Abstract: The study on Rice yellow mottle virus (RYMV) infection and reproductive losses in rice was carried out under screenhouse condition in Nigeria. Thirty-five rice varieties were evaluated for RYMV resistance. Percent productive tillers, date to 50% flowering and percent spikelets fertility per plant were between 43.2-96.7%, 57.67-112 days and 0-71.8%, respectively. Number of panicles per plant, number of grains per panicle and 1000 grain weight per plant were between 8.33-45.67, 0-77 and 0-27.57 g, respectively. Yield losses of between 17-100% were obtained from all the rice varieties evaluated. Out of the 35 rice varieties studied, only Gigante (18%), Moroberekan (19%) and NERICA-L 42 (32%), have the least yield losses and RYMV resistance characteristics. The three varieties (Gigante, Moroberekan and NERICA-L 42) are known to possess stable resistance characteristic to RYMV disease and will comparatively be suitable for cultivation in areas where RYMV incidence is endemic and on a long term be used by rice breeders as sources for breeding for durable resistance to RYMV disease in Nigeria.
INTRODUCTION
Rice is the most important crop in the developing world in terms of production and in contribution to diet (Peng, 2007). Notwithstanding, efforts aimed at increasing rice production especially within the African continent is seriously affected by the militating effects of Rice yellow mottle virus (RYMV) (Abo et al., 2005). RYMV belongs to the sobemovirus group, it is very stable and highly infectious to rice (Abo et al., 2005), especially the Asian Oryza sativa (indica type) in the lowland and irrigated ecosystem. The disease is wide spread in almost all the West African states including Nigeria (Banwo et al., 2004; Rossel et al., 1982).
Typical symptoms of the disease include mottling and yellowing of leaves, delayed flowering with poorly exerted panicles and bearing sterile and discolored spikelets (Gnanamanickam, 2009). Severely infected plants may be stunted or become dead, whereas, profuse tiller and leaf formation, as well as delayed senescence are associated with the disease.
RYMV is transmitted by insect vectors and through mechanical contact (Abo et al., 2000; Nwilene et al., 2009; Sere et al., 2008). Yield losses of 50-100% have been recorded on rice crops in farmer’s fields in Africa and surrounding islands (Banwo et al., 2004; Fomba, 1988; Abo et al., 2002; Onasanya et al., 2006). Analysis of yield and yield component is essential in cereals since the growing population is currently depending on the consumption of starch from these crops (Singh et al., 2007a, b; Khan et al., 2007). RYMV came to limelight in Nigeria with the introduction of exotic rice varieties from Southeast Asia coupled with intensification of cropping practices without dry season gaps (Thresh, 1991; Abo et al., 2002). This situation as well as lack of extensive adaptive testing of the exotic rice varieties in their environments led to the disruption of the apparent equilibrium that had been established between host rice and RYMV (Awoderu, 1991; Fomba, 1988; Onasanya et al., 2006).
Screening of Oryza germplasm in Nigeria has indicated the existence of potential sources of resistance to RYMV disease in the indigenous African Oryza species, as well as traditional African upland rice varieties (Abo et al., 2005). Therefore, the objective of this study was to evaluate the resistant status of inter-specific and intra-specific rice genotypes in order to establish the impact of RYMV on the establishment, growth and yield component of the rice plants. And this will invariably help to identify rice varieties comparatively suitable for cultivation in areas where RYMV incidence is endemic and on a long term, to identify sources for breeding for durable resistance to the disease among elite varieties in Nigeria.
MATERIALS AND METHODS
Location of experiment: The experiment was conducted under an artificial environment in a screen house at the National Cereals Research Institute Badeggi, Niger state from August 2008 to March 2009.
Virus isolate: Leaves of RYMV infected rice plants were collected from farmers’ fields at Wushishi, Niger state, Nigeria. The collected RYMV infected leaf sample was propagated on the highly susceptible Bouake 189 following mechanical inoculation of 21 old plants in the screen house (Onasanya et al., 2006). Three weeks after inoculation, leaves bearing typical yellow mottle symptoms were harvested and used for inoculating the rice varieties.
