Screenhouse studies were conducted using 10 RYMV isolates from 6 different localities in Mali against 8 WARDA differential rice genotypes to investigate the possible existence and classification of different pathotypes of RYMV in Mali. The reaction of 8 rice genotypes to the 10 RYMV isolates was different in terms of SPAD and yield reductions. The interaction between isolates and rice cultivar was also significant. AMMI cluster analysis revealed the existence of two pathotypes (HPI and MPI) of RYMV isolates in Mali. Of 8 rice genotypes studied, only Bouake 189 was highly susceptible to the two pathotypes. This information could be useful in the rice breeding programs aiming at deployment of RYMV resistant genotypes to different rice ecologies and localities in Mali.
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Rice Yellow mottle virus (RYMV), genus sobemovirus, is the only known virus disease of rice in Africa and it is indigenous to the continent. RYMV was first described in 1966 in Kenya (Bakker, 1970) and has subsequently been reported throughout West Africa, Madagascar, Tanzania, Zanzibar and most recently Mozambique. The spread of the disease has been facilitated by intensive agriculture husbandry practice (Awoderu, 1991) and this disease is limited to rainfed and irrigated lowlands. The virus is highly infectious, environmentally stable and is transmitted both mechanically and by Chrysomelid beetle vectors in the field (Hull, 1988; Abo et al., 1998; Nwilene, 1999), but not transmitted by seed (Konaté et al., 2001). The estimated yield reduction due to RYMV infection in susceptible lowland cultivars was up to 97% (Reckhaus and Adriamasintseheno, 1995) and as high as 54% in a tolerant upland cultivars (Fomba, 1988). RYMV is known to be one of the most economically damaging diseases of rice in sub-Saharan Africa (Ndjiondjop et al., 2001). The virus, depending on the genotype, causes yellowing, mottling and stunting of infected plants with narrowing of emerging leaves and when infection occurs early, the plant normally dies.
Varietal resistance seems the most promising control mechanism. However, two types of resistance had been found so far: a high natural resistance in Oryza glaberima land race and a local indica cultivar Gigante (Ndjiondjop et al., 2001) and a partial resistance in japonica varities (Albar et al., 2003; Ghesquière et al., 1997). Irrigated rice farmers generally prefer the higher yielding indica rice which are susceptible to the virus. Therefore, all major rice varieties grown in West African lowlands, such as Bouaké 189, Jaya, BG 90-2 and IR 1529-680-3, are highly susceptible to RYMV (Séré and Sy, 1997).
The existence of different RYMV strains in the field (Konate et al., 1997; NGuessan et al., 2000) that are different in their pathogenicity is often a matter of considerable practical importance. Therefore reliable criteria are needed for distinguishing, identifying and pathotyping these strains. Screening for durable resistance need to be done with the most virulent pathotypes as, most often, the breakdown in resistance is attributed to a poor prerelease challenge with appropriate pathogen population (Mekwatanakarn et al., 2000).
In this study, our aim was to investigate the existence of a highly virulent pathotype of Rice yellow mottle virus in Mali to be used in screening for durable resistance. Besides, identification of different virulence strains and pathotypes from different localities will be used to identify and characterize rice genotypes for stable resistance to RYMV as such information is useful for developing rice varieties with durable resistant to RYMV in Mali.
MATERIALS AND METHODS
Rice genotypes: Eight differential rice genotypes (Table 1) used in this study were established by WARDA plant pathology unit to identify difference in virulence among RYMV isolates (WARDA, 2001).
RYMV isolates: Ten RYMV isolates (Table 2) used for this study were collected from rice in 6 different localities in Mali. Before used, each isolate was first propagated in the susceptible rice variety Bouake189, following mechanical inoculation of 21 old plants in the screenhouse. Four weeks after inoculation, leaves from each RYMV isolate bearing typical yellow mottle symptoms were harvested and used for inoculating rice genotypes. By this way, the inoculum of the isolates were standardized.
Inoculation of rice genotypes: The eight young differential rice genotypes were inoculated mechanically (Fauquet and Thouvenel, 1977) with the 10 isolates in the screenhouse 21 days after direct seedling in 3 replicates. Another sets of same eight young differential rice genotypes in 3 replicates not inoculated were used as controls. Infected leaf samples of each RYMV isolate were 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. Each rice plant was inoculated thrice same day.
SPAD and yield measurement: Chlorophyll (SPAD) and yield reductions due to RYMV disease were evaluated.
|Table 1:||Identity of differential rice genotype used|
|Table 2:||Identity of RYMV isolates used for pathological study|
Chlorophyll content was measured using SPAD 502 Chlorophyll Meter (Monje and Bugbee, 1992; Martines and Guiamet, 2004) at 42 days after inoculation. SPAD and yield measurement were obtained both for test and control genotypes.
Data analysis: Using SPAD and yield data from both test and control genotypes, percentage SPAD and yield reductions due to RYMV disease were determined for each genotype. IRRISTAT version 4.3 statistical software was used for all the analyses. Variance and mean comparison of percentage SPAD and yield reductions were performed. Genotype (cultivar) by environment (isolate) interaction effects on SPAD and yield reductions was carried out using additive main effect and multiplicative interaction (AMMI) analysis (Ebdon and Gauch, 2002). Cluster dendograms showing classification of genotype (cultivar) levels of resistance to environment (isolate) and classification of environment (isolate) pathogenic level to genotype (cultivar) were plotted using AMMI analysis.
