infection greater than 85%.

The data were processed with Microsoft Office’s Excel 2016 software and analysed with XLSTAT 2014 and STATISTICA version 7.1. Excel 2016 was used for data entry, data arrangement, frequency calculation, sector realization and curves. XLSTAT, on the other hand, was used to analyse the performance of varieties in the presence of the virus through analysis of variance (ANOVA). Finally, STATISTICA was used for correlation analysis.

The diagram in Figure 3 shows the variation in the average number of tillers per m² at 60 days after sowing (DAS), the semi-heading cycle and the sowing-maturity cycle 85% of the different varieties. The number of tillers varied according to the variety but also from one replicate to another. The maximum average number of tillers was obtained with variety ARC1-20-B-1 (383 tillers) and the minimum average number was obtained with variety ARC1-235-B-1 (235 tillers). That is a difference of 148 tillers. The shortest semi-heading cycle (50%) was obtained at 84.66 DAS with the ART315-BC1-6 varieties, and the longest cycle was obtained at 99 DAS with the variety IR88638-62-1-1-1-1-1-1-1, a difference of 14.34 days. The shortest seeding-maturity cycle (85%) was obtained at 110.33 days with the varieties ART347-9-1-1 and the longest cycle was obtained at 129.66 days with the variety FKR 56N, a difference of 19.33 days.

Figure 3.Variation in the average number of tillers (60 DAS), the CSE (50%) and the CSM (85%) of the different varieties.

Figure 4 shows the average heights at 60 DAS and at maturity for the different rice varieties. The greatest mean height at 60 DAS was 95.06 cm observed in the FKR 56N variety. The smallest mean height at 60 DAS of 56.86 cm was noted in variety ART349-1-1-1.

The greatest mean height at maturity was 129.66 cm observed in FKR 56N. The smallest mean height at maturity of 110.33 cm was noted in variety ART347-9-1-1.

Figure 4. Average heights (60 DAS and at maturity) and the average yield of the varieties.

The best average grain yield in the presence of RSNV was 4852.87 kg/ha for FKR 19 and the lowest average yield of 1837.39 kg/ha was observed for WAB2104-WAC5-2-TGR1-BAT8-1.

The analysis of the different agronomic performances revealed the existence of variability within the varieties studied in the presence of RSNV in the environment. The analysis of variance showed the existence of a significant difference (P < 0.05) within varieties for the mean number of tillers at 60 days after sowing: the average plant heights at 60 DAS and the average grain yield. On the other hand, the analysis of variance showed the absence of a significant difference within the varieties sown for the number of panicles (F = 0.8006; P = 0.7404). At the level of traits such as the 50% seed-heading cycle; the sowing cycle at 85% maturity; mean plant height at maturity, a highly significant difference (P < 0.001) existed within varieties for these variables.

The statistical analysis also revealed the existence of a very strong correlation (P < 0.01) between incidence and certain agro-morphological parameters. The incidence is positively correlated with the number of tillers per m² (P = 0.001; R = 0.3405) and the number of panicles per m² (P = 0.01; R = 0.3477). It is negatively correlated with height at 60 DAS (P = 0.033; R = 0.2255) and height at maturity (P = 0.001; R = −0.3737). There is also a positive correlation between the height at maturity and the grain yield (p = 0.003; F = 0.3140).

Analysis of variance revealed a highly significant difference (P < 0.001) in the mean incidence of RSNV that varied by variety (Table 2). Thus, the highest average incidences were observed on the varieties ARC1-20-B-1; WAB2066-WAT21-1-B-1-TGR2 and WAB2081-WAC2-2-TGR2-WAT1-9-TGR3 with average incidences of around 32.80%; 31.73% and 29.86%, respectively.

The ART346-10-2-1 varieties; ART347-9-1-1; ART346-10-4-1 and WAB2135-WACB-2-TGR3-WAT8-1 had the lowest mean incidence of 0.80%. In contrast, ART346-10-1-1 and NERICA-L19 did not show any symptoms of RSNV.

Table 2.Impact of RSNV and average grain yield of varieties.

The analysis also revealed that the varieties ART348-4-1-1 (V5); FKR19*TSF6-36 (V8); WAB2138-WACB-2-TGR2-WAT5-1 (V7); FKR 19 (V19); NERICA-L19 (Check) (V30); ART347-9-1-1 (V10); WAB2152-TGR2-WAT1-2 (V15); ART347-1-1-1 (V27); FKR 56N; ARC1-235-B-1 (V26); ARICA 2 (CHECK) (V28) showed a very low incidence against RSNV and gave both good yields. However, the first 4 varieties mentioned above gave the best yields with a very low incidence of RSNV.

Components assigned to the same letter in the same column are not significantly different at the 5% probability threshold. Highly resistant (HR) for 0% - 10% infection, resistant (R) for 11% - 20% infection, moderately resistant (MR) for 21% - 30% infection, susceptible (S) for 31% - 85% infection, and highly susceptible (HS) for infection greater than 85%.

Figure 5 shows the frequency of RSNV impacts observed on the 30 varieties. Indeed, the majority of varieties, 77%, have a very low incidence compared to the RSNV, between 0 and 9.9%. Low incidences of 12.8% for one group and between 14.17% and 17.1% for another were obtained for 13% of the varieties. Finally, the highest incidences concern 10% of varieties, including an incidence of 29.9% for one group and incidences between 31.7% and 32.8% for another group.

Figure 5. Frequencies of varieties in the groups for the incidence of RSNV.