Rice varieties: Thirty-five rice varieties were used for the study (Table 1). The NERICA varieties were obtained from Africa Rice Center (AfricaRice), Cotonou, Republic of Benin. Bouake 189 and FARO 29 were used as Susceptible Checks (SCK) and Moroberekan and Gigante as Resistant Checks (RCK) (Onasanya et al., 2006).
Experimental design, treatment and treatment allocation: The experiment was a factorial in a Complete Randomized Design (CRD). One level of Factor A (virus treatment) represent the control with zero doses. The 2 by 35 factorial with 70 treatment combinations was replicated thrice. Two hundred and ten plastic buckets each measuring 16 cm diameter and filled with 2.5 kg sterilized fadama soil were used. The pots were laid 0.5 m apart on the screen house tables (1 m above ground level). Three seeds each of every entry were sown directly in the plastic buckets and later thinned to one seedling per pot. Pots were constantly supplied with fresh tap water in the mornings and evenings until maturity. Then 2 g Nitrogen-Phosphorous-Potassium (NPK) fertilizer was applied to the plants at 28 days after sowing (DAS). This was followed by a split application of 2 g of urea at 45 DAS and at early flowering stage.
| Table 1: | List of variety used for the study |
| RCK: Resistant check; SCK: Susceptible check | |
Inoculation of rice varieties: The virus mechanical inoculation method was according to Onasanya et al. (2006). Infected leaf samples of the RYMV isolate was ground with 0.01 M phosphate buffer pH 7.0 at the ratio of 1:10 (w/v) and the resulting homogenate filtered through cheesecloth. Carborundum powder (600 mesh) was added to the inoculum to aid the penetration of the virus into leaf tissues. The virus extract was finger-rubbed on the plant leaves at 40 DAS. For each test variety, three entries were inoculated with the virus and three were left un-inoculated to serve as control.
Data collection: The parameters such as percent productive tillers, date to 50% flowering (days), number of panicles per plant, number of grains per panicle, 1000 grain weight per plant and percent spikelets fertility per plant were collected from both control and test varieties (Onasanya et al., 2006; Nwilene et al., 2009).
Data analysis: Using the yield data from both test and control varieties, percentage yield loss due to RYMV disease was determined for each variety. All the data were subjected to statistical analysis using IRRISTAT software (Xiaoping and Ognjen, 2005).
RESULTS AND DISCUSSION
The study on RYMV infection and reproductive losses in rice was carried out under screen house condition. Percent productive tillers, date to 50% flowering and percent spikelets fertility per plant were between 43.2-96.7%, 57.67-112 day and 0-71.8%, respectively (Table 2). Besides, number of panicles per plant, number of grains per panicle and 1000 grain weight per plant were between 8.33-45.67, 0-77 and 0-27.57 g, respectively (Table 3). Due to RYMV infection, 13 NERICAs, two FAROs, 2 NCROs and Moroberekan produced significantly lower percentage productive tillers per plant at maturity in relation to their respective control entries (Table 2). It is evident that virus infection delayed date to 50% flowering (days) in 24 rice varieties against their un-inoculated control entries (Table 2).
| Table 2: | The effect of virus infection on reproductive capacity and yield components of rice genotypes under screenhouse condition |
| RCK: Resistant check; SCK: Susceptible check; V1: RYMV inoculated entries; V0: Entries not inoculated (control); CV: Coefficient of variation; SE: Standard error of mean | |
Besides five varieties (NERICA-L 22, NERICA-L 25, NERICA-L 34, NERICA-L 41 and NCRO 26) gave a significantly lower mean effect compared to their control entries, while six other varieties were not affected significantly by virus infection with respect to date to 50% flowering (days). The study also indicated that virus infection significantly affected all rice varieties causing a lower mean effect on percent spikelets fertility per plant (Table 2). NERICA-L 14 gave the lowest mean effect of 3.70% spikelets fertility per plant, which was found to be significantly lower than mean effects obtained from other rice varieties screened.
However, 48.6% of the rice varieties used in the study produced higher mean effect on the number of panicle per plant as compared with their un-inoculated control entries (Table 3). Seven rice varieties (NERICA-L 6, NERICA-L 17, NERICA-L 23, NERICA-L 38, NERICA-L 49, FARO 52 and Bouake 189) produced significantly lower mean effects on the number of panicle per plant against their un-inoculated control entries (Table 3).
| Table 3: | Effect of virus infection on the production and grain quality of rice genotypes under screenhouse condition |
| RCK: Resistant check; SCK: Susceptible check; V1: RYMV inoculated entries; V0: Entries not inoculated (control); CV: Coefficient of variation; SE: Standard error of mean | |
The higher number of panicles per plant observed in eleven rice entries for inoculated plant did not translate to higher number of grains per panicle as all the virus infected rice plants gave significantly lower mean number of grains per panicle against their un-inoculated control entries (Table 3). Except for the resistant indica and japonica types (Gigante and Moroberekan), only NERICA-L 38, NERICA-L 41, NERICA-L 47 and NERICA-L 49 gave between 20-40 grain per panicle in relation to their controls (Table 3). It is evident also that the 1000-grain weight per plant was significantly lower for all virus-infected entries as compared to their un-inoculated control entries (Table 3). Yield losses of between 17-100% were obtained from all the rice varieties screened (Fig. 1). Gigante and Moroberekan have the least yield losses and followed by NERICA-L 42 which has performed better than other rice varieties in their RYMV resistance under screenhouse condition (Fig. 1).
Prolonged vegetative lag phase as well as anatomical and histological changes resulting from virus infection might possibly cause the delay in date to 50% flowering observed in this study (Gnanamanickam, 2009). Previous study have shown that spikelets sterility have been associated with virus diseases meaning that virus infection might interfere with the carbohydrate build-up necessary for the spikelets development, triggering the degeneration of pollens and drying-up of the stigma resulting in spikelets sterility (Gnanamanickam, 2009). This could possibly explain why in the present study virus infection significantly affected all rice varieties causing lower mean effect on percent spikelets fertility per plant.
Grain weight differences might be attributed to the virus causing reduction of starch deposited in the endosperm (Abo et al., 2002). This is evident in the present study as the 1000 grain weight per plant was significantly lower for all virus-infected rice varieties as compared to their un-inoculated control varieties. Yield losses usually depend on many variables including virus strain or isolate and host cultivars (Onasanya et al., 2004; Bailiss and Senananyake, 1984; Onasanya et al., 2006). In the current study, yield losses of between 17-100% was obtained from all the rice varieties screened thus establishes the fact that the varietal yield losses was due to RYMV infection (Bailiss and Senananyke, 1984; Onasanya et al., 2004; Onasanya et al., 2006). The is consequently explain why the higher number of panicles per plant observed in inoculated rice varieties did not translate to higher number of grains per panicle as a results of the significant effect of the RYMV infections. The least yield losses obtained in this study for Gigante, Moroberekan and NERICA-L 42 suggest their possible resistant characteristic to RYMV disease under screen house condition (Onasanya et al., 2004; Onasanya et al., 2006).
| Fig. 1: | The contribution of Rice yellow mottle virus (RYMV) to yield losses in 35 rice varieties under screenhouse condition |
However, similar results could be obtained under natural viral infestation under field condition (Heinrichs et al., 1997; Rossel et al., 1982).
CONCLUSION
The study on RYMV infection and reproductive losses in rice brings much hope about the availability of donor rice cultivars with durable resistance to RYMV in Nigeria. The three varieties such as Gigante, Moroberekan and NERICA-L 42 as revealed by this study are known to possess stable resistance characteristic to RYMV disease in Nigeria. And these identified RYMV resistant rice varieties will comparatively be suitable for cultivation in areas where RYMV incidence is endemic and on a long term be used by rice breeders as sources for breeding for durable resistance to the disease in Nigeria.