Considerable difference was observed in the reactions of 8 rice genotypes to 10 RYMV isolates from 6 different localities in Mali in terms of SPAD and yield reduction (Table 3).
|Fig. 1:||Genotype (cultivar) by environment (isolate) interaction effects on SPAD reduction using additive main effects and multiplicate interaction (AMMI) analysis. Genotype: V1 = Gigante; V2 = Bouake189; V3 = Faro11; V4 = Moroberekan; V5 = Lac 23; V6 = ITA235; V7 = PNA647F4-56; V8 = H232-44-1-1. Environment: MA = PR44; MB = PR35; MC = PR28; MD = PR17; ME = PR16; MF = PR76; MG = PR92; MH = PR95; MI = PR121; MJ = PR124|
|Table 3:||Analysis of means comparison for percentage SPAD reduction due to RYMV disease|
|In a column, means followed by a common letter are not significantly different at the 5% level by Duncans Multiple Range Test|
|Table 4:||Analysis of means comparison for percentage yields reduction due to RYMV disease|
|In a column, means followed by a common letter are not significantly different at the 5% level by Duncans Multiple Range Test|
|Fig. 2:||Genotype (cultivar) by environment (isolate) interaction effects on yield reduction using additive main effects and multiplicate interaction (AMMI) analysis. Genotype: V1 = Gigante; V2 = Bouake189; V3 = Faro11; V4 = Moroberekan; V5 = Lac 23; V6 = ITA235; V7 = PNA647F4-56; V8 = H232 - 44-1-1. Environment: MA = PR44; MB = PR35; MC = PR28; MD = PR17; ME = PR16; MF = PR76; MG = PR92; MH = PR95; MI = PR121; MJ = PR124|
|Fig. 3:||Cluster dendogram showing classification of environment (isolate) pathogenic level to genotype (cultivar) using additive main effects and multiplicate interaction (AMMI) analysis. Environment: MA = PR44; MB = PR35; MC = PR28; MD = PR17; ME = PR16; MF = PR76; MG = PR92; MH = PR95; MI = PR121; MJ = PR124|
|Fig. 4:||Cluster dendogram showing classification of genotype (cultivar) level of resistance to environment (isolate) using additive main effects and multiplicate interaction (AMMI) analysis. Genotype: V1 = Gigante; V2 = Bouake189; V3 = Faro11; V4 = Moroberekan; V5 = Lac 23; V6 = ITA235; V7 = PNA647F4-56; V8 = H232-44-1-1. R = Resistant; MR = Moderately resistant; S = Susceptible|
According to AMMI analysis, MA and MD isolates were responsible mainly for unfavorable interactive conditions leading to significant yield and SPAD reduction in all the rice cultivars (Fig. 1 and 2). Based on cluster dendrogram classification for isolates pathogenic and genotypes viral resistance levels, MA, MD, MH were classified as Highly Pathogenic Isolates (HPI) and MB, MC, ME, MF, MG, MI, MJ were classified as Mildly Pathogenic Isolates (MPI). Two genotypes (Gigante and Lac 23) were highly resistant, while the five others were moderately resistant and one genotype (Bouake 189) was susceptible (Fig. 3 and 4).
This study revealed the existence of two pathotypes of RYMV isolates in six localities in Mali. The two pathotypes consist of the Highly Pathogenic Isolates (HPI) and the Mildly Pathogenic Isolates (MPI). The Additive Main Effect and Multiplicative Interaction (AMMI) analysis seemed effective in understanding and explaining complex genotype by environment (GE) interactions between the rice genotypes and RYMV pathotypes (Ebdon and Gauch, 2002; Onasanya et al., 2004). Such interactions could generate complex data sets difficult to understand with ordinary analysis of variance (ANOVA). In the current study, 10 RYMV isolates used covered major rice ecologies from six different localities in Mali leading to very high RYMV interactions among rice genotypes. The existence of HPI and MPI RYMV pathotypes obtained in this study (NGuessan et al., 2000) have led to differential interactions among genotypes with heavy implications on the genotype resistance and yield stability.
As revealed by this study, genotypes pathogenic resistance to HPI and MPI RYMV pathotypes first occurs at the level of the individual and involves physiological or behavioral tolerance or adaptability. Subsequent response to increasing viral pathogenicity may involve survival only of the better-adapted genotypes (Ebdon and Gauch, 2002; Barrett and Rosenberg, 1981). HPI pathotypes, which consist of three isolates, could be described as possessing both stable and high level of virulence affecting genotypes resistance to RYMV across 6 localities in Mali. Under different rice ecologies in Mali, V1 and V5 genotypes possessed heterogenous viral resistance characteristics making them to be more stable, adaptable and more resistant to stress induced by HPI pathotypes originated from different localities. Genotypes that have adapted to endure variable isolates or strains infestations are more likely to tolerate an independent stress compared to those genotypes that are only adapted to a fixed isolate or strain (Barrett and Rosenberg, 1981; Annicchiarico and Perenzin, 1994).
As RYMV isolates population increases, there is probability that MPI pathotypes population will be more than that of HPI and possible interactions between these two pathotypes could lead to the emergence of new highly virulent strains. The use of highly resistant genotypes (V1, V5) will potentially reduce HPI and MPI pathotypes population and their interactions. There is probability that the two resistant genotypes (V1, V5) obtained in this study will survive and evolve through combinations of genes present in the population (Barrett and Rosenberg, 1981; Crossa et al., 1990) since population resistance is enhanced by genes polymorphism that may result in short-term selection of more tolerant genotypes in stressful viral environments (Ebdon and Gauch, 2002; Barrett and Rosenberg, 1981).
Conclusively, the high genotypes by environment interactions in the reactions of rice genotypes to RYMV revealed the existence of two pathotypes of RYMV in Mali. This information could be useful in the rice breeding programs aiming at deployment of RYMV resistant genotypes to different rice ecologies and localities in Mali.